Sometimes air handlers are set up to provide more outside air than is necessary. Space use or occupancy rate may have changed since the original installation. Applicable ventilation codes may have changed. A quick check of the relevant standards will indicate the minimum outside air that must be provided for the current use pattern.
The existing percentage of outside air can be determined from the simple function
OA% = (RA temp – mixed air temp)/(RA temp – OA temp)
OA = outside air temperature
RA = return air temperature
During the normal course of operation (e.g., at full speed if the motor has a variable-speed drive) the above temperatures are measured at the return-air and outside-air intakes to the AHU, and at the discharge duct for the mixed-air temperature. Substituting these values in this equation generates the approximate percentage of outside air supplied to the unit.
There usually is a damper in the outside-air intake duct. This damper can be adjusted to provide the necessary volume of fresh air. If there’s no damper, then it is easy to install one. One exception is if the outside air is only slightly excessive. This means that the duct size may be small enough to limit the airflow and the cost of installing a damper probably cannot be justified.
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Excerpt from Energy Conservation Projects.