Return Grill sizes
What 's the easy way to calculate return grill size ?
tape measure's pretty accurate
CFM divided by Velocity = Free Area in sq ft.
EG: 900CFM/300FPM=3sq ft.
3 sq ft X 144 = 432 sq in free area.
4 sq ft X 144 = 576 sq in free area.
this old one for me has worked fine.....grille size x 2 = cfm. Example 30x20 grille 30x20x2=1200 cfm. I know all about rules of thumb and use them everyday for load caculating..ha ha.
Depends on the PD of the grille,and what you can afford to deduct from the ESP of the indoor unit,to have enugh ASP for the rest of the duct system.
The above our good ideas for noise and velocity,but you might have to go larger,depending on the rest of the duct system.
Here's an example of a job that required to Carrier Infinity Air Purifiers,due to the pressure drop of the Purifiers,same goes from return grilles and any filters,upsize as needed ,depending on the rest of the duct system.
Last edited by dash; 03-08-2009 at 12:12 AM.
You need to get the free air area & pressure drop of the grille at various CFMs from the Mfg, then (as Dash stated), you can figure what area is needed based on the ESP of the air handler & the remaining Available Static Pressure (ASP).
Especially with larger tonnage A/Cs; when using filter racks at the rooms, & not the one at the furnace, I believe in over-sizing them, &/or, adding more RA racks.
It is a good idea to Size the Filter Rack(s) for a Free Air Area resulting in a Velocity of 300-fpm or less, that improves filtering while reducing the pressure drop as the filter loads.