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Old 01-11-2013, 01:45 AM   #1
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Flex Duct


Can flex duct be used in place of a normal return plenum?
I've read it can so long as it is not the only return in the house but not sure how good the source was.

8" is probably the biggest I can use but the plenum takes up 2 joist cavities, so run 2 flex ducts?

Thank you.

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Old 01-11-2013, 03:15 PM   #2
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No, two 8 inch flex returns would not be near enough for any system.

Yes it is ok to use flex for a return and the size depends on your system. Out here, I prefer 14" for two ton, 16" for 2.5 and 3 ton and 18" for 3.5 and 4 ton and so on.

And remember, two 8" returns would not equal 16."

Need to list the equipment size for further help.

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Old 01-11-2013, 03:33 PM   #3
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2-8" flex would be good for almost 1 ton of A/c, or just a bit under 400 CFM.
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Old 01-11-2013, 04:55 PM   #4
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Ok I found the serial and model number. It's a brand new furnace.

It is rated at:
96% efficiency rating
80,000 BTU/h
17.5" cabinet width
1600 CFM
ECM Variable Speed
2-stage

If I understand the numbers correctly it's about a 7 ton? Am I right?

What if I ran flex duct from the actual vent return only 4 feet then had it dump in to the regular plenum? Would this affect the furnace significantly?
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Old 01-11-2013, 05:25 PM   #5
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Its an 80,00 BTU furnace, it has no real tonnage rating. but the blower is rated to move enough air for a 4 ton A/C.

Post pic of the regular plenum. What size return duct are you running to it.
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Old 01-11-2013, 05:45 PM   #6
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Flex Duct


yeah I believe my AC unit is a 2.75 ton.
There is a pic of the original return plenum. It took 2 ceiling joist cavities.

What do you mean what size return duct am I running to it?

This particular plenum is in the basement ceiling for the living room directly above it. It runs about i'd say 12 feet.
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Old 01-11-2013, 06:16 PM   #7
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Ok. Where are you running this flex to and from.
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Old 01-11-2013, 08:17 PM   #8
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from the floor return vent all the way across the 12' and tie in to the rest of the return trunk. So it won't be directly tied in to the furnace. I just want it above the one side of the basement. It would tie in to here if possible(see pic).
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Old 01-11-2013, 08:26 PM   #9
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So you ant to remove the panning on those joist, and run flex in place of that panning.

Panning is what the metal on the joist is called.
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Old 01-11-2013, 08:29 PM   #10
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yes the sheet metal is already off on one side. That picture is of the same return just on the other side of the basement. The flex would replace the sheet metal just for the ceiling in the one room(12 feet) then transition back to the sheet metal on the other side of the wall.
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Old 01-11-2013, 08:41 PM   #11
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So what size is the floor return.
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Old 01-11-2013, 08:44 PM   #12
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I measured the grate that goes over both joist cavities and it's 8" deep by 30" wide.
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Old 01-11-2013, 09:02 PM   #13
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Allowing for the 1 5/8" width of the joist. That makes the register opening in 1 joist space roughly 8X14 which would need a min of a 10" flex duct. 12" would be better.
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Old 01-11-2013, 09:05 PM   #14
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dang. Can the 10" duct be..."flattened" a little to fit up in the joist cavity or does it have to stay in it's round shape? I only have 8 inches from the subfloor to the bottom of the ceiling joist but 14" width as you stated.
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Old 01-11-2013, 09:11 PM   #15
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No, flattening the duct will decrease its air flow ability.

Your other choice is to add more returns elsewhere. So you can use a smaller duct.

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