flex ducts

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Posts: 5
Joined: Thu Jan 02, 2020 5:14 pm

Fri Jan 10, 2020 8:27 am


I repaired my air supply duct replacing the existing 10" run with new from the AC unit to where it connects to the plenum. Put it 4" off the ground to keep moisture from rotting it.

The return air supply (a 12" flex duct) is rotted away on the bottom exposing the insulation. What are the ramifications of replacing this with 10" flex duct? I ask as there is not enough clearance under the unit to put the 12" duct and it would be on the ground.


Posts: 238
Joined: Tue Feb 26, 2008 8:33 am
Location: Anna, Tx

Fri Jan 10, 2020 2:01 pm

yuk. not a good choice. ***Disclaimer*** I am not an HVAC guy in any professional capacity. I know what i know from having to stick my head into the beast....and even that is questionable. It was aggravating, stressful, and ultimately i had to just take the bull by the short hairs. The bull regrets it.

While it is somewhat odd that both sides aren't the same...I am thinking the first issue is the 10 inch. Most of the charts i came across had return and supply the same size.

See if you can get the model number for your blower unit - and then look up the specs for its rated output (CFM and Velocity). Sometimes they will list 3 rated CFM outputs (low, med, hi) - go with the middle one. (A general rule of thumb is 400cfm per ton. So a 3.5 ton would be 1400CFM)

There are several different "charts" posted out on the internet - and you will have to sort through them to get a satisfactory answer. What is even worse is if you call in a contractor - their focus is on the green verses what you really need - or at least that was my experience.

Personally - I would consider all metal ducting in any "exposed" spaces. Critters can't chew through it. It has the best rating for air flow (less restrictive), and it is pretty much "do it once and be done". The round duct can be converted to rectangular duct with ease. This would make it require less height for mounting. (FWIW i replaced my 12" flex duct 3 times before i finally ripped it all out and started over with all metal. haven't had to go under since it was done)
Opportunity has a shelf life.

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Posts: 5673
Joined: Wed Feb 28, 2007 8:01 pm
Location: Weedsport, NY

Fri Jan 10, 2020 6:06 pm

I agree with the metal duct upgrade, A 10 or 12 inch rectangular duct should give about the same air volume as your 12" round flex.

"If I can't fix it, I can screw it up so bad no one else can either."

Posts: 5
Joined: Thu Jan 02, 2020 5:14 pm

Sat Jan 11, 2020 8:49 am

thx for the info. the AC unit is 17 yrs old and has worked very well thus far since we bought it. The AC flow is good now that the fresh air supply flex tube is new. I totally agree with going all metal on the supply / return ducts. When the AC unit dies, I will bite that bullet and do it right.

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