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Bathroom fan - vent issue

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Oct 10, 2013, 11:12 AM
Bathroom fan - vent issue
Morning. I am installing a new vent fan. Output from that is 4". The existing vent pipe is rigid 3" pipe up thru the roof. The location of the existing pipe is also problematic - its terminus is right next to the fan. Can I replace the 3" pipe with 4" and/or flexible vent pipe? Or should I stick with rigid? I'm in VA and have not had a chance to check codes yes.

Oct 10, 2013, 08:40 PM
The longer the run the more important it is to have a larger size (4") and to use rigid & smooth vent pipe. If you are going to run a new route, look for the most direct way to get the vent outside. Try to avoid a through the roof vent if you can. What happens with a bath vent is that the moisture gets trapped in a long vertical run, then condenses and drips back into the fan.

Oct 11, 2013, 09:44 AM
Persistent little bugger isn't he?

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Any advice given here is general in nature and is not necessarily valid for your given area. If in doubt check with your local codes enforcement department for what is required when doing electrical, plumbing or structural work on your house. Permits may or may not be required in your area and home owners may not be able to DIY some tasks. I have no way of knowing if you have the skills needed to complete the tasks you are asking about, when in doubt seek professional assistance.

My advice may be worth exactly what you pay me for it. :-) For the record I did not stay in a Holiday Inn Express last night.
Oct 11, 2013, 01:38 PM
Thanks guys. I've replaced the 3" rigid with 4" rigid up thru the existing vent hole in the roof. Pretty simple. Had to use insulated flex pipe to the fan. Worked like a charm. I'll also get some insulation around the vertical run also - should that address condensation issues?
Oct 11, 2013, 05:42 PM
Insulation around the vent pipe will help but not totally cure the condensation issue. It's just a factor of having a long vertical run - lots of area to hold moist air that can eventually cool and run back downhill.

Oct 12, 2013, 08:08 AM
True, the shorter the run the better. Actually Tommy Silva preached the importance of insulated ductwork on a ATOH episode. And it was a very short run {say 4'} and I generally see eye to eye with his top notch advise. And yes money is no option on either TOH and ATOH, so factor that in as well.
He also mentioned in soffit venting condensation will wreck havoc on nearby wood work, and that venting through the roof was preferable. I am not a big fan of poking holes in three tabs, but installed correctly venting should stay high & dry.
Many ways to skin a cat....

This message has been edited. Last edited by: CommonwealthSparky,

Popeye only reached for the Spinach can as a last resort...
Oct 13, 2013, 01:22 AM
the vertical was only 3 feet as the bathroom is on an outside wall. the fan is only about 40" from the outside wall.