AHU – Return Air Fan

A return air fan is optional on small systems but is essential for the proper operation of air economizer systems for free cooling from outside air if the return path has a significant pressure drop (greater than about 75 Pa) It provides a positive return and exhaust from the conditioned area, particularly when mixing dampers permit cooling with outdoor air in intermediate seasons and winter. The return air fan ensures that the proper volume of air returns from the conditioned space. It prevents excess pressure when economizer cycles introduce more than the minimum quantity of outside air. It also reduces the static pressure the supply fan has to work against. (The use of a return fan can increase system energy use if the system is so arranged that the return fan works against more static pressure than the incremental static pressure the supply fan would have to provide to overcome the return path pressure drop) The supply fan(s) must be carefully matched with the return fans, particularly in variable air volume (VAV) systems. The return air fan should handle a slightly smaller amount of air to account for fixed exhaust systems, such as the toilet exhaust, and to ensure a slight positive pressure in the conditioned space. Chapter 45 of the 1999 ASHRAE Handbook—Applications has design details.

One Reply to “AHU – Return Air Fan”

  1. I am designing the ductwork for an existing 1600 sq ft two-story residence in Durham, NC to incorporate energy efficiency renovations, and am trying to use a Bosch 3-ton 18.5 SEER package unit. My question is: can I supplement the capacity of the 4-speed X13 ECM blower by adding a smaller variable speed fan in the return duct to help lower the external static pressure seen by the blower, so it can operate over its’ full four speeds? My duct system static will be something like 0.5” but the blower really wants 0.2”. Blower can operate at 0.5”, but only with 2 of four possible speeds.

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