Duct Return Transition


Old 10-27-20, 08:13 PM
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Duct Return Transition

We moved the old and have a new oil burner, the plenum size is 24x24, the burner calls for a 16x25 filter rack, but the existing return is 8x16, can we transition to this size with duct, to a 14" flex, or leave 1/2 open to basement and the other half pulling from 8x16 ? Is it choking it down too much ?
Old 10-28-20, 02:24 PM
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By "We moved the old and have a new oil burner", I assume you replaced an existing oil furnace with a new one. Please confirm.

A 16x8 return duct is only good for 560 CFM, about 1-1/2 tons of AC and 50,000 BTU/Hour heating input, whereas the 16x25 is good for 800+ CFM. Sounds like you'll have to increase the return duct and probably add a return grille too. Please tell us equipment M/N's: Furnace and outdoor unit if there's AC. It'd also be good to know how many and what size(s) of return grilles there are now. Images of furnace and attached ducts would be good too.

Yes, it is definitely "choking it down too much"!

Leaving the return open to the space in which the furnace is located is prohibited by Code, because of the risk of circulating combustion products to the living space.

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