Friday, February 13, 2015

Figuring out CFM and Grille Sizes (including Return Grille) for Central HVAC systems

A HVAC system is sized for the volume of air in your home. If your house is 1200 sqft and the ceilings are 7 ft tall, the air volume is 8400 cubic feet. A properly sized HVAC system will perform at least 6 full air changes per hour, so it would need to move 8400x6=50400 cubic feet of air per hour. 50400/60 minutes then would equal 840 CFM.
(A real HVAC installer would also want to measure leakage to tweak the CFM rate for your specific house construction but that's beyond the scope of this post.)
To find the size of the compressor in tons, divide 840 by 400 (400 CFM per ton), giving 2.1 tons. Always round up, in this case to a standard size 2.5 ton unit.  If you are going to use a heat pump you've already done your heater sizing. (A house with no insulation or poor insulation should be figured at 300 CFM per ton.)
How big to make the return air grill? You don't want so much air rushing past the grill that it is noisy but you don't want a large grill either. The grill should be sized for an air speed of 300-500 FPM, or feet per minute. 400 FPM is the usual target.
We know you'll need to push 840 CFM. We'll divide by 400 FPM giving 2.1 sqft as the necessary filter face size. Convert to inches: 2.1 x 144 = 302.4 sqin. Take the square root of 302.4 = 17.389. So let's look at some standard size filters near 18in:

18x20 = 360sqin
16x20 = 320sqin

A 16x20 return grill would provide the necessary air flow and actually take us a little under 400 FPM.

If you already have an HVAC system you can work the numbers and see how well your system is sized. Also nice to know if you are planning on adding square footage somehow (new floor, finishing the basement, enclosing a porch, etc.) You can check your HVAC guy's recommendations this way.

You'll want to further assure you can get a full 840 CFM by checking the technical documentation for the air handler. You want to find the max external static pressure drop (intake resistance to air) that still allows the air handler to deliver 840 CFM. Then purchase filters that do not exceed that drop. Ideally, purchase the filter with the highest MERV rating that has a pressure drop for your size filter that is below this value. Most filter manufacturers don't provide detailed pressure drop information, unfortunately.

The air handler will also not push the right CFM if the pressure is not balanced throughout the home. This usually happens in a closed room with no air return (very common). All closable interior doors leading to living spaces (office, bedroom, bathroom, den) should be shortened by 1 1/2 inches on the bottom, or have an air vent installed in the door, or a jump vent.

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