Large air-handling units (AHUs) have dampers in the return air duct and outside air duct. These control devices regulate the flow of air to be treated by the AHU. Often the outside air damper has a minimum position to ensure that required volume of fresh air is supplied to each zone. The volume of outside air may be increased when it is colder than the return air, in the cooling season; or when it is warmer than the return air, in the heating season.
The outside air louvers should be fully closed when a unit is not in operation to prevent infiltration of conditioned air into the building. Or exfiltration can happen if other units are on in the building serving zones having an interconnection that permits the movement of air into the zone conditioned by the disabled AHU. The operating units, supplying a minimum 20 percent outside air, induce a positive pressure in the building. Building air will therefore be forced through the outside air dampers of the unit that is not in operation.
Closing the outside air dampers when the AHU is off is a simple matter of modifying the existing damper actuator controls, although preventing exfiltration completely is also a function of the condition of the damper blades and the gasketing. Quality dampers are airtight against a small static pressure, but they must be properly maintained and adjusted to perform according to specifications. This is possible only if there is an access door built into the ductwork for inspection and maintenance of these louvers. This access port should be insulated the same as the duct and should have a tight-fitting access door.
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Excerpt from Energy Conservation Projects.