EPA Energy Star
Exhaust Ventilation Systems

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The following came from the EPA’s Energy Star web site:

https://www.energystar.gov/ia/new_homes/features/MechVent_062906.pdf

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 exhaust ventilation works in a small home.  Indoor air is continuously exhausted from a central fan (shown) or remote fans usually located in bathrooms.  Fresh outdoor air can be drawn into the house through remaining leaks in the building envelope.  Homes built with extremely tight envelopes may require the installation of room wall ventilation openings or specially designed windows that allow outdoor air to enter.

These opening are sized and located to allow the proper amount of fresh air to enter homes without causing uncomfortable drafts and to prevent indoor pressurization.  Kitchens should have separate, manually operated, exhaust fans.

The advantages of exhaust ventilation are control and consistency.  Moisture, odors and pollutants are removed continuously, regardless of weather conditions.  High indoor air quality is maintained due to the constant infiltration of outdoor air.

Exhaust ventilation systems are most suitable for moderate climates.  Care must be taken during design and installation to prevent these systems from “back drafting” dangerous combustion gases from fireplaces and gas appliances into homes.

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