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What size grill and duct do I need for my cold air return to fit between wall studs in the following scenario?

I have an open-air return in my basement mechanical room with no cold air return ducts or transfer grills. Thus I'd like to install a cold air return duct to the floor above.

I have a Rheem R96VA0602317MSA 3 ton 56,000 BTU furnace rated for HEATING 1025 CFM @ .2" [.049 kPa] conditioning a 1000 sqft single first floor apartment with 8 foot ceilings.

http://www.deltatms.com/wp-content/uploads/rheem-r96v-spec-sheet.pdf

I'd like to punch up through the ceiling of the mechanical room above the furnace and add a cold air return.

How big of a grill and duct do I need for the cold air return? Looking online at some calculators it seems like I need 421 sq inch grill - but is that for a supply? Using another calculator, I have 8000 sqft of total air volume with 6 changes an hour which suggests a 2.5 ton system, but since I already have the spec saying 1025 cfm in a 3 ton system should I just go off the 3 ton 1025 cfm value I have? Another calculator says I need 360 sq inches of grille?

I couldn't find calculators on return duct size - just supply total duct area. What duct size do I need if it only travels 9 feet, from basement to ceiling? I'd like the duct and grill to be something that can fit between wall studs so that I can have it come up into an interior wall on the first floor.

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  • You must accommodate at least the minimum size duct and grill for the cfm of the air handler
    – Kris
    Nov 24, 2021 at 13:45
  • learnmetrics.com/cfm-duct-sizing-chart. If using flex round duct 18” will accommodate 16” is slightly undersized
    – Kris
    Nov 24, 2021 at 13:55
  • What size for rigid rectangular duct? Nov 24, 2021 at 15:10

2 Answers 2

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3 ton makes no sense when talking about a furnace. Ton is a measure of A/C capacity and your blower motor needs 400 CFM per ton. If you have paired this furnace with a 3 ton A/C unit then that is a big mistake. 2.5 ton would be the correct size.

Anyways, your furnace maxes out at 1,025 CFM so you need one of the following:

  • 16" flex duct (this is rated for 1,000 cfm so it should be fine)
  • 16" rigid duct (this is rated for 1,200 cfm but 15" doesn't exist)
  • 18"x12" rigid rectangle duct (rated for 1,100 cfm)

If you plan to use a standard wall cavity with 2x4s 16" on center then you would have to run six 14x4 ducts through the walls or seven 12x4s.

duct size calculator

Your return should match the supply, period.

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  • If I use an 18X12" rectangle duct to the plenum from the ceiling, what kind of "boot" would I need? I think I can pop up into the back of a closet but would need to angle my way 45 degrees to the back of perpendicular wall adjacent to the closet to cut out the vent opening... Nov 24, 2021 at 15:14
  • @MonkeyBonkey Plenums are usually oversized so a reducer is common. You need a local sheet metal shop to fabricate the desired transition piece (reducer). I only chose 18x12 for my example because it was convenient on that chart. You should pick your duct size based on whatever size grill is available; usually they are square. You can get away with a 14x14 duct but 15x15 would be best. The grill is usually larger than the duct.
    – MonkeyZeus
    Nov 24, 2021 at 15:21
  • @MonkeyBonkey For example, the plenum on my furnace is 17x20 (1,930 cfm) which is way oversized for the 1,200 cfm motor so I had a transition piece fabricated for my trunk.
    – MonkeyZeus
    Nov 24, 2021 at 15:25
  • Looking at its twin unit with a return - I see that the other unit has an 8x22 rectangular duct for the return - is that undersized? Nov 26, 2021 at 0:07
  • @MonkeyBonkey Look at the duct size calculator which I linked. If you can't make heads or tails of it then you should hire a professional.
    – MonkeyZeus
    Nov 29, 2021 at 13:44
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Minimizing flow restriction on the suction side of a fan improves efficiency, so bigger is better. I have over 1000 square inches of filter area (duct is larger) for a 5 ton AC. I think it is more a matter of "how big can you make it" than what minimum will work.

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  • Well I have very limited space since I have to make it fit between another system and a lot of existing ductwork - the grill I can probably make pretty big, but the actual ducts have to squeeze past some joists, studs and other pipe, so looking for the minimum thinest size Nov 26, 2021 at 0:02

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