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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.

  • Are we talking about a single point return, or something more sophisticated? Where in the house is the furnace mounted? – ThreePhaseEel Dec 8 '19 at 4:52
  • @ThreePhaseEel - Yes, it is a single point (well, split to two points) return. Furnace is against an exterior basement wall of a 3 story home (base + 1 + 2). – MTAdmin Dec 8 '19 at 5:02
  • Multi story just threw a curve ball as the length will also be a factor. heat only the lower duct will be larger what climate zone are you in? What type of insulation? – Ed Beal Dec 8 '19 at 18:17
  • @Ed Beal - Using the ASHRAE map, Zone 6 (NW MT). Insulation is 5.5" fiberglass R21 in the walls. Either R21 or R30 in the roof/ceiling. No attic. – MTAdmin Dec 8 '19 at 23:48
  • How many turns ? is it a straight shot after coming out of the furnace or total number of 90’s with an up flow system sometimes the trunk goes straight up and the opening is on the wall. At each floor 1st floor registers in the floor and 2nd in the ceiling? Just trying to get a better understanding of your system. – Ed Beal Dec 9 '19 at 14:19
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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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It is important to note the space pressure requirements. For most ducts,supply air=return air. But there are special design that require positive pressure,like in hospital theatre,return air>supply air

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  • Hello, and welcome to Home Improvement. Thanks for the answer; keep 'em coming. – Daniel Griscom May 5 at 13:09

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