Flex duct length restrictions

[William M. Corbett]

Can any one tell me the acceptable length on 6" flex duct?

[David P. Valley]

I don't think there's a code relating to length of duct work. I've seen ducts run across the entire attic space and I would get adequate readings at the supply.

[Russel Ray]

It would depend on what it's being used for. There are some limitations on dryer ducts, but I'm not aware of any for exhaust fans.

[Jae Williams]

Maximum length for HVAC application is "from here to there".

[Robert J. OConnor]

No restrictions on flex length for residential, and 6'-0" is considered a good maximum for commercial. However a system with a very short metal duct supply plenum and flex everywhere ("octopus system") is considered by many (myself included) to be poor practice. Flex has a much higher resistance to flow than metal duct, which restricts the supply output.

The only problem is you have to actually know what you are doing to correctly size/layout a metal duct supply plenum ... not likely when the HO just gets the cheapest guy to do the job ...