furnace return duct sizing question

Old 06-11-05, 02:18 AM
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furnace return duct sizing question

I'm looking for some help regarding furnace return duct sizing. My Title 24 report calculates a total heating load of 44950 Btuh. I could not find my Bryant model number 345MVA036090 on their website, but their other models’ specifications suggest that the “036” indicates 1200cfm, and the “090” indicates 90,000Btuh input for the high stage, with perhaps output of 63000 Btuh for the low stage, and 84000 Btuh for the high stage. I found several “rules of thumb” for FAU return duct sizing on the web. It was not clear to me whether to use the Title 24 load of approx 45000Btuh, or the high stage input of 90000Btuh. Using the 45000 value, I found the following formulas:

(6sq” / 1000Btu) x 45000Btu = 270sq”

(60 to 80sq” / ton) x (1 ton / 12000Btu) x 45000 Btu = 225 to 300sq”

My HVAC subcontractor recommended a 20x30" = 600sq" duct when I was drawing the plans for our 2600sq ft home in southern Calif., but he installed a 9x14" = 126sq" return. When I protested, he changed it to 14” x 16” yielding 224sq”. It is 3 feet long. It is the only return in the whole house, and rather centrally located. Is this adequate? Thanks!
Old 06-11-05, 12:19 PM
Ed Imeduc's Avatar
Join Date: Aug 2002
Location: Mountain Williams Missouri
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A real good job for heat and cool will have 80sq" per ton and a very tight job 60sq" per ton . So a 3 ton AC I like 244sq "in and out of the unit.


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