In many situations, a relief (or exhaust) air fan may be used instead of a return fan. A relief air fan relieves ventilation air introduced during air economizer operation and operates only when this control cycle is in effect. When a relief air fan is used, the supply fan must be designed for the total supply and return static pressure in the system, since the relief air fan does not operate during the non-economizer mode of operation. During the economizer mode of operation, the relief fan must be controlled to exhaust at a rate that tracks the quantity of outside air introduced, to ensure a slight positive pressure in the conditioned space, as with the return air fan system cited previously. The section on Economizers describes the required control for relief air fans.
