Sunday, December 26, 2010

Ventilation That You Need in Your House

How to Get the Ventilation That You Need in Your House

 The old farmhouses where people lived several generations ago had little resistance to air. They were drafty, uncomfortable and very dry in winter due to high air-change rates. Modern housing restricts air entry through good air barriers and sheet materials, such as plywood, oriented strand board (OSB) and drywall. In fact, new Canadian houses and many retrofitted existing houses are so airtight that you cannot count on incidental leakage for good indoor air quality. You must induce or augment the house air-change rate using mechanical ventilation, a requirement in the National Building Code for new houses since 1990.

What Does This Mean to You?

Figure 1 — Infiltration and exfiltration of air in a house
Figure 1 — Infiltration and exfiltration of air in a house

If your house is stuffy, odours linger, or humidity is high in fall and winter, it is likely that your house does not have adequate fresh air. If you or your children have respiratory conditions, such as asthma, bronchitis or chronic colds, getting the proper amount of fresh air is even more important. Opening windows can be part of the solution, but open windows can be a security risk (in some neighbourhoods), can cause comfort problems and can increase heating and cooling costs. Furthermore, opening windows may not improve indoor conditions under all circumstances. Using a mechanical ventilation system, such as an exhaust fan or a heat recovery ventilator (HRV), can be more effective.


Ventilation is often defined as a means of providing fresh air. However, the word “ventilation” can describe several different types of air movement.


Figure 2 — Distribution of air throughout a house
Figure 2 — Distribution of air throughout a house

We get some fresh air from natural infiltration. This is the amount of fresh air that comes into your house through leaks, and is sometimes shown as house air changes per hour (ACPH). An air-change rate of 0.5 ACPH means that half the house air is changed every hour, or that the amount of fresh air that enters the house every two hours equals the volume of the house. Exfiltration, or the amount of air that exits the house, always equals infiltration — if it didn’t, the house would either implode or explode.


The fresh air needs to be moved around the house, particularly to rooms with closed doors (such as bedrooms). This distribution usually requires fans and ducting systems. Imagine a two-storey house with all the fresh air infiltrating or being delivered into the basement where the clothes dryer is running. Without distribution, the fresh air would be removed from the house by the clothes dryer before it reached the occupants on the floors above. Only the basement would receive fresh air.


Figure 3 — Circulation of air in a house
Figure 3 — Circulation of air in a house

Even if fresh air is introduced to a room, it will often need some help to be circulated to all parts of the room. Rooms with lots of furniture or stored items are susceptible to having under-ventilation in some parts of the room. Fans can help here, too.

Is Ventilation Necessary?

Ventilation and good air quality are sometimes under-appreciated. If your furnace breaks down in the winter and the house starts getting cold, you will notice that problem within a couple of hours at the most. Insufficient ventilation will generally not be noticed as quickly as it takes time for symptoms, such as stuffy air, to develop.
A good time to check your indoor air quality is when you enter your house, before you get accustomed to the indoor air. Does it have a distinctive odour? Is it fresh and neutral? People moving to a house with good ventilation from a house with bad ventilation will recognize that the indoor air quality in their previous residence was not as good as in their new home.
People need fresh air all the time, but the need for additional ventilation will change. In the middle of winter, when it is very cold or windy outside, the natural air-change rate of the house will be highest and you may not require additional mechanical ventilation.
However, most times in fall, winter and spring, having some mechanical ventilation may make sense. Mechanical ventilation is beneficial for mid-summer if you are using an air conditioner and do not open windows for extended periods of time. In fact, summer is when houses have the lowest natural air change rate. Figure 4 shows the results of recent research that monitored air change rates in an Ottawa home during the summer. Most of the time, the house air change rate was below 0.3 air changes per hour — a recognized threshold for good indoor air quality.

Providing Good Ventilation

All houses can be provided with good ventilation. It is easier to do so when you have ducted air-moving equipment. Here is advice for a variety of houses, starting from the most difficult.
Figure 4 — Summer natural air infiltration rates for an Ottawa home not using its mechanical systems and with its windows closed
Figure 4 — Summer natural air infiltration rates for an Ottawa home not using its mechanical systems and with its windows closed

Older Houses with No Ducts or Fans

Intentional ventilation was a foreign concept to homeowners of houses built 60 or more years ago. Open windows could sometimes induce a breeze in hot summer periods. Open windows were also the standard ventilation for most bathrooms. Houses were so air leaky that the common goal was to reduce ventilation, rather than to promote it.
However, ventilation should be considered if these houses have been tightened as a result of renovations and energy retrofits. While opening windows will still play a part in a ventilation strategy, people are reluctant to leave them open long enough to ensure adequate air quality control. At the very least, bathroom or kitchen fans vented to the outside can be installed to control moisture in these areas. Sometimes, ducting can be retrofitted into chases or installed on the basement ceiling, if the homeowners want the benefits of an air distribution system. Do not install ducting systems in the attic, as the temperatures in Canadian attics are inappropriate for ducting in both the summer and winter. Cross-ventilation, through windows on either side of the house, can contribute to an increased ventilation rate.

Houses with Bathroom Fans and Forced-Air Furnaces

This is the basic system for older houses. The bathroom fan vents air from the house. Infiltration matches the rate at which the exhaust fan vents air outdoors, and outdoor air enters through various leaks in the house envelope. The furnace fan and ducting system can mix this fresh air with house air and distribute it around the house. In new houses in some parts of the country, the “ventilation” fan switch is located by the thermostat so that the furnace distribution fan can be turned on at the same time as the ventilation fan to ensure ventilation air is distributed and circulated evenly throughout the house.
Is this the best way to run a ventilation system? Probably not, but it can be made to work. First, install a good bathroom fan vented to the outside. Make sure that it is highly energy efficient (less than 50 watts) and quiet (1.5 sones or less), so you can use it without getting annoyed. Make it small (25 L/s or 50 cu. ft./min.), or ensure that you can control the speed. Leave it running all the time, or at least when the house is occupied. That ensures that some fresh air is entering the house. If your furnace or air conditioner is running frequently, then the fresh air is being distributed. You can leave your furnace fan on to ensure distribution, but that will usually cause high electrical consumption. New furnaces can be purchased with DC motor fans that can be controlled to run at under 100 W at low speed. Older furnace fans have power consumption of 400 – 600 W.
If you have an inefficient furnace fan and need to use the furnace fan for distribution, consider getting a fan-cycling control device. This controller can be used to turn the furnace fan on for 20 minutes per hour, for instance, to reduce the fan motor electrical consumption. Even 20 minutes an hour should ensure adequate circulation. Having the ventilation fan electrically linked to the furnace fan will ensure that when you turn your ventilation system on the furnace fan comes on too and the fresh air gets to where it is needed.

Heat Recovery Ventilators (HRV) Connected to the Furnace Ducting System

HRVs are efficient devices that provide good ventilation without a big energy cost for heating the outside air. The HRV uses the air being exhausted to preheat incoming air. They are able to recover 60 – 80% of the heat in the outgoing air which means, in winter, the incoming air will be much warmer than outside air (but still cooler than house air). The balanced ventilation provided by an HRV does not usually create house depressurization that could effect the proper and safe functioning of fuel fired appliances in the home.
Since the HRV in this case uses the furnace ductwork, the furnace fan has to be on, or at least cycling on and off, for the fresh air to get distributed around the house. The same considerations apply about getting a furnace with an efficient fan motor or a furnace-fan cycling controller. Note also that all HRVs need maintenance and cleaning.
See CMHC’s About Your House fact sheet Maintaining Your HRV for advice.

Separately Ducted HRVs

This system is the most efficient way to ventilate your house, as the HRV does the air preheat and the HRV fan motor distributes the fresh air and collects the stale air through its own ducting system. This means that the furnace fan motor does not need to run as part of the ventilation distribution system which represents significant energy savings.
The HRV should be used anytime the house is normally closed up. Run it continuously at low or medium speed, and switch to “high” for parties or other times when you want more ventilation. If you are buying an HRV, pick one that has been independently certified (e.g. choose one with a Home Ventilating Institute or HVI certification sticker). Choosing one with a high “sensible recovery efficiency” and a fan motor with low energy consumption will ensure lowest operating costs.  Also ensure that the contractor installing the HRV has been certified to do so through an organization such as the Heating, Refrigeration and Air Conditioning Institute of Canada (HRAI).
As mentioned in the previous section, make sure that the HRV is regularly maintained.

Running the Ventilation System

Ventilation is not as critical when the house is unoccupied, although some houses require ongoing mechanical ventilation to keep the windows from fogging up in winter and to prevent the subsequent damage to window frames, trim and walls. It is especially important to have high ventilation rates for:
  • The first fall and winter for a new house, to get rid of construction moisture.
  • Houses with high numbers of occupants, either temporary or permanent.
  • Houses in which renovation activities (drywalling, painting, floor re-finishing and so on) or new furniture may be creating high concentrations of pollutants.
  • Houses in which bedroom doors are generally closed during sleeping hours. Open doors help ensure that the bedroom air has the same quality as the air in the rest of the house. Closed-door bedrooms require higher ventilation rates or good distribution systems.
  • Houses whose residents have respiratory problems (people allergic to outdoor pollutants require filtered outdoor air).


Most Canadian houses will benefit from the fresh air supplied by mechanical ventilation. In new houses, that fresh air is most efficiently delivered by a separately ducted HRV. In existing houses, quiet and efficient bathroom and kitchen fans, or HRVs when practical, can improve air quality. Using efficient furnace fan motors or furnace fan cycling controls will help to distribute fresh air to all rooms of the house at a reasonable cost.


Sunday, December 5, 2010

Mechanical Ventilation for Medical Students

Patients may require mechanical ventilation for chronic or acute respiratory failure.  When a patient is intubated and placed on ventilation, there are several settings a physician may have to manage on the mechanical ventilator.  These ventilator settings include the positive end expiratory pressure (PEEP), the fraction of inspired oxygen (FiO2), the respiratory rate and the tidal volume.

As a medical student in the ICU, you will surely get asked several basic questions on mechanical ventilation.  Wrong answers to these questions will bore your attending, amuse your residents, and feed that desperate fear you have of accidentally killing your future patients through your own ineptness.
With that being said, correctly answering questions on the mechanical ventilation of your ICU patients could make your team nod proudly, yes, proudly.  The simplified basics of mechanical ventilation are described below.  Once you get the basics, move on to the second article in this series, “How to determine the initial settings for mechanical ventilation”.

There are two aspects medical students must understand about ventilation control settings:
1. Oxygenation and pH. 
2. Volume and Pressure
The reason we want to control inspiratory volume and pressure is because we are trying to walk a fine line between assisting the patient with breathing without allowing their respiratory muscles to atrophy or drastically decreasing the cardiac preload.
We want to control the oxygen levels and pH of our patient’s bodies to sustain healthy tissues and metabolic function.  The pH is reflected in our carbon dioxide and bicarbonate balance.  So we adjust oxygen and carbon dioxide levels as needed.

In the ICU, an oxygen concentration on the arterial blood gas (pO2) can be as low as 70 and still be considered acceptable.  How do we keep the pO2 at or above 70 mmHg?  Adjust the FiO2 and PEEP on the ventilator settings. Altering the patient’s pO2 can be just that simple- FiO2 and PEEP
The FiO2 is the fraction of inspired oxygen, while PEEP is the positive end expiratory pressure.
Carbon dioxide and pH:

The blood pH in your patient should be 7.4 plus or minus 0.5 and the carbon dioxide levels on the arterial blood gas (pCO2) should read 40 mmHg plus or minus 5.  How do we keep the pH and CO2 at safe levels? Adjust the respiratory rate and tidal volume.  Again, it’s just that simple.  You can alter the pH and pCO2 for your patient by adjusting the respiratory rate and the tidal volume on the ventilator. 

[Remember: The rate multiplied by the tidal volume will give you the minute ventilation (in units of L/min).  You may or may not get pimped on minute ventilation but it’s easy enough to remember: tidal volume x respiratory rate.]

So in short, the two basic goals of mechanical ventilation are to get the oxygen and carbon dioxide on the arterial blood gas close to perfect, and to do this with a pressure and volume that is appropriate for the medical situation the patient is in.  Alter the oxygen with the FiO2 and PEEP.  Alter the carbon dioxide (and thus pH) with the respiratory rate and tidal volume

Thursday, November 11, 2010


A belt is a looped strip of flexible material, used to mechanically link two or more rotating shafts. They may be used to move objects, to efficiently transmit mechanical power, or to track relative movement. Belts are looped over pulleys. In a two-pulley system, the belt may either drive the pulleys in the same direction, or the belt may be crossed so that the shafts move in opposite directions. A conveyor belt is built to continually carry a load between two points.

Power Transmission

Belts are the cheapest utility for power transmission between shafts that may not be parallel. Power transmission is achieved by specially designed belts and pulleys. The demands on a belt drive transmission system are large and this has led to many variations on the theme. Belts run smoothly and with little noise, and cushion motor and bearings against load changes, albeit with less strength than gears or chains. However, improvements in belt engineering allow use of belts in systems that formerly allowed only chains or gears.

Pros and cons

A belt drive is simple, inexpensive, and does not require parallel shafts. It helps protect a car from overload and jam, damping it from noise and vibration. Load fluctuations are shock-absorbed (cushioned). They need no lubrication and only little maintenance. They have high efficiency (90-98 percent), higher tolerance of misalignment, and are relatively inexpensive. Clutch action is activated by releasing belt tension. Different speeds can be obtained by step or tapered pulleys.
However, the angular-velocity ratio is not constant or equal to that of the pulley diameters, due to slip and stretch. Heat accumulation is present, and speed is limited to approximately 7000 feet per minute (ft/min), and a power of only 500 horsepower (hp). Temperatures ranges from -31 to 185°F. Adjustment of center distance or addition of an idler pulley is crucial for balancing the wear and stretch. To install endless belts, the relevant assembly must be dismantled first.

Flat belts

Belts on a Yanmar 2GM20 marine diesel engine.
Belts on a Yanmar 2GM20 marine diesel engine.

Flat belts were used early in line shafting to transmit power in factories. It is a simple system of power transmission that was well suited to its time in history. It delivered high power for high speeds (500 hp for 10,000 ft/min), in cases of wide belts and large pulleys. However, these drives are bulky, requiring high tension leading to high loads, so vee belts have mainly replaced the flat-belts (except when high speed is needed over power). The Industrial Revolution soon demanded more from the system, as flat belt pulleys need to be carefully aligned to prevent the belt from slipping off. Because flat belts tend to slip towards the higher side of the pulley, pulleys were made with a slightly convex face (rather than flat) to keep the belts centered. The flat belt also tends to slip on the pulley face when heavy loads are applied. In practice, such belts were often given a half-twist before joining the ends (forming a Möbius strip), so that wear was evenly distributed on both sides of the belt (DB). A good modern use for a flat belt is with smaller pulleys and large central distances. They can connect inside and outside pulleys, and can come in both endless and jointed construction.

Round belts

Round belts are a circular cross section belt designed to run in a pulley with a circular (or near circular) groove. They are for use in low torque situations and may be purchased in various lengths or cut to length and joined, either by a staple, gluing or welding (in the case of polyurethane). Early sewing machines utilized a leather belt, joined either by a metal staple or glued, to great effect.

Vee belts

The Vee belt (also known as V-belt or wedge rope) provided an early solution to the slippage and alignment problem. It is now the basic belt for power for the transmission. It provides the best array of traction, speed of movement, load of the bearings, and longer service life. It was developed in 1917 by John Gates of the Gates Rubber Company. They are generally endless, and their general cross-section shape is trapezoidal. The "V" shape of the belt tracks in a mating groove in the pulley (or sheave), with the result that the belt cannot slip off. The belt also tends to wedge into the groove as the load increases—the greater the load, the greater the wedging action—improving torque transmission and making the vee belt an effective solution, needing less width and tension than flat belts.
V-belts trump flat belts with their small center distances and high reduction ratios. The preferred center distance is larger than the largest pulley diameter but less than three times the sum of both pulleys. Optimal speed range is 1000-7000 ft/min. V-belts need larger pulleys for their larger thickness than flat belts. They can be supplied at various fixed lengths or as a segmented section, where the segments are linked (spliced) to form a belt of the required length. For high-power requirements, two or more vee belts can be joined side-by-side in an arrangement called a multi-V, running on matching multi-groove sheaves. The strength of these belts is obtained by reinforcements with fibers like steel, polyester or aramid (e.g. Twaron). This is known as a multiple-belt drive.
When endless belts do not fit the need, jointed and link vee-belts may be used. They are, however, weaker and speed up to only 4000 ft/min. A link v-belt is a number of rubberized fabric links held together by metal fasteners. They are length adjustable by dissasembling and removing links when needed.

Film belts

Though often grouped with flat belts, they are actually a different kind. They consist of a very thin belt (0.5-15 millimeters or 100-4000 microns) strip of plastic and occasionally rubber. They are generally intended for low-power (ten hp or seven kW), high-speed uses, allowing high efficiency (up to 98 percent) and long life. These are seen in business machines, tape recorders, and other light-duty operations.

Timing Belts

Timing belts, (also known as Toothed, Notch or Cog) belts are a positive transfer belt and can track relative movement. These belts have teeth that fit into a matching toothed pulley. When correctly tensioned, they have no slippage, run at constant speed, and are often used to transfer direct motion for indexing or timing purposes (hence their name). They are often used in lieu of chains or gears, so there is less noise and a lubrication bath is not necessary. Camshafts of automobiles, miniature timing systems, and stepper motors often utilize these belts. Timing belts need the least tension of all belts, and are among the most efficient. They can bear up to 200 hp (150 kW) at speeds of 16,000 ft/min, and there is no limit on speed.
Timing belts with a helical offset tooth design are available. The helical offset tooth design forms a chevron pattern and causes the teeth to engage progressively. The chevron pattern design is self-aligning. The chevron pattern design does not make the noise that some timing belts make at idiosyncratic speeds, and is more efficient at transferring power (up to 98 percent).
Disadvantages include high starting price, grooving the pulleys, less protection from overload and jam, no clutch action, and backlash.

Specialty Belts

Belts normally transmit power on the tension side of the loop. However, designs for continuously variable transmissions exist that use belts that are a series of solid metal blocks, linked together as in a chain, transmitting power on the compression side of the loop.
"T belts" that simulate rolling roads for wind tunnels can be made to reach speeds of up to 250 km/h.

Thursday, October 28, 2010


I have never failed to register my dislike of ‘small lofts’ particularly those mean and undersized structures which dictatorial, bureaucratized urban district councils sometimes permit their unfortunate tenants to erect in their back gardens. Believe it or not, these bureaucrats actually specify the size the loft is to be, yet they know nothing about racing pigeons or their permitted habitat nor about the hygiene without which animals cannot be kept as they should be. For instance, what do those form-filling bureaucrats know about ventilation and its affect on animals? Sweet Fanny Adams!

At the same time, what do fanciers know about ventilation and its effect on hygiene in the loft? I regret to have to say that the majority of fanciers have no idea at all! They think you can bung racing pigeons into any old shed, or disused barn, and then proceed to monopolize the prizes. Such thinking is so wide of the mark as to be laughable.

The first product of inadequate ventilation in a loft is the production of gases. What I want you all to understand is that gases have the facility of diffusion, a function which does not apply to everything.

It is in order for me to give you a further example, this time of non-diffusion. Let us help ourselves to a large glass jar and fill a third of its capacity with mercury, which is an extremely heavy metallic liquid. We now take up a can of water and pour this into the glass container until the liquid has taken up its third of the accommodation. Finally, we fill the remaining third of the glass jar with oil. So, we have oil, water and mercury stacked up inside the jar and wonder of wonders! Each liquid stays in its own space forbearing to mix with the other liquids in the jar. Even if we shake the contents of the glass jar in an attempt to make the contents diffuse, the three elements sort themselves out and stratarise with a third of the jar occupied by the mercury at the bottom, then the water, and finally the oil occupying a layer on top of the water. So, we have demonstrated the fact that certain elements won’t diffuse or, to put it another way, they won’t mix, representing a perfect case of class distinction!

Let us carry out another experiment which is as simple and informative as the non-diffusion demonstration. Once again we help ourselves to a capacious glass jar and into ft we pour a comparatively heavy gas, such as oxygen. Next, we pour into this jar a light gas, such as hydrogen. Right, we have two gases corked up in the Jar, a heavy one and a light one, and we leave them alone for a while. Soon, by chemical examination (because we cant see these gases which are invisible) we test the contents of the jar to discover if the two gases we put in it are infact not still separate and we find that the two gases had, in fact, diffused (mixed). This mixture would be the same throughout the jar, a perfect mixture of the two gases. This property (the mixing together of several gases) has a considerable bearing on ventilation, as we shall see. In fact, we have seen that one gas, which is sixteen times heavier than the other, has diffused with ft without difficulty!

Let us now tackle the aspect of loft ventilation by using the owner as a guinea pig. So, you are sitting in the corner of a room which has been sealed up, with you inside. You are breathing, of course, taking in air and exhaling carbonic acid gas. Although you are using up the air and replacing it with a poisonous gas, the latter gas does not work on you at once to affect your inhalement of air. This is because the carbonic acid gas you exhale from your lungs tends to diffuse with the air in the room, mixing freely with it. This process could continue for a long time before breathing became difficult or laboured because the air in the whole room would not deteriorate to a state where it affected breathing until the foul air given off by the lungs had practically exhausted the oxygen content of the gases.

We know, because we were taught at school, that air expands with heat and contracts with cold. When heat is put to air it tends to become lighter and because it is less dense than colder air it is forced upwards by the cool air which continues to press against it. Thus, we define the well-known statement that ‘hot air rises’ for the simple reason that it is being pressurised by the cooler air round about and below it. We note how air moves in a room ‘under pressure’ and it is a fact that winds are caused the same way viz under cooler pressure.

If we were naive and simple (which we most definitely are not) we would kid ourselves that all we need in a loft are inlets along the lower sections of walls to allow the cooler air to pass into the loft and outlets at higher altitudes to permit the cooler air to pressurise the hotter air and force it out of the loft, the hotter air being the carbonic acid gas breathed out by the pigeons in the loft after they had inhaled the cooler, purer air. As I have said, we are far too clever to fail for that idea because your ‘Old Hand’ knows very well that you can’t change the air efficiently in one large compartment (or in one small one) more than three times per hour without setting up unwanted draughts which could be of a harmful nature. The last thing we wish to do is put your birds in jeopardy but you can be quite sure that your old preceptor has far more sense than to commit an elementary error of this kind.

Let us bear In mind that gases, including harmful ones, freely mix together, thereby diffusing impurities as well as purities, so we must get to know more about the art of ventilation before we can hope to provide our bird with a safe and proper home. Perhaps we had better take the example once again of a man sitting in a seated room, breathing the air in it. We know from our tables and statistics that a man can turn out enough carbonic acid gas from his lungs and his skin, each hour, sufficient to render about 3,000 cu.ft. of air unfit for further respiration. A simple calculation will show that this person must be provided with a room l2ft x l0ft x 8ft as an alternative to being the target of nasty draughts. So, we now come to the inescapable basic fact of all schemes of ventilation viz that an animal must be given sufficient room viz ample air space.

If you care to look at ft in another way, you can say that ‘air space’ is really ‘lung space’. We must understand that if there is insufficient room, or air space, the metabolism must suffer from toxic gases which quickly put impurities into the bloodstream. How can any fancier hope to excel when racing pigeons whose blood has been poisoned by carbonic acid gas through loft overcrowding? You can feed your birds on the finest food money can buy and change the drinkers every few minutes, but without benefit to birds who are living in overcrowded accommodation. They would be steadily gassed every day and every night which, though not killing them immediately, would exert a subtle but lethal effect on them. Perhaps now you will realise why I detest ‘small lofts’ and in particular those heartless bureaucrats who are probably the country’s main contributors to the poisoning of racing pigeons.

If I lived in a ‘council house’ and was therefore at the mercy of dictatorial bureaucrats I would not erect a loft at all! Instead, I would construct an aviary with four walls of wire mesh. Then I would drape some transparent polythene sheeting over it. Incidentally, I’m not quite sure about modern council regulations governing the erection of ancillary buildings but I know that up to recent times the council had no Jurisdiction over property that is transparent. In other words, I hold the opinion that anyone could build an aviary with a transparent roof without needing permission from the local authority but please don’t act on this advice without getting good legal opinion, or an opinion from the RPRA, which probably knows the ins-and-outs of modem local by-laws.

I would then insert a wire-mesh floor some 12in above ground level so that birds could not reach the ground below the wire-mesh floor. One could stick a wooden rod or two through both wails of the mesh to provide perches. Nestboxes could be put in the aviary in the proper season and I maintain that birds living in this structure would be healthier and fitter than any birds kept in a loft or structure with wooden or solid walls.

It would be almost impossible for birds living in this way to contract respiratory disease, or anything like it. They would have to be healthy to live, anyway.

The only birds I lost in such conditions were some Belgian squeakers which I suspected of suffering from respiratory disease. I wasn’t sure so decided to take no chance. There were thirteen of them and they went into the aviary one November night. Four of them toppled from their perches so they were indeed affected, but the remaining nine stayed to thrive and prosper. If the four had contracted the disease so had the other nine but the healthy conditions in which they were compelled to live cleared up the trouble once and for all.

As we have seen, a cubic air space of about 3000 cu.ft. is necessary for the well-being of one human but the average quota for animals is 25 cu.ft. for each pound of body weight. As the average racing pigeon weighs only l6oz (1-1 lb) altogether then 9 cu.ft. would appear to be quite sufficient. On the basis of a pigeon requiting about one third of the 25 cu.ft. of air, a small loft 9ft x 9ft x 6ft high would accommodate about 54 pigeons. So much for theory!

However, as pigeon fanciers we know that we are not just beset by the production of carbonic acid gas through pigeon respiration but there are other sources of obnoxious loft gas build up. For instance, what about the pigeons’ droppings? These fall onto the loft floor where they build up all through the night, giving off ammonia.

This brings me back to what I was saying about the diffusion of gases. No matter how much individual gases weigh, they mix freely and instantly, to produce yet another type of gas, some productions being worse than others. In a pigeon loft, where perched birds spend the night building up a floor or perch layer of wet droppings, the said droppings give off ammonia gas which diffuses (mixes freely) with the carbonic acid gas to create an entirely but even more obnoxious gas, known as carbamate gas. This additional hazard militates against the sums we have just been doing in respect to air space per bird. Therefore, a much more liberal amount of air space must be provided if we are to counteract this inevitable drawback.

How is a fancier to know if the production of obnoxious gas to generate atmospheric impurity has reached a dangerous level, affecting the general loft ventilation system? Well, nature fitted him with a very reliable obnoxious gas detector - his nose! If you can smell impurities in the air, then the ventilation is inadequate, in fact, it is downright dangerous. No one should be able to smell ‘pigeons’ in a pigeon loft, nor should any nose be assaulted by the abominable stink of ammonia from droppings. If you can smell either pigeons or droppings, or both, the loft ventilation system should be overhauled at once. To delay the work is to inflict respiratory disease on all the loft’s inmates.

Take your own home for example. If you can sniff unpleasant odours, by way of a general mustiness, two possibilities are imminent: (1) The ventilation system is inadequate; (2) There is ‘rising damp.’ The risk of (2) above is inherent in every pigeon loft which has been erected without adequate damp-proofing, which is to say that a waterproof-course has not been laid between the piers which support a structure on the ground and the loft Itself. Before standing the loft on anything one should first cover the pier or prop with slate, lead impregnated damp course, polythene, or some material which is waterproof and has lasting qualities. Otherwise, rising damp will reach up into the loft and begin to poison the internal air.

Fanciers should know that rising damp’ is not merely moisture moving up the wall of the loft through capillary attraction but it is a living, seething vile bacteria, which destroys as it progresses, with its single task of multiplying its spores (cells) in its course of encroachment on the structure. No good racing pigeons should be exposed to this damnable risk.

The ‘nose detection’ of obnoxious gas can only operate when one is on the point of entering a loft. After some time spent in the loft’s interior the nose stales in its quest for odours and tends to get used to the vitiated air. Therefore, practice sniffing when you enter the loft and if you can smell atmospheric impurities decide to do something about the situation immediately.

As I said, it is necessary for us to do our sums again by taking into consideration not merely the carbonic acid gas exhaled by perched pigeons but also the ammonia gas exuded by the damp droppings. According to the standard adopted by scientists, a loft 9ft x 9tt x 6ft high (486 sq.ft.) would accommodate 24 pigeons. Judged by what I have seen on my past travels most fanciers are keeping double that number of birds in their lofts, thereby Indulging In blatant overcrowding with the worst possible results. Indeed, most of the birds in these lofts will be suffering from respiratory disease which, in most cases, will have escaped the notice of the owner.

When calculating air space in order to arrive at figures which show there is sufficient air to ventilate the structure there is a limit to how high we can go inside the loft. For instance, if we supposed that the loft roof was some 18ft above the floor, the air in the upper layer of some lift would not be deemed air that was available for respiratory purposes to the inmates of that loft. Although the actual accommodating air space height is somewhat of an arbitrary nature (science poses pros and cons) I think we can discount air that stacks up more than 7ft from the floor. This requirement Indicates, in no uncertain manner that floor area is of the greatest factor, not height. This means that loft designers and builders should be encouraged to provide depth (or width) as an important provision towards the ventilation problem.

Air space in buildings which contain chimneys is less critical than buildings which do not contain chimneys, such as pigeon lofts. The chimney does not merely discharge the smoke from fires, it also has the effect of sucking out the used air so that new, fresh air can penetrate the rooms and replace the vitiated air which is being extracted via the flue. I saw ducted ventilation (square wooden ducts installed) in the loft of Van Den Bosche of Ghent. Although the loft was in the attic (loft) of a house which peaked up to a considerable height to the centre ridge, the partners had installed the duct to bring fresh air down from the height to a level some 6ft above floor level. I complimented them on their cleverness and pointed out the remarkable condition of their birds as their response to the purer air they were breathing. It is a fact which so few fanciers will subscribe to, that pigeons in most lofts would react to Improvement made to the ventilation system by assuming a high level of ‘Condition’ of a kind few fanciers have ever seen. I hereby invite my reader to take his nose to his loft and, if he is not entirely satisfied with what he encounters, he sets about the introduction of a system of ventilation which can bestow real benefits on his pigeons.

My own birds occupy loft sections which are 8ft deep, 5ft wide and 6ft 61n high. In a section of this size I allow six pairs of birds to breed, no more. However, it should be borne in mind that apart from the above dimensions I have also installed fittings which I have proved definitely assist the ventilation by helping to keep the fresh air moving through the loft. I will not permit any air inside the loft to ‘dwell’ viz remain static. I require the air to flow into the loft and keep moving until it passes out to give perfect ventilation.

I know there are those who will cry out against this system by pointing out that I have already said that when a loft’s internal air is changed more than three times in an hour draughts are created. Well, what are draughts? A draught is a stream of cold air which is playing into an interior containing warm air. But there is no warm air in my loft! Like those other fanciers who have created ‘east wind pigeons’, I deliberately built my loft to face east and it is wide open to this cold wind. The result is that the air inside my loft is of the same temperature as that of the coldest which is found outside it, therefore no draughts are possible. I wish my reader to understand that you can’t create an ‘east wind’ family of racing pigeons if you pamper your birds and give them heated or ‘protected’ interiors. Why not? Because an ‘east wind’ strain of pigeons can only become so if it develops the kind of plumage which can be guaranteed to keep its body temperature at the correct level (107’F) in the coldest of conditions but no pigeon is going to grow such a thick plumage unless its environment demands growth of that kind. Hence the exposure to east winds.

The secret of keeping the air moving in the loft is to install louvers at floor level and a 4in gap running the length of the rear wall at the position where it meets the roof. In a loft fitted up in this way the air streams from the front and both ends (like the front, the end walls of the loft must also be louvered at floor level) Incidentally the gap in the rear wail must not be louvered but be covered with fine mesh wire.

If this kind of ventilation is installed (and it is the best) the fancier should again use his nose as an air impurity detector, especially when he has nestlings in the nest. Obviously, the hatching out of nestlings means something like a doubling of those sources of supply of carbonic acid gas and the ammoniac gases from the droppings. Can you still smell pigeon and/or ammonia when you enter the loft? If so there is only one thing for it and that is to increase the number of louvers in front and side wails until enough air is streaming through the loft, from front to rear, to keep the air clean and sweet.

In the past, few fanciers were willing to consider the effect of bad ventilation on racing pigeons. They tended to blame a number of other external ponderables for their sad lack of success. Fortunately, more and more fanciers have seen the light and are taking notice of loft design as an influential factor in pigeon racing success. Some things one can ignore in the hope that they will go away but no amount of indifference will relieve a loft of the burdens it imposes on its inmates because of its bad design and lack of real ventilation.

Thursday, October 21, 2010

Principles of ventilation

Principles of ventilation

The basic principle of ventilation in dwellings is to create air circulation from the living space to the service or wet rooms (sweeping principle). Fresh air entrances are placed in bedrooms and living room while air extraction is placed in toilet, bathroom and kitchen. To allow air circulates in the dwelling, transfer grilles can be placed on doors. Air can also circulate below doors when there is enough space between doors and the floor.
A common shortcoming found in Serbian dwellings is the installation of an intermittent small extraction fan in the bathroom without any air supply in the living part. Useless to say that this does not qualify as a ventilation system

Ventilation solutions

Any ventilation solution has two parts: on one hand the fresh air supply and on the other, the evacuation of inside stale air. A ventilation solution must provide enough fresh air but not too much. That would result in waste of heating energy. Based on that common understanding, several systems exist, each of them having pluses and minuses.

Natural supply and extract (Passive Stack Ventilation)

This ventilation system is based on the natural air movement through the dwelling as a result of internal and external temperature differences and wind induced pressure differences. Temperature and pressure differences cause moist air to be drawn up the ducts to be replaced by fresh air through inlet vents situated in the walls or window frames of habitable rooms. A free flow of fresh air from 'dry' to 'wet' areas creates whole house ventilation. 

Continuous mechanical supply and extract. 1. Fresh air distribution system, 2. Grilles for air transfer (air can also pass below doors), 3. Warm stale air extraction, 4. Warm stale air exhaust, 5. Fresh air intake, 6. Filters, 7.8. Ventilator, 9. Sound insulation system, 10. Flow management system (humidity sensitive), 11. Ventilation ducts for air supply and extraction Pre-heating of fresh air (optional),

From uncontrolled air leaks to controlled ventilation

Ventilation used not to be a concern in residential buildings. They were poorly insulated and air leaks on the windows and walls were plentiful. Keeping the place warm was difficult and cost a lot of energy, but air rewenal was naturally done, altought in a completely uncontrolled way. Air leaks are unequally distributed and are not controllable. Their flow vary in time and season and is either insufficient to provide enough fresh air or provide too much.
In a search for better comfort and rationalization of energy spending (after the energy crisis of the 70s), buildings started to be better insulated. Air leaks were greatly reduced and the quest for air tightness was launched. Achieving a low-energy building, requests outstanding thermal insulation, no thermal bridges and excellent air tightness. The natural ventilation of the past, based on construction defects, is not an option anymore.
The only way to maintain a healthy interior and control the spending of energy is to control the ventilation. In modern dwellings, were construction defects are minimal, a controlled ventilation system can provide air renewal that is able to adapt its flow to the inside air humidity and to the occupancy level. In fact, ventilation is such an important system that most European countries have already passed legislation to make ventilation compulsory. Serbia is not at that stage yet.

The fact that passive stack ventilation depends on natural motors, is a big plus: energy saving (no fan), no noise (low air speeds and no fan) and simplicity of maintenance. It is the simplest of all ventilation systems and when it is possible to implement it, it is a very good ventilation solution. Its main drawback also lies in the fact that it relies on natural motors. Passive stack ventilation might be hard to control as temperature, air pressure and humidity level vary greatly outside. It is possible to improve it further by using humidity sensitive air inlets and extract grilles. 

Continuous mechanical supply, natural extract. 1. Fresh air distribution system, 2. Grilles for air transfer (air can also pass below doors), 3. Warm stale air extraction, 4. Natural warm stale air exhaust, 5. Fresh air intake, 6. Filters, 7. Pre-heating of fresh air (optional), 8. Ventilator, 9. Sound insulation system, 10. Electronic management system, 11. Ventilation ducts for air supply and extraction.



Natural supply / Mechanical extract

This solution is similar to the passive stack ventilation where the exaust of stale air is complemented with a mechanical extract to better regulate its flow. Fresh air is coming from air inlets located in the windows or in the walls of bedrooms and living rooms, while stale air is extracted from wet rooms such as the kitchen and bathrooms. 

The main advantage of this type of ventilation is the fine control it gives on the flow of air extracted from the dwellings. This can be coupled with advanced humidity sensitive regulation systems that can adapt the airflow generated by the fan according to the needs of each wet room. Some systems even include presence detection to regulate the airflow. 
n apartment buildings, this can be implemented through an individual extraction system in each apartment or with a single extraction system for the building.

Mechanical supply / Natural extract

In this solution, the fresh air supply is provided using a ventilator while the air exhaust is natural. This makes possible to filter or pre-heat the air before it is injected in the dwelling. The air intake can also be located anywhere (such as on the roof) which can solve the problem existing with pollution or noise getting through air inlets on the windows, if the building is located in a busy street for instance. 

Because the fresh air supply has to be conducted in all rooms, this solution requires more ventilation ducts and more work. It can be a good compromise mainly if filtering of the air is necessary. 

 Individual or collective treatment of the ventilation in residential building.


Mechanical supply and extract

This is the most complex and expensive system in which both flows of air are controlled and regulated using a ventilator. The main advantage of such a system is that it can be implemented with a heat recovery unit. 

The heat recovery unit exchange the heat contained in the hot stale air taken out of the dwelling to warm up the cold fresh air getting in. Depending of the system, as much as 95% of the heat contained in the hot air can be recovered, limiting the loss of energy due to the ventilation system to a minimum

Natural supply, continuous mechanical extract. 1. Natural fresh air intake system, 2. Grilles for air transfer (air can also pass below doors), 3. Warm stale air extraction, 4. Warm stale air exhaust, 5. Ventilator, 6. Sound insulation system, 7. Electronic management system, 8. Ventilation ducts for air supply and extraction.


Ventilation in modern quality dwellings is a necessity for having a comfortable and healthy interior. Yet, in Serbia, it is very often overlooked or implemented badly with a single extract fan in the bathroom and no air intake. Different solutions exist to properly implement a ventilation system and the best one is function of the specifics of any given project. Nevertheless, a good choice has to balance cost, complexity, maintenance and energy spending. When it is possible, the Passive Stack Ventilation offers a simple and economical alternative and that is the one we decided to implement in Amadeo.

Tuesday, October 19, 2010

Ventilation with a Face Mask

Providing positive-pressure ventilation with a face mask and a bag-valve device cna be a lifesaving maneuver. Although seemingly simple, the technique requires an understanding of the airway anatomy, the equipment, and the indications.Face-mask ventilation is used in patients who have respiratory failure but arestill breathing spontaneously and in patients with complete apnea. Face-mask ventilation can be indicated in any situation in which spontaneous breathing is failing or has ceased, including cardiopulmonary arrest.

Face-mask ventilation is rarely contraindicated. However, caution is advised in patients with severe facial trauma and eye injuries. In addition, foreign material (e.g., gastric contents) in the airway may lead to aspiration pneumonitis. In these circumstances, alternative approaches, including endotracheal intubation, may be necessary.
There are many types of face masks, varying in design, size, and construction materials.Transparent masks are preferred because they allow for inspection of lipcolor, condensation, secretions, and vomitus. To maintain a good seal, the mask’s size and shape must conform to the facial anatomy. Thus, several mask shapes and sizes should be readily available.
Various bag-valve designs are available. All have a self-inflating bag and a nonrebreathing, unidirectional valve. The valve is designed to function during both spontaneous and manually controlled ventilation. Because bag-valve devices can operate without an oxygen source, it is important to ascertain that supplemental oxygen is flowing through the bag-valve device when supplemental oxygen is indicated and available. Test the bag-valve device’s capability for delivering positive-pressure ventilation before use. This can be achieved by sealing the bag-valve device connector with your thumb and squeezing the bag with reasonable force. If it is difficult to compress the bag or if air is forced between the connector and your thumb, positive pressure can be delivered.
Whenever possible during face-mask ventilation, suction should be readily available.
You may need to use airway-management adjuncts, such as disposable oral or nasal airways. Before beginning face-mask ventilation, examine the patient’s oral cavity. If possible, remove any dental prostheses or other foreign bodies that might be swallowed
or aspirated

Thursday, September 16, 2010

Packaged Air Conditioners

Packaged Air Conditioners

The window and split air conditioners are usually used for the small air conditioning capacities up to 5 tons. The central air conditioning systems are used for where the cooling loads extend beyond 20 tons. The packaged air conditioners are used for the cooling capacities in between these two extremes. The packaged air conditioners are available in the fixed rated capacities of 3, 5, 7, 10 and 15 tons. These units are used commonly in places like restaurants, telephone exchanges, homes, small halls, etc.
As the name implies, in the packaged air conditioners all the important components of the air conditioners are enclosed in a single casing like window AC. Thus the compressor, cooling coil, air handling unit and the air filter are all housed in a single casing and assembled at the factory location.
Depending on the type of the cooling system used in these systems, the packaged air conditioners are divided into two types: ones with water cooled condenser and the ones with air cooled condensers. Both these systems have been described below:

Packaged Air Conditioners with Water Cooled Condenser

In these packaged air conditions the condenser is cooled by the water. The condenser is of shell and tube type, with refrigerant flowing along the tube side and the cooling water flowing along the shell side. The water has to be supplied continuously in these systems to maintain functioning of the air conditioning system.
The shell and tube type of condenser is compact in shape and it is enclosed in a single casing along with the compressor, expansion valve, and the air handling unit including the cooling coil or the evaporator. This whole packaged air conditioning unit externally looks like a box with the control panel located externally.
In the packaged units with the water cooled condenser, the compressor is located at the bottom along with the condenser (refer the figure below). Above these components the evaporator or the cooling coil is located. The air handling unit comprising of the centrifugal blower and the air filter is located above the cooling coil. The centrifugal blower has the capacity to handle large volume of air required for cooling a number of rooms. From the top of the package air conditioners the duct comes out that extends to the various rooms that are to be cooled.
All the components of this package AC are assembled at the factory site. The gas charging is also done at the factory thus one does not have to perform the complicated operations of the laying the piping, evacuation, gas charging, and leak testing at the site. The unit can be transported very easily to the site and is installed easily on the plane surface. Since all the components are assembled at the factory, the high quality of the packaged unit is ensured.

Package AC with Water Cooled Condenser


Packaged Air Conditioners with Air Cooled Condensers

In this packaged air conditioners the condenser of the refrigeration system is cooled by the atmospheric air. There is an outdoor unit that comprises of the important components like the compressor, condenser and in some cases the expansion valve (refer the figure below). The outdoor unit can be kept on the terrace or any other open place where the free flow of the atmospheric air is available. The fan located inside this unit sucks the outside air and blows it over the condenser coil cooling it in the process. The condenser coil is made up of several turns of the copper tubing and it is finned externally. The packaged ACs with the air cooled condensers are used more commonly than the ones with water cooled condensers since air is freely available it is difficult maintain continuous flow of the water.
The cooling unit comprising of the expansion valve, evaporator, the air handling blower and the filter are located on the floor or hanged to the ceiling. The ducts coming from the cooling unit are connected to the various rooms that are to be cooled.

Wednesday, September 1, 2010

Types of Air Conditioning Systems

Types of Air Conditioners

There are various types of air conditioning systems. The application of a particular type of system depends upon a number of factors like how large the area is to be cooled, the total heat generated inside the enclosed area, etc. An HVAC designer would consider all the related parameters and suggest the system most suitable for your space.

Window Air Conditioner

Window air conditioner is the most commonly used air conditioner for single rooms. In this air conditioner all the components, namely the compressor, condenser, expansion valve or coil, evaporator and cooling coil are enclosed in a single box. This unit is fitted in a slot made in the wall of the room, or often a window sill.

Split Air Conditioner

The split air conditioner comprises of two parts: the outdoor unit and the indoor unit. The outdoor unit, fitted outside the room, houses components like the compressor, condenser and expansion valve. The indoor unit comprises the evaporator or cooling coil and the cooling fan. For this unit you don’t have to make any slot in the wall of the room. Further, the present day split units have aesthetic looks and add to the beauty of the room. The split air conditioner can be used to cool one or two rooms.

Packaged Air Conditioner



Packaged air conditioner: An HVAC designer will suggest this type of air conditioner if you want to cool more than two rooms or a larger space at your home or office. There are two possible arrangements with the package unit. In the first one, all the components, namely the compressor, condenser (which can be air cooled or water cooled), expansion valve and evaporator are housed in a single box. The cooled air is thrown by the high capacity blower, and it flows through the ducts laid through various rooms. In the second arrangement, the compressor and condenser are housed in one casing. The compressed gas passes through individual units, comprised of the expansion valve and cooling coil, located in various rooms.

Central Air Conditioning System

4) Central air conditioning system: The central air conditioning system is used for cooling big buildings, houses, offices, entire hotels, gyms, movie theaters, factories etc. If the whole building is to be air conditioned, HVAC engineers find that putting individual units in each of the rooms is very expensive initially as well in the long run. The central air conditioning system is comprised of a huge compressor that has the capacity to produce hundreds of tons of air conditioning. Cooling big halls, malls, huge spaces, galleries etc is usually only feasible with central conditioning units.

Monday, August 16, 2010

HVAC Systems

Heating, Ventilation and Air-Conditioning (HVAC) Systems
The main purposes of a Heating, Ventilation, and Air-Conditioning (HVAC) system are to help maintain good indoor air quality through adequate ventilation with filtration and provide thermal comfort. HVAC systems are among the largest energy consumers in schools. The choice and design of the HVAC system can also affect many other high performance goals, including water consumption (water cooled air conditioning equipment) and acoustics (See Acoustics).

The following actions detail how engineers can design a quality system that is cost-competitive with traditional ventilation designs, while successfully providing an appropriate quantity and quality of outdoor air, lower energy costs, and easier maintenance.

Codes and Standards

The national consensus standard for outside air ventilation is ASHRAE Standard 62.1-200, Ventilation for Acceptable Indoor Air Quality  and its published Addenda. This standard is often incorporated into state and local building codes, and specifies the amounts of outside air that must be provided by natural or mechanical ventilation systems to various areas of the school, including classrooms, gymnasiums, kitchens and other special use areas.
Many state codes also specify minimum energy efficiency requirements, ventilation controls, pipe and duct insulation and sealing, and system sizing, among other factors. In addition, some states and localities have established ventilation and/or other indoor air quality related requirements that must also be followed
Design in accordance with ASHRAE standards Design systems to provide outdoor air ventilation in accord with ASHRAE Standard 62.1-2007  and thermal comfort in accord with ASHRAE Standard 55–1992 (with 1995 Addenda) Thermal Environmental Conditions for Human Occupancy

Potential for Natural Ventilation and Operable Windows

In some parts of the country, where temperature and humidity levels permit, natural ventilation through operable windows can be an effective and energy-efficient way to supplement HVAC systems to provide outside air ventilation, cooling, and thermal comfort when conditions permit (e.g., temperature, humidity, outdoor air pollution levels, precipitation). Windows that open and close can enhance occupants' sense of well-being and feeling of control over their environment. They can also provide supplemental exhaust ventilation during renovation activities that may introduce pollutants into the space.

However, sealed buildings with appropriately designed and operated HVAC systems can often provide better indoor air quality than a building with operable windows. Uncontrolled ventilation with outdoor air can allow outdoor air contaminants to bypass filters, potentially disrupt the balance of the mechanical ventilation equipment, and permit the introduction of excess moisture if access is not controlled.

Strategies using natural ventilation include wind driven cross-ventilation and stack ventilation that employs the difference in air densities to provide air movement across a space. Both types of natural ventilation require careful engineering to ensure convective flows. The proper sizing and placement of openings is critical and the flow of air from entry to exit must not be obstructed (e.g., by closed perimeter rooms).

Designers should consider the use of natural ventilation and operable windows to supplement mechanical ventilation. Consider outdoor sources of pollutants (including building exhausts and vehicle traffic) and noise when determining if and where to provide operable windows.

If operable windows will be used to supplement the HVAC system, ensure that:

openings for outdoor air are located between 3-6 feet from the floor (head heigh
the windows are adjustable and can close tightly and securely;

the windows are placed to take maximum advantage of wind direction, with openings on opposite sides of the building to maximize cross-ventilation.
Selection of HVAC Equipment

In most parts of the country, climatic conditions require that outdoor air must be heated and cooled to provide acceptable thermal comfort for building occupants, requiring the addition of HVAC systems. The selection of equipment for heating, cooling and ventilating the school building is a complex design decision that must balance a great many factors, including heating and cooling needs, energy efficiency, humidity control, potential for natural ventilation, adherence to codes and standards, outdoor air quantity and quality, indoor air quality, and cost.

Where feasible, use central HVAC air handling units (AHUs) that serve multiple rooms in lieu of unit ventilators or individual heat pumps.

Although there are many different types of air handling units, for general IAQ implications in schools, air handling units can be divided into two groups: unit ventilators and individual heat pump units that serve a single room without ducts; and central air handling units that serve several rooms via duct work. Unit ventilators and heat pumps have the advantage of reduced floor space requirements, and they do not recirculate air between rooms. However, it is more difficult to assure proper maintenance of multiple units over time, and they present additional opportunities for moisture problems through the wall penetration and from drain pan and discharge problems. Central air handling units have a number of advantages as compared to unit ventilators and heat pumps serving individual rooms. They are:

Quieter, and therefore more likely to be turned on or left on by teachers and staff;

Less drafty due to multiple supplies and a return that is away from occupants;

Better at controlling humidity and condensed moisture drainage;

Easier to maintain due to reduced number of components and few units to access;

More space around units and can be accessed without interfering with class activities;

Space for higher efficiency air filters, and more surface area;

Made of heavier duty components;

Less likely to have quantity of outdoor air supply inadvertently reduced.

Specify the following features for all air handling units:

Double-sloped drain pan and drain trap depth

Double-sloped drain pan - A double-sloped pan prevents water from standing and stagnating in the pan.

Non-corroding drain pan - Made from stainless steel or plastic. Prevents corrosion that would cause water to leak inside the AHU.

Easy access doors - All access doors are hinged and use quick release latches that do not require tools to open. Easy access to filters, drain pans, and cooling coils is imperative.

Double wall cabinet - The inner wall protects the insulation from moisture and mechanical damage, increases sound dampening, and is easier to clean.

Tightly sealed cabinet - Small yet continuous air leaks in and out of the AHU cabinet can affect IAQ and energy. The greatest pressure differentials driving leaks occur at the AHU.

Double wall doors with gaskets - Double wall doors provide better thermal and acoustic insulation, and will remain flatter, allowing a better seal against door frame gaskets

Minimum 2 inch thick filter slots - For better protection of the indoor environment, as well as the equipment and ducts, the filters slots should be able to accommodate 2 in. or thicker filters.

Extended surface area filter bank - To reduce the frequency of filter maintenance and the cost of fan energy, the bank is designed to allow more filter area, such as the deep V approach or bags.

Air filter assemblies (racks & housings) designed for minimum leakage - The filter bank should have gaskets and sealants at all points where air could easily bypass the air filters, such as between the filter rack and the access door. Use properly gasketed manufacturer supplied filter rack spacers.

Air filter monitor - A differential pressure gauge to indicate the static pressure drop across the filter bank. This feature could easily be installed as an option in the field.

Corrosion resistant dampers & links - All moving parts such as pivot pins, damper actuators, and linkages are able to withstand weather and moisture-induced corrosion for the full life of the system
Energy Recovery Ventilation

Consider specifying energy recovery ventilation equipment.

Indoor air can be 2 to 5 times more polluted than outdoor air; therefore, most HVAC system designers understand that increased amounts of outdoor air supply is generally better for IAQ. Yet there are concerns over the implications that this added amount of outdoor air supply has on the first cost and operating cost of the HVAC system, as well as moisture control for the school (too wet or too dry). As a result, school designers often try to reduce the amount of outdoor air equal to – or even below -- 15 cubic feet per minute (cfm) of outside air per person, the minimum for school classrooms, as established by the American Society of Heating, Refrigerating and Air -conditioning Engineers (ASHRAE) . In many parts of the country these concerns can easily be addressed by application of basic engineering principles and off-the-shelf HVAC equipment.

First cost, energy costs, and moisture control do not have to be at odds with good IAQ. Energy recovery ventilation equipment can make the negative implications of 15 cfm per person of outdoor air behave like 5 cfm, while retaining the IAQ advantage of 15 cfm. This approach has been proven in many schools in various regions east of the Rockies, where advanced HVAC systems cost roughly the same as conventional systems, yet provide significant operating cost savings and IAQ advantages.

EPA has developed the School Advanced Ventilation Engineering Software (SAVES) package as a tool to help school designers assess the potential financial payback and indoor humidity control benefits of Energy Recovery Ventilation (ERV) systems. See also:

Overview of ERV Systems

Financial Aspects of ERV Systems

Location of Outdoor Air Intakes and Exhaust

Sloped Intake Plenum and Accessible Intake Screen

Proper location of outdoor air intakes can minimize the blockage of airflow and intake of contaminated air.

The bottom of air intakes should be at least 8 inches above horizontal surfaces (generally the ground or the roof) to prevent blockage from leaves or snow. In northern locations, more separation may be needed due to greater snow depths or drifting snow.

Intakes should not be placed within 25 feet of any potential sources of air contaminants, including sewer vents, exhaust air from the school, loading docks, bus loading areas, garbage receptacles, boiler or generator exhausts, and mist from cooling towers.

If the source is large or contains strong contaminants, or if there is a dominant wind direction in the area, the minimum separation distance may need to be increased. Air admittance valves, an inexpensive and code-approved one-way air valve, can be added to sewer vents to eliminate the potential for release of gases into the surrounding air.

Grilles protecting air intakes should be bird- and rodent-proofed to prevent perching, roosting, and nesting.

Waste from birds and other pests (e.g., rats) can disrupt proper operation of the HVAC system, promote microbial growth and cause human disease. The use of outdoor air intake grilles with vertical louvers, as opposed to horizontal louvers, will reduce the potential for roosting.

Intake Screens must be accessible for inspection and cleaning.

In existing schools, an insufficient amount of ventilation air is often the result of clogged intake screens that are inaccessible for inspection and cleaning. Screens hidden by an intake grille should be designed with a grille that is easily opened, such as a hinged grille with two quick-release latches, or in the worst case, a grille with four one-quarter turn fasteners. All screens should be easily removable for cleaning.

Consider adding a section of sloped intake plenum that causes moisture to flow to the outside or to a drain if intake grilles are not designed to completely eliminate the intake of rain or snow.

Outdoor Air Quantity
Classrooms and other school spaces must be ventilated to remove odors and other pollutants. The national consensus standard for outside air ventilation is ASHRAE Standard 62.1-2001 -
If outside air is provided through a mechanical system, then at least 15 cubic feet per minute (cfm) of outside air must be provided for each occupant. A typical classroom with 30 people requires a minimum of 15 x 30 or 450 cfm of outside air.

In spaces where the number of occupants is highly variable such as gyms, auditoriums and multipurpose spaces, demand controlled ventilation (DCV) systems can be used to vary the quantity of outside air ventilation in these spaces in response to the number of occupants. One technique for doing this is to install carbon dioxide (CO2) sensors that measure concentrations and vary the volume of outside air accordingly. If an auditorium fills up for school assembly, then CO2 concentrations will increase, a signal will be provided to the HVAC system and outside air volumes will be increased accordingly. When the spaces served by an air handler have highly variable occupancy, this type of control can both save energy and help control moisture (and mold) by reducing the quantity of humid outside air when it is not needed for ventilation. CO2 and other sensors must be periodically calibrated and maintained.

Air Filtration

In addition to "atmospheric dust," airborne particulates can include pollen, mold (fungal) spores, animal dander, insect proteins, pesticides, lead, and infectious bacteria and viruses. Designers can integrate features into the ventilation system that will provide benefits for the school occupants as well as the efficiency and longevity of the HVAC system. In addition, these features can reduce the need for expensive cleaning of the duct work and air handling units.

Filter Efficiency

Air filters should have a dust-spot rating between 35% and 80% or a Minimum Efficiency Rating Value (MERV) of between 8 and 13.

The higher the rating, the better the protection for the equipment and the occupants. It has been estimated that a 30% increase in static pressure across a coil results in a $200 per 10,000 cfm of air movement (at 7 cents per KWH). This does not include the added cost of cleaning dirty heating or cooling oils, drain pans, or air ducts. Designers should consider specifying a low efficiency (~10%) pre-filter upstream of the main filters. The pre-filters are generally easy and inexpensive to change, and will capture a significant amount of the particulate mass in the air thereby extending the useful life of the more expensive main filters.

Pressure Drop

Design more filter surface area into ventilation systems.

This has two advantages: the number of filter changes each year is reduced, thereby reducing the cost of labor to properly maintain the filters; and static pressure loss is lower, which saves money by reducing the amount of power needed to operate fans and blowers. Since different filter media are approximately proportional in their efficiency/pressure drop ratio, the most effective method for reducing pressure drop is to design more filter surface area into the filter system. This can be done by the specification of a filter with larger amounts of surface area, such as a pleated filter or bag filter. The next method is to increase the number and/or size of the filters in the airstream, for example, by mounting the filter slots in a "V" pattern, rather than a filter rack that is simply flat and perpendicular to the airstream.

Monitoring Pressure

Consider installing a simple pressure differential gauge across all filter banks.

This will prevent school facilities personnel from having to guess whether the filter is ready for replacement. A gauge with a range of zero to 1.0 in. w.g. can save money and the environment by preventing premature disposal of filters that still have useful life, and can prevent health and maintenance problems caused by overloaded filters that have blown out. The gauge should be easily visible from a standing position in an easily accessed location near the air handling unit.

Air Cleaning for Gaseous Contaminants

The most effective means of reducing exposure of occupants to gases and VOCs is to manage and control potential pollution sources. Filters are available to remove gases and volatile organic contaminants from ventilation air; however, because of cost and maintenance requirements, these systems are not generally used in normal occupancy buildings or schools. In specially designed HVAC systems, permanganate oxidizers and activated charcoal may be used for gaseous removal filters. Some manufacturers offer "partial bypass" carbon filters and carbon impregnated filters to reduce volatile organics in the ventilation air of office environments. Gaseous filters must be regularly maintained (replaced or regenerated) in order for the system to continue to operate effectively.

Ventilation Controls

Although a typical HVAC system has many controls, the control of outdoor air quantity that enters the building can have a significant impact on IAQ, yet typically is not part of standard practice. Demand controlled ventilation is addressed as a method of humidity control, but is not otherwise discussed here because its primary use is to reduce the supply of outdoor air below the recommended minimum for the purposes of saving energy, not for improving IAQ.

Outdoor Air Volume Monitoring and Control

Supplying acceptable quantities of outdoor air to occupied spaces is a critical component of good indoor air quality. Yet nearly all school ventilation systems cannot indicate whether outdoor air is even being supplied to the school, much less gauge the quantity of that air. Virtually all existing school ventilation systems rely upon a fixed damper to regulate the amount of outdoor air. Yet wind, stack effect, unbalanced supply and return fans, and constantly changing variable air volume (VAV) systems can cause significant under- or over-ventilation, which can affect IAQ and energy costs. Combinations of these effects can even cause the intake system to actually exhaust air.

Specify the addition of a measuring station that actively controls the amount of outdoor airflow by modulating the outdoor air damper and the return (recirculation) damper, if needed to overcome wind and stack effects.

These measuring stations are designed to work in limited duct space and with low air velocities. This is an easy task, as some manufacturers offer their airflow measuring stations in separate packages with dampers and actuators, and others are built into the AHU at the factory.

Moisture and Humidity Control
Uncontrolled moisture indoors can cause major damage to the building structure, as well as to furnishings and to finish materials like floors, walls, and ceilings. Uncontrolled moisture can trigger mold growth which not only damages the school facility, but can lead to health and performance problems for students and staff.

Primary causes of indoor moisture problems in new schools include:

Use of building materials that were repeatedly or deeply wetted before the building was fully enclosed

Poor control of rain and snow, resulting in roof and flashing leaks

Wet or damp construction cavities

Moisture-laden outdoor air entering the building

Condensation on cool surfaces

Controlling moisture entry into buildings and preventing condensation are critical in protecting buildings from mold and other moisture-related problems, including damage to building components.

Follow these links for more moisture information:

Building Materials

Precipitation Control

Building Envelope

Controlling Moisture in Ventilation Air

Energy Recovery Ventilators

Summer Breaks and Humidity Control


Air Distribution and Duct Insulation

Dirt and moisture should not be present in duct systems, and must be controlled to prevent mold growth. However, it is not always possible to assure that ducts remain dirt and moisture free. In many existing schools, sheet metal ducts, as well as those constructed of or lined with insulation products, are often contaminated with mold because dirt and moisture found their way into the system.

Duct board and duct liner are widely used in duct systems because of their excellent acoustic, thermal, and condensation control properties. If the HVAC system is properly designed, fabricated, installed, operated and maintained, these duct systems pose no greater risk of mold growth than duct systems made of sheet metal or any other materials.

However, the very properties that make duct board and duct liner superior insulators (e.g., a fibrous structure with large surface area that creates insulating air pockets), also makes them capable of trapping and retaining moisture if they do get wet (though the fibers themselves do not absorb moisture).

While there is an ongoing debate about the wisdom of using insulation materials in duct systems that might retain moisture longer, all sides agree that extraordinary attention to preventing moisture contamination of the duct work should be the primary strategy for preventing mold growth. See ANSI/ASHRAE Addenda 62t and 62w, Addenda to ANSI/ASHRAE Standard 62-2001, Ventilation for Acceptable Indoor Air Quality

As a secondary strategy, designers should consider methods of reducing the potential for future problems to occur due to unforeseen moisture contamination by investigating insulation products now on the market that minimize the potential for moisture to penetrate the insulation material. These include foil vapor retarders, tightly bonded non-woven vapor retarders, butt or shiplap edges, and other techniques that have been developed by insulation manufacturers to address concerns about moisture.

Pay special attention to preventing moisture from entering duct work.

Preventing moisture from entering duct work is critical to preventing mold problems in all types of ducts. Moisture in ducts is usually due to penetration of precipitation through inlet louvers, excess moisture in outdoor air, or condensation droplets from cooling coils that are not properly drained or ducts that are not properly sealed. Under certain circumstances, when exceeding recommended maximum cooling coil face velocity, water droplets can escape cooling coils and be carried into the air stream, saturating any dirt or dust downstream. Because dust and dirt serve as a food source for mold and are usually present in all but brand new duct systems, mold will grow on any duct surface that remains wet.

If specifying duct board or internal duct lining for thermal and/or acoustical control, be sure to consider the potential for uncontrolled moisture to enter the duct over the life of the system. Select products that will minimize the potential for moisture retention in the event of unforeseen contamination of the duct system, such as those with properties that reduce the potential for moisture to penetrate the air stream surface. Ensure that all duct systems are properly fabricated and installed.

Degrease sheet metal air ducts.

The sheet steel used to make ducts has a thin petroleum or fish oil coating primarily intended to inhibit corrosion during transportation and storage of the steel. This coating may trap dirt particles, some people find the odor objectionable, and there are concerns that the emissions from the coating could affect individuals with asthma or allergies. One solution is to remove the coating from the duct using a mild cleaning agent, such as a household dishwashing liquid, in conjunction with a heated high-pressure sprayer.

Seal air ducts to prevent HVAC system air leakage
In addition to significant energy losses, air leakage from HVAC ducts and air handling units can cause significant IAQ problems due to unexpected airflow between indoors and outdoors, and between areas within the school. Air leakage from supply or return duct work contributes to the condensation of humid air in building cavities and/or on the neighboring surfaces. Air leakage can be especially problematic for ducts or AHUs that are located outside the conditioned spaces. The primary goals for the designer are to keep all air ducts within the conditioned space, and to specify that the joints and seams of all ducts, including return ducts, are sealed using an appropriate material.

Types of Air Distribution

Nearly all schools currently use the mixed-airflow method for distribution and dilution of the air within the occupied space. Designers should investigate a method called vertical displacement ventilation or thermal displacement ventilation. This approach successfully uses natural convection forces to reduce fan energy and carefully lift air contaminants up and away from the breathing zone.

 Cool supply air (blue) slowly flows out of the two heating/cooling registers in the corners of the room, and spreads across the floor. As it is warmed by people (brown columns represent students) and other warmer objects in the room, it rises upward, continuously lifting polluted air up and away from the occupants. It is then collected and exhausted outdoors.

Exhaust Air

Quick removal of concentrated air contaminants and building pressurization are two ways that exhaust systems affect IAQ. Special use areas such as science labs, vocational/technical shops, cafeterias, and indoor pools already have well established regulatory codes regarding ventilation with outdoor air and negative pressure requirements with respect to adjacent spaces. Less well recognized areas in schools where special exhaust ventilation is desirable are janitor closets, copy/work rooms and arts/crafts preparation areas where off-gasing from significant quantities of materials or products may occur. These areas should be maintained under negative pressure relative to adjacent spaces.

Provide exhaust ventilation for janitor's closets.

If housekeeping and maintenance supplies are properly stored in janitor closets, only enough air need be exhausted to place the closet under negative pressure relative to surrounding rooms. As long as air does not easily leak into or from the closet through openings such as plenums or utility chases, 10 CFM of air exhausted from the room will typically make it negative, and prevent the buildup of air pollutants.

Provide exhaust ventilation for copy/work rooms.

In addition to the code-required amount of outdoor air being supplied to this room for general ventilation, it is desirable to determine what types of equipment and activities the school plans for this room, and to supply special exhaust ventilation for concentrated pollutant sources. Two examples of sources are copy machines and work areas for adhesives. Most copier manufacturers can provide an optional vent kit, which is usually a simple plastic fitting, that allows a piece of 3" or 4" diameter flexible duct to be connected between the copier and an exhaust fan. This captures much of the heat, particles, ozone, and other pollutants and exhausts them outdoors before they can spread throughout the workroom. A small exhaust hood over a work surface, similar to a fume hood in a science lab, would also be helpful to reduce exposure when adhesives, sprays, paints, and solvents are being used in the workroom.

Provide exhaust ventilation for arts and crafts preparation areas where off-gassing from significant quantities of materials or products may occur.

Consider specifying a differential pressure monitor to monitor building pressurization.

IAQ problems are often traced to improper pressurization, which causes unexpected airflow between indoors and outdoors, and between areas within the school. To reduce introduction of unconditioned moist air and pollutants from outdoors, the building should be designed to operate between zero and 0.03 in. w.g. (0 to 7 Pa) positive, relative to outdoors.

Do not operate exhaust systems when the HVAC system is turned off to avoid bringing in unconditioned moist air that may condense on cooler indoor surfaces.

Designing for Efficient Operations and Maintenance

Ensure that all system components, including air handling units, controls, and exhaust fans are easily accessible.

To help ensure that proper operation and maintenance of HVAC system components will be performed, it is critical that the designer makes the components easily accessible. AHUs, controls, and exhaust fans should not require a ladder, the removal of ceiling tiles, or crawling to gain access. Rooftop equipment should be accessible by way of stairs and a full-sized door, not a fixed ladder and a hatch.

Label HVAC system components to facilitate operations and maintenance.

Labeling of HVAC components is an inexpensive and effective method for helping facilities personnel properly operate and maintain the HVAC systems. The labels should be easy to read when standing next to the equipment, and durable to match the life of the equipment to which they are attached. At a minimum, the following components should be labeled in each ventilation zone of the school and should correspond with the HVAC diagrams and drawings. "AHU" refers to any air handling unit that is associated with outdoor air supply.

The number or name of the AHU (e.g., AHU ##, or AHU for West Wing)

The outdoor air (OA), supply air (SA), return air (RA), and exhaust or relief air (EA) connections to the AHU, each with arrows noting proper airflow direction

The access door(s) for the air filters and the minimum filter dust-spot (or MERV) efficiency (Air Filters, minimum xx% dust spot efficiency)

The filter pressure gauge and the recommended filter change pressure (Filter Pressure, max 0.x in. w.g.)

The access door(s) for the condensate drain pan (Drain Pan)

Other pertinent access doors such as to energy recovery ventilation wheels or plates (Energy Recovery Ventilation Unit)

The minimum amount of outdoor air for each AHU (### CFM minimum during occupied times)

The outdoor air damper (OA Damper), with special marks noting when the damper is in the fully closed (Closed), fully opened (open), and minimum designed position (Min)

If a motorized relief damper is installed (EA Damper), note the same positions as above.

The access door to any outdoor air controls (OA Control(s)) such as damper position adjustments, outdoor airflow measuring stations, resets, fuses, and switches)

Breakers for exhaust fans AHU, unit ventilators

Access doors for inspection and maintenance of air ducts

Any dampers and controls for air side economizers (as appropriate)

The number or name of all exhaust fans, including the air quantity exhausted


Building commissioning is a quality assurance program that is intended to show that the building is constructed and performs as designed. Click here for more information on commissioning HVAC and other building systems.

Commission key building systems.

Engage a commissioning agent (the person responsible for implementing the commissioning plan) during the schematic design phase or earlier. The agent may be a member of the design team, an independent contractor, or a member of the school district staff;

Collect and review documentation on the design intent;

Make sure commissioning requirements are included in the construction documents;

Write a commissioning plan and use it throughout design and construction;

Verify installation and functional performance of systems;

Document results and develop a commissioning report.

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