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Sizing a Return
December 17, 2009
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.
| Filter Size | Area (in²) | Ton (cfm) |
| 12 | 12 | 144 | n/a |
| 12 | 20 | 240 | 1 (400) |
| 12 | 24 | 288 | 1.5 (600) |
| 12 | 30 | 360 | 1.5 (600) |
| 14 | 14 | 196 | 1 (400) |
| 14 | 20 | 280 | 1.5 (600) |
| 14 | 24 | 336 | 1.5 (600) |
| 14 | 30 | 420 | 2 (800) |
| 16 | 20 | 320 | 1.5 (600) |
| 16 | 24 | 384 | 2 (800) |
| 20 | 20 | 400 | 2 (800) |
| 20 | 25 | 500 | 2.5 (1000) |
| 20 | 30 | 600 | 3 (1200) |
| 20 | 36 | 720 | 3 (1200) |
| 24 | 24 | 576 | 3 (1200) |
| 24 | 30 | 720 | 3 (1200) |
| 24 | 36 | 864 | 4 (1600) |
| 25 | 25 | 625 | 3 (1200) |
| 30 | 30 | 900 | 4 (1600) |
| 30 | 36 | 1080 | 5 (2000) |
info@hartcool.com
G.R.D
