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house is at a comfortable temperature and humidity level. However, natural ventilation is unpredictable and therefore may not meet needs at all times. During the heating season, natural ventilation results in an unacceptable level of heat loss and comfort problems—hence a controlled mechanical ventilation system is required. The NBC specifies the minimum unobstructed operable area to the outdoors for rooms that are not mechanically ventilated. Where air is exhausted from or supplied to a room by mechanical means, no operable windows are required. In practice, however, occupants normally expect operable windows in most rooms, and in some cases, operable windows are required as a means of egress in case of fire. If the type, location and orientation of operable windows are carefully planned, some measure of natural ventilation and summer cooling can be achieved. However, in locations where natural ventilation does not provide adequate temperature or humidity moderation, mechanical air conditioning can be used to provide the required level of comfort.
MECHANICAL VENTILATION It is well understood that the combination of an airtight building envelope and an effective and efficient mechanical ventilation system can outperform a leaky house with no mechanical ventilation system. A house that is built tight and ventilated right will have lower heating costs and greater year-round comfort. For this reason, the NBC requires the installation of a mechanical ventilation system in houses intended for year-round occupancy. As a minimum, the system must include a principal fan capable of providing an air flow rate suitable for the number of bedrooms; in most cases, supplemental exhaust fans to ventilate high moisture generation in areas such as kitchens and bathrooms; and protection against depressurization that
might otherwise allow fuel gases from fuel-fired appliances (for example, furnaces, water heaters and fireplaces) to be drawn back into living spaces. Mechanical ventilation systems are required for occupant health and comfort, as well as to maintain the integrity of the building envelope. Ventilation systems must be carefully designed to ensure they can manage the full range of normal household activities without overventilating or underventilating. In households with frequent moisture-generating activities such as cooking, bathing and floor washing, continuous mechanical ventilation at a low speed may be needed to control indoor humidity and odours. There may be occasions when operating the mechanical ventilation system at its peak capacity on a continuous basis is needed to control contaminants arising from activities such as painting or social events. In households with sedentary occupants, or when the house is not occupied for extended periods of time, intermittent operation of the mechanical ventilation system may be all that is required to maintain acceptable indoor air quality.
System Design Options Though mechanical ventilation systems are often designed and installed by qualified mechanical contractors, it is important for builders to understand the basics. There are two options for designing mechanical ventilation systems for houses. The first option is to apply the requirements stated in the NBC. The second is to develop a design based on CAN/CSA-F326-M91: Residential Mechanical Ventilation Systems. The discussion that follows is based on the NBC. There are three types for mechanical ventilation systems: Ventilation system coupled with a forced-air heating system—The ventilation and heating system share ductwork to supply fresh and heated air to