Adding fresh air to a heating or cooling system accomplishes two primary indoor air quality goals: It pressurizes a building, and increases indoor air quality by diluting polluted or stale indoor air. Adding a fresh air inlet to most systems is typically a simple and relatively inexpensive proposition.
Outside air enters the system through a roof or sidewall vent inlet that collects air from a clean outside location. The air is pulled into the system through a filter, and then a duct connected to the return plenum. Smaller systems usually use the suction side of the air handler fan to pull in the outside air. The air mixes with the return air, and then is dispersed evenly throughout the building through the supply duct system.
This drawing shows additional accessories that could be included in a fresh air system. Each fresh air duct should contain a volume damper to control the volume of the airflow. Another option is a motorized shutoff damper that seals off the duct from the outside when the blower motor isn't running. Since these ducts carry unconditioned outside air, insulation should be added to the outside of the ducting to contain the heat or cold from entering the building.
Since the fresh air enters the duct before the fan, the volume of return air being pulled from the building is less than the supply air the system delivers into the building. Since supply air exceeds return air, a positive pressure will be maintained in the building when the fan is in operation.
Positive pressure reduces infiltration of outside air from undesirable locations and prevents duct, irritants and other pollutants from entering the building. This control of infiltration may reduce the heat loss or gain sufficient to offset the cost of heating or cooling the outside air.
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