What size of insulated flex hose do I need to install for bathroom exhaust. Fan is 110cm. Length of hose in attic is approx 15-20 ft, going just about straight to the exit site. Bathroom size is 100 sq ft. There is a 5" one there now and we are replacing it. Both fan entrance and fan exit to outside is 4". We do not know why the builders did it this way. House is 5 years old.

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    They must have gotten a special on big pipe. I would use what is there if the adapters are in good shape. – Ed Beal Feb 15 at 20:01

110 cm fan requires 5" by code and manufacturer's standards. I think you can possibly go up to 4" at 100 cm if you use rigid duct and it is a relatively short distance - probably 4-8' at the most.

With flex duct you cannot for sure go out at 4" more than a few feet and also 4" flex I think maxes out at around 80 cm.

Honestly I would not worry about the 4" if you can go rigid. However if you can't that is a super long distance for flex and expect the duct to fail or pool water and possibly put strain on fan motor. (This is not end of the world stuff you giving you my opinion) If being inspected 4" flex will not pass unless your inspector just doesn't care about that at all - some just care that you vent outside, some want it done exactly right.

IRC Chart is below.

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Now would I install a 4" flex duct in my own home with a 100 cfm fan. Yes if there were not a lot of bends and it was generally straight. It also depends on how often the bathroom will be used and the bathroom size. If it is a heavy use bathroom and smaller in size - well wow a bend in a flex duct and you have a small pond after a week and mold and whatever kind of issues once things start breaking down.

  • We just bought this house and it was inspected before buying. The inspector never mentioned anything about the hose being flex or rigid. As per comment above, most people here use flex house in Vancouver area, BC, Canada. – donna Feb 16 at 4:22
  • Donna - understand but it isn't code or manufacturers recommendation. This is not something a city inspector would normally catch, although trust me some do. Whether it is normal or not in your area doesn't matter. Flex duct is fine but generally has to be oversized due to friction lose. Also you having it inspected is just your inspectors decision/opinion... Looks like he missed something and that is why you had to write this question. Would I worry about a 4" flex duct, no. Is it code or recommended, no. – DMoore Feb 16 at 5:25
  • Bathroom is 100 sq ft with a shower that is used about once every other day. – donna Feb 17 at 9:45

Either follow "just about stright" to use hard smooth duct at 4", or figure the reason the builders used 5" was that increased size somewhat compensates for flex duct having terrible flow friction compared to smooth duct.

  • There are code charts for flex/rigid and span per cm. Flex duct at 4" that long for sure isn't code. – DMoore Feb 15 at 23:18

In HVAC an 8" insulated flex duct is equivalent to a 6" hard pipe so they maybe compensated for flex.

Check the install manual for your bathroom fan exhaust and proceed accordingly.

Do you have a specific gripe with upsizing?

  • Will have to find the manual online. No grip with going bigger, but the other hose is 4". Thanks – donna Feb 16 at 4:20

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