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Sizing a Return

January 04, 2010

Return air grilles. They don't have to be a pain when a few simple rules are applied.

Golden Return Rule #1:

Air volume going back to the air handler (fan) must equal what is supplied from the air handler.

Sizing (Keep face velocity low)

Returns should be at 400-600 fpm maximum
Filter Returns should be at 450 fpm maximum
*ACCA recommends 300 max for filter grilles and 500 max for non-filter grilles.
The rule of thumb is 2 cfm per square inch of filter size. See table.
Low velocity reduces noise, especially on stamped face grilles (672/673); fixed-bar grilles can handle more velocity without noise (94A/96AFB/RH45/RHF45/RCB).
You really can't oversize a single point return like you can with a supply. The system will not be affected adversely, only improved. *This does not apply to multiple return locations where balancing is more critical to pull in relevant amounts from each room.
Static pressure is also reduced. S.P. works against & reduces blower delivery volume (cfm)
No one expects noise from a return.

Location

Returns should be put in stagnant air locations that need to be reconditioned.
High for cooling mode (hot air rises)
Low for heating mode (cold air falls)
Both modes, choose a primary season
Returns should not be near a supply register's throw range. If at all possible place the return at an opposite corner of the room.

Room Air Movement

Returns do NOT have much effect on a room's air movement, regardless of face velocity. They only grab air about a duct diameter away from the face.