How large should a return HVAC duct be relative to the size of the supply

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I have a 4 ton American Standard electric forced air furnace (with no A/C – just heat) with a single speed blower motor, circa 2006.

My supply main trunk is 12"x20".

The return side of the system has not yet been completed. Is there a standard rule of thumb for sizing the return side of the duct system?

EDIT: Ok, clearly this is a more difficult question and answer than I anticipated. Does anyone have a general rule of thumb for the ratio of a single trunk supply to a single trunk return? i.e. In any given installation should the return/supply ratio be 1:1? Should it never be less than x:1? Any input would be appreciated.

Best Answer

OK, I have heard from multiple HVAC contractors that a general rule of thumb in a single trunk return, single trunk supply duct system, the RETURN:SUPPLY ratio should be 1:1 or greater.

In other words, the return should always be at least as large as the supply. And preferably the return should be larger than the supply.

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