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Exhaust Fan Question

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Jan 04, 2014, 03:12 PM
Exhaust Fan Question
So I just got done repainting our bathroom because of brown mold growing on the walls / ceiling above the shower. I have checked out the exhaust fan and noticed it wasn't pulling any air. Climbed up inside the attic crawl space and disconnected the exhaust duct and fan now pulls air. When I checked the exhaust duct, water came pouring out at the joints. This is an non-insulated crawl space so I know the water is coming from the exhaust condensing and was more than likely my cause for the fan not pulling any air. My question is this though. While up there, the exhaust for both second floor bathrooms go through the same side vent which is located about 16 feet away for the first fan and 20 feet away for the second fan. The builders used the metal flex hose to route the fan to the side wall vent. Is it worthwhile to separate the exhausts of both fans and run them to the rear wall of the house (lengths would then be 4' and 8')? Would that reduce moisture build up inside the duct? Or would other options be better (replacing flex duct with solid duct, installing an automatic timer for the fans, simply insulating the ducts better)?

Some more information on our house if needed:
Master bathroom (where the mold was growing) - 72 sf, 70 cfm fan
Main bathroom - 45 sf, 50 cfm fan

Jan 04, 2014, 05:01 PM
Everything you are scoping out would be a good idea. General guidelines for fans is to have no more than 15' of solid 4" vent. every bend counts as 2'. Granted, you can go a little further and it will still work but it's efficiency will be cut down.

Flex pipe will not work as well as solid.

Never a good idea to combine multiple vents into one unless you go up in size. Here too, under most cases it will work OK but if more than one fan is operating at the same time it will not work well. Plus, there will be backblow from one fan into the other - even with a flapper.

So if you can reduce your runs from 15'/20' with flex line to 4'/8' of solid, you will likely solve your problem.

Jan 04, 2014, 05:58 PM
If it is really cold in your attic, the vent should either be insulated or run it under your insulation. (This is where ours runs, under insulation)
This above location can cause moisture to freeze in the vent, and leak out when it thaws...causing problems including water damage to ceilings.
Jan 06, 2014, 04:30 PM
run your vent straight up to the ridge vent.using 1 90 on the fan body.looking straight turns not bends
you want the steam to go out, if you turn the pipe the steam will cool at that point and puddle
Jan 07, 2014, 11:46 AM
Thanks for all the information. Based on what I know about the builder, it looks like the easy route was chosen as they didn't have to deal with the soffit with the venting which is why they went to the farthest wall when they laid the ducts down. It also explains why there is only one outside vent compared to two.

We didn't have an issue in the past but since my folks moved in with us, we are using both bathrooms more frequently. Looks like I will be fixing that this spring when the weather warms up.
Jan 08, 2014, 02:46 AM
mind if i add a little something ****
if you do as conrad suggests,,,that is ** coverr the duct with insulation
make sure it is sloping ** the out doors..
the reason being...your fan pulls steam out of your bathroom
steam is a vapor,when the ambiant temp cools the steam will turn back in** water...if your pipe is laying on the joists it will puddle and accumulate till it drips down ** your floor. if not run on an angle
steam rises by its own accord, and disapates, so it is my oipinion ** run your vent straight up, ** a ridge vent. no offsets except the 90 degree turn on the fan itself..install piping as you would a dryer veny with no screws just silver backed duct tape..[the steam can collect on the screw ends and drip back ** the fan]

this flaging of the word "two" is really upsetting me!!!!!!
i like the word "two" its a handy word
aint never done me no wrong..

This message has been edited. Last edited by: Frodo,
Jan 08, 2014, 08:46 AM
You are right Frodo. I ran our vent tube horizontally and under the insulation and created a vent box in the in the soffit area w**** it could exhaust.
Jan 08, 2014, 11:54 AM
What the heck is with the random asterisks?
A S T e r i s case this word causes more of them.
They seem a bit random, and of course make no sense.
Jan 08, 2014, 07:12 PM
oh my my my, what a potty mouth Big Grin

They make it look so easy on tv, don't they
Jan 08, 2014, 08:19 PM
Okay, thanks for the clarification, GardenSprite. And Re-modlr this is a Bathroom forum, so Potty mouth is rather appropriate?

Anything to reduce the spammers is good news to me, Thanks again GS!
Jan 08, 2014, 11:19 PM
Conrad, Jaybee is responsible for this latest success.