"Mechanical" or "forced" ventilation is used to control indoorairquality. Excess humidity, odors, and contaminants can often be controlled via dilution or replacement with outside air. But in humid climates, much energy is required to remove excess moisture from ventilation air.
Kitchens and bathrooms typically have mechanical exhaust to control odors and sometimes humidity. Factors in the design of such systems include the flow rate (which is a function of the fan speed and exhaust vent size) and noise level. If the ductings for the fans traverse unheated space, the ducting should be insulated as well to prevent condensation on the ducting. Direct drive fans are available for many applications, and can reduce maintenance needs.
Heatrecoveryventilation systems employ heat exchangers to recover some heat from exhausted air, to preheat the incoming outside air.
Ceiling fans and table/floor fans circulate air within a room for the purpose of reducing the perceived temperature because of evaporation of perspirations on the skin of the occupants. Because hot air rises, ceiling fans may be used to keep a room warmer in the winter by circulating the warm stratified air from the ceiling to the floor. Ceiling fans do not provide ventilation as defined as the introduction of outside air
Fresh air is needed inside the home to help eliminate odors and pollutants harmful to human health. Fresh air also helps eliminate excessive moisture that harms the building structure and furnishings and is the source of mold and mildew growth. It is also important to replace the air that is expelled out of the home by kitchen range fans, clothes dryers, and other exhaust equipment. Failure to replace exhaust air decreases air pressure inside the home, causing outside air to be pulled into the home through leaks and other openings. In Minnesota, where many homes are fairly air tight, this depressurization can result in back drafting of the furnace and other combustion appliances: carbon monoxide and other dangerous gases are pulled back into the house rather than being expelled up the chimney. Although open windows are often relied on to supply fresh air, in a climate like Minnesota's this is not practical year round. Here, homes are "built tight" to ensure comfort and keep monthly energy bills as low as possible, especially during the winter. Although tightening up a home to prevent air leaks and then bringing in outdoor air through ventilation may seem like a contradiction, it is not. Tightening is essential for comfort and energy efficiency; controlled ventilation is necessary to ensure that the proper amount of fresh air is brought indoors in all seasons.
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