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

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? Dec 8, 2019 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). Dec 8, 2019 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? Dec 8, 2019 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. Dec 8, 2019 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. Dec 9, 2019 at 14:19