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HVAC Calculating Return Air

HVAC Calculating Return Air

HVAC Calculating Return Air

(OP)
Question: From toolbag
Is there a formula or standard in the HVAC industry that explains the correct size opening in sq.in.,on the return air going back to a Lennox roof top unit. The Lennox unit i have is 10 tons.

I was told by an expert in the field that the unit is starving for air. He talked about the static air and didn't explain how i can check the static pressure.

The filters are clean and the evaporator is clean. I also cleaned the condenser several times.

Any help would be great. Thanks

RE: HVAC Calculating Return Air

Speaking approximately, a 10 ton unit is probably around 5000 cfm, for 5000 cfm you'd like to see around a 24x24 duct serving. The actual static pressure will depend on the length of duct.

You can check the static pressure with a pitot tube, or inclined manometer.

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