Thread: How to deterine the proer size of return grill?

How do I determine what is the proper size for a return grill. Usually I see 20x20 or multiple small grills. This house had a single that was about 10x16. Seemed small to me. Had low temp diffs too

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Very general rule of thumb
The return(s) should be larger than all the supply vents added together.

Another very general rule of thumb - the returns should be larger than the return at the bottom of the air handler unit ...

If not, yo' in fo' a heap o' troubl' ...

This is what I do when set up like this: If air handler unit is an a closet with a louver return air (or louvered door), open the door part way and see if the door is sucked closed, if it is, open the door further, and further, and further ... until the door remains open - now measure the space from the edge of the door to the door casing, measure the height, measure the width of the door (the triangle space at the top, and at the bottom if the door is not close to the floor), add those areas up and that is how much more return air area the unit is trying to get - okay, rule of thumb as that does not take the static pressure designed and intended for the return into account, but it does make a lasting impression on the occupants when you open the door halfway, let the door go ... and the door slams itself shut! Yeah, there be a problem with that one.

Another "rule of thumb": If it was built before 2001, the return is too small.

Originally Posted by Dom D'Agostino

Another "rule of thumb": If it was built before 2001, the return is too small.

Dom,

What changed in 2001 to make sure the return are still not too small?

Originally Posted by ren ramsey

How do I determine what is the proper size for a return grill. Usually I see 20x20 or multiple small grills. This house had a single that was about 10x16. Seemed small to me. Had low temp diffs too

ACCA Manual D (second edition) recommends 300 fpm velocity at the face of the return grille. That edition if from the 1980s, but I don't think there have been significant changes in design since then. Use 400 cfm/ton for blower capacity. So a 3 ton system would need 4 square feet of total return grill area. The recommended velocity at the main return plenum is 600 fpm (700 max).

Originally Posted by Mark Reinmiller

ACCA Manual D (second edition) recommends 300 fpm velocity at the face of the return grille. That edition if from the 1980s, but I don't think there have been significant changes in design since then. Use 400 cfm/ton for blower capacity. So a 3 ton system would need 4 square feet of total return grill area. The recommended velocity at the main return plenum is 600 fpm (700 max).

I agree with this and would add that a higher velocity would be acceptable but would start to generate noise above 400 FPM. Even at a higher velocity the system would (should) perform adequately.

Good answer Mark.