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Old 09-23-2009, 01:35 PM   #1
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Basement return air ducting - flex or metal?


I am in East Tennessee and building the return from a new basement room to the air handler about 12 ft away, also in the basement. I have been told at my local hardware outlet that once I place the return register (which includes an air filter) in the wall of the room I can fabricate a plywood box behind the return register and cut a 6” diameter hole in the back of the box so as to accept a metal collar to which I can then attach 6” flexible ducting to carry the return air back to the air handler. The place where I will tap into the air handler will be just a few feet from the circulating fan. My concerns are a) is it acceptable to use plywood for the return air box behind the return air register b) is it acceptable to use flexible ducting for return air in a basement? c) since the flexible ducting would tap into the main return air ductwork for the house close to the fan, is there any danger of the flex collapsing under the reduced pressure? d) if I use metal pipe returns all the way, wouldn’t this accentuate vibrations/noise?
Thanks for your thoughts.

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Old 09-23-2009, 01:50 PM   #2
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Basement return air ducting - flex or metal?


1) There is nothing wrong/unsafe with wood for return air. We use a wood joist space here all the time. Not acceptable for hot air/fire hazard.
2) Flex duct is not so good. Will eventually collect a lot of dust in the valleys and be unsanitary. Metal is better AND I would put a damper in it and close it halfway. You do not want to suck LOTS of air thru it or it will whistle at the register. Also you want to take most of the return air from upstairs.
3) Vibration and noise is not a issue unless your furnace is noisy/vibrating at which point it should be fixed anyway.

Good Luck

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Last edited by yuri; 09-23-2009 at 01:53 PM.
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Old 09-28-2009, 12:21 PM   #3
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Basement return air ducting - flex or metal?


Thanks yuri!
I’m now going with the metal pipe plus damper into the top of the wall (which minimizes the pipe length & keeps it high up) & using the space between the studs as a pathway to the filter register at near floor level.
Can you tell me, is it usual to use e.g. caulk/adhesive along the two studs comprising the pathway so that they seal against the sheetrock and minimize unfiltered air leaks into the returning air?
Thanks for the information.
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Old 09-28-2009, 03:59 PM   #4
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Basement return air ducting - flex or metal?


Must be a small area, that your only using 6"

Caulk is normal.
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Old 09-28-2009, 06:08 PM   #5
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Basement return air ducting - flex or metal?


Caulk is good. I would start with the damper 100% open and blow some cigarette smoke at the register with the furnace running to see how fast it gets sucked in. Close the damper a bit if you feel the velocity is too high. Close it in Summer when the A/C is on. Your choice in the winter how much air you want to remove from the basement. Its a compromise with comfort upstairs. Take too much from downstairs defeats air moving upstairs. I like electric baseboard heaters for basements and leave the furnace for upstairs use.
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Old 10-29-2009, 01:01 PM   #6
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Basement return air ducting - flex or metal?


Thanks guys for the comments & advice!

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