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Flex size VS hard size exhaust ducts.

KZQuixote

KZQuixote

Sawhorse
Joined
Nov 9, 2010
Messages
503
Location
Bend, Oregon
Hi Folks,

Is there a specific code section that mandates a larger exhaust duct size if the contractor chooses to use a flexible duct to vent a bath fan? Or is this controlled by the manufacturer's instructions. I'm working with the 2006 IRC.

TIA

Bill
 
Must meet a given CFM, and the manufacturer's performance charts will list cfm vs pressure. Flex duct manufacturers publish the pressure drop data for their product.

This is no different than selecting a fan for a short vs a long duct run or for different duct sizes. The same fan that is code compliant on 10 feet of 6" duct may not be on 100 feet of 4" duct.
 
Thanks Dr. J,

I thought as much. Another case of the local inspector inventing the code.

Bill
 
Last edited by a moderator:
There is a length limitation of 14' for some but not all of the flex duct material. I know of nothing that would require an up size unless that is what the fan model is designed for.
 
Thanks Guys,

I installed a 80CFM exhaust fan in my bathroom. It took one 24" section of 4" aluminum hard duct to get through the blocking at the bearing wall. Outside of that I attached a 12" piece of flex so that I could connect the soffit hood. After I was done I was able to look through another attic vent and see that that the flex was well arranged and not kinked at all. The inspector told me that if I wanted to use flex I had to go up to five inch. This for a total assembled length of three feet.

I'm not changing it.

Bill
 
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