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The air within homes can become stale from moisture, odors, and pollutants that penetrate the home or are generated internally by human activity and out gassing from building materials and furnishings. A constant supply of fresh, outdoor air can provide greater assurance of good indoor air quality and improved comfort.
In most homes, ventilation is provided accidentally when air leaks through the building envelope. Accidental ventilation is unreliable because it is dependent on a pressure difference between indoor and outdoor spaces caused by temperature or wind variations. Too much fresh air often enters a house during cold weather, causing uncomfortable drafts and high heating bills. Not enough fresh air may enter during mild weather which can lead to poor indoor air quality.
Air leakage through the building envelope accounts for between 25 percent and 40 percent of the energy used for heating and cooling in a typical residence. Many new homes are being air sealed to reduce this energy use. Where tighter construction reduces air leakage and accidental ventilation, active ventilation systems may be needed to provide fresh air.
Figure 1 shows how supply ventilation works in a small home. Outdoor air enters through a single intake and is distributed through ducts to the living room and bedrooms. Stale air is removed by leakage throughout the building and through exhaust fans located in the kitchen and bathrooms. The supply air intake should be located away from sources of pollution, odor or dust—such as the ground, garages, driveways and plumbing or dryer vents. Supply systems can be turned off when homes are not occupied.
Fresh outdoor air is provided continuously regardless of weather conditions. Indoor air quality is improved where fresh outdoor air, low in pollutants, mixes with indoor air, which has become stale from human activity. Fresh air is provided to the living spaces within a house through properly sized and located vents without causing uncomfortable drafts. Filters and dehumidifiers can be added to the system near the intake to further remove pollutants and provide humidity control needed in hot, humid climates. Thus, they can be used safely with all types of heating and cooling equipment.
Supply ventilation creates positive indoor pressure. This is advantageous in moderate and hot climates because positive pressure avoids pulling hot, humid air into wall cavities where condensation problems can occur. In cold climates, positive pressure can possibly lead to moisture problems if hot, moist air is forced into wall cavities where condensation is likely to occur. In addition, supply ventilation systems avoid “back drafting” combustion gases from appliances and fireplaces into homes.
Resources for this article:
The Consumer Guide to Home Energy Savings (Wilson and Morrill), available from the American Council for an Energy Efficient Economy at 510-549-9914
Moisture Control in Homes fact sheet available from the Energy Efficiency and Renewable Energy Clearinghouse (EREC), POBox 3048, Merrifield, VA 22116, (1-800-363-3732)