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Tuesday, February 15, 2011

How do I get the ventilation that I need in my 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.

The design, construction and maintenance of a home determine the amount of exchange between indoor and outdoor air. Since most home pollutants come primarily from indoor sources then bringing in outdoor air can help lower the concentration of pollutants in the home.

Today’s modern homes are relatively air tight and are constructed to resist air entry through use of air barriers, vapour barriers and sheet materials such as plywood, oriented strand board (OSB) and drywall. All this was done in the name of energy efficiency but little thought was given to the effect on air quality. Homes have in fact become so air tight that you must induce air change by the use of mechanical ventilation in order to maintain good indoor air quality.
Older homes did not have this problem. They were often drafty and very dry in winter due to high air change rates. Sealing up these homes is usually the major issue in order to reduce heating requirements.

There are a number of ventilation mechanisms that occur in any home:

1. Air infiltration: This is the air that naturally comes into the house through leaks in doors, windows, openings and gaps.

2. Mechanical ventilation: Air pulled into the house by using ventilation fans such as kitchen fans, bathroom fans and clothes dryers.

3. Stack effect: We know that hot air rises. When cold air comes into the basement and is warmed up, it rises and moves through the house eventually leaving through the attic to the outside.

4. Distribution: Fresh air coming into the house needs to be moved around. This usually requires fans and ducting systems.

At certain times of the year, when temperatures are mild and there is a light breeze then opening windows can provide good ventilation. In older homes open windows were the standard ventilation even for bathrooms, but these should be upgraded when remodelling to provide a fan that vents to the exterior. In newer homes or remodelled older homes, using mechanical ventilation or a heat recovery ventilator can be more effective particularly during cold winter months or when air conditioning is in use.

Bathroom fans provide basic ventilation. The fan removes stale air from the house and natural infiltration through the various leaks allows outdoor air to enter. The furnace fan and ducting system, if present, mixes this fresh air with house air and distributes it around the house. In many new houses a ” ventilation switch” is installed next to the thermostat which operates the bathroom fan. In some cases this is also electrically connected to the furnace fan.

Ventilation fans should be run for a few hours a day. These fans should be relatively low flow (about 50 cu ft/minute) and as quiet as possible so that they are not annoying. Sound levels less than 1.5 sones are best.

Heat Recovery Ventilators (HRV) are becoming increasingly common particularly in new homes. They provide good ventilation without too much additional energy cost. The HRV vents out stale air and brings in the same amount of fresh air. This is known as a balanced system and should be calibrated yearly. The units are designed so that some of the heat in the outgoing air is used to warm up the incoming fresh air. Basic systems are connected into the existing furnace ductwork. Efficiencies of 60 to 80% are quoted but I doubt that this level is achieved in normal operation. (Just my opinion!!) The furnace fan also needs to run to distribute the fresh air around the house.

Separately ducted HRV’s are also available where the HRV fan motor distributes the fresh air and collects the stale air through its own ducting system. Although more expensive to install, this system does not need to run the furnace fan and is therefore more energy efficient in the long term.
HRV’s should be used anytime the house is normally closed up. They should be run continuously at low speed and switched to high for parties or other times when you want more ventilation.

Improving ventilation may lower the concentration of pollutants in your home. Most homes will benefit from the fresh air supplied by mechanical ventilation. Opening windows and doors will increase the natural ventilation and exhaust fans will remove moisture and draw in fresh air by infiltration. For tightly sealed houses a HRV system may be necessary.

Safety Note: Be aware that over sizing kitchen or bathroom exhaust fans is not advisable. High exhaust rates can cause lower pressures within the home. This in turn can cause dangerous Carbon Monoxide to be drawn into the home from gas fired appliances such as furnaces or hot water heaters or from wood burning stoves etc. If in doubt, check with a qualified Heating/Ventilation expert.