Can any one tell me the acceptable length on 6" flex duct?
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.
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.
Maximum length for HVAC application is “from here to there”.
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 … :roll: