Page 1 of 1

Bathroom exhaust vent tube getting condensation buildup.

Posted: Thu Nov 29, 2018 9:37 pm
by JohnDS
Hello. I have a first floor bathroom with an unheated attic above. I wanted a quiet exhaust fan and did not want to hear the motor running so I had purchased a 250cfm centrifugal exhaust motor that is mounted in the attic. On the one end(inlet side) of the motor, I have 6” flexible dryer hose running to bathroom. I installed an 8”x8” central A/C ceiling box in in bathroom ceiling in which the 6” flexible dryer hose is connected. Below, in bathroom ceiling, I have a 4way central A/C ceiling diffuser installed to have it look neat. On the outlet side of the motor in attic, the 6” dryer hose continues through attic and out the side of the house.

I noticed during winter, the vent house was sagging in attic. Come to find out there was water in it. I had emptied about 2 gallons out of it!!!!! My thoughts are that the warm humid air is condensing on the inside of exhaust hose as it travels through the cold attic and exits. I’m thinking while the motor is off, there may be a constant draft with the warm air constantly running through the system.

To combat this, I was thinking of installing a damper that closes when in off position. The problem is that all the ones I’ve found are cheesy and are not sealed 100%, so I did not even attempt to buy. Another idea I had was to use insulated central A/C flex hose, but I don’t think that would help because though insulated, the inside will still be cold from sitting in attic. I was also thinking to shorten the run where it enters attic and tie right into the 4” pvc plumbing vent that vents through roof. Would this be ok? It kind of sucks choking the fan from 6” to 4”, but I’m running out of ideas.

1) Am I correct in thinking this is what’s happing?
2) Will a damper fix this issue or is the condensate happening while its in the on position too?
3) Tie into 4” plumbing vent.
4) How can I fix this issue? It’s a headache.

Re: Bathroom exhaust vent tube getting condensation buildup.

Posted: Thu Nov 29, 2018 10:01 pm
by Shannon
Do not hook to the plumbing vent! That is a sure way to allow sewer gases to enter the house easily.

A damper will for sure help as well as insulated ducting.
Where does the vent finally exit the attic? Through the soffit ? Through the wall? Through the roof?

Re: Bathroom exhaust vent tube getting condensation buildup.

Posted: Thu Nov 29, 2018 11:05 pm
by JohnDS
Thanks, I did not think about sewer gases. The bathroom is in the middle of the house, so the vent hose enters into the attic, into the fan motor, then runs along the peak of the roof, then out the side of the house. Probably about a 15’ to 20’ run. I’m not experienced with roof penetrations or that would have probably been better.

Yea, I will search more for a decent damper. They all look kind of cheap though. As for insulating, I’ll see if they have maybe double wall insulated flex hose.

Re: Bathroom exhaust vent tube getting condensation buildup.

Posted: Fri Nov 30, 2018 12:33 am
by A. Spruce
The biggest problem is the length of the duct, which allows the air to cool too much before it exits, the size doesn't help either, as there is more surface area for heat to dissipate and condensation to collect. Insulation will help, but this is going to be a constant problem with this length of duct.

The next problem is that the duct is not rigid and it does not angle downward towards the exit point. Flex duct is full of ridges, which dissipate heat, restrict airflow, and traps water, so even if it were angled properly, it's still going to collect water, hence why solid ducting is a better idea.

No, a damper isn't going to help, unless it is somehow an automatic one. How many people in your house can reach the ceiling to operate a damper, much less remember to do so when they turn on the fan? Dampers you find in AC grills don't close tight enough to stop air flow, you would have to come up with something else and adapt it to your needs. A one way damper isn't going to work either, since you want to keep air in, it won't open when you turn on the fan because the air is flowing the wrong way. A one way damper is typically found in the exhaust hood hanging on the exterior wall, preventing wind from blowing in, but easily allow air out.

Re: Bathroom exhaust vent tube getting condensation buildup.

Posted: Fri Nov 30, 2018 7:55 am
by Shannon
Ya that is a long way! Now you have a much larger fan than usual , 250CFM ,so that will help but Spruce is correct the 6" duct size is likely deleting any benefits you get from the higher CFM size. I would really be temped to go 6" to the fan and 5" after the fan. I would also keep the fan and rigid duct lower and slope towards the exterior exhaust vent and insulate the duct.
The damper will need to be something Inline in the duct and have to be just air flow/gravity operated like the ones in normal fan housings. Not really sure where you would find something like that but I would start at Hvac shop. the other option is a mechanical operated one that activates when the fan turns on and off.

In all honesty they make really quiet regular bath fans now I would have just installed a regular fan and vented through the roof in a closer location to the bathroom. Im sure your fan was pricey and now this set up does not work well either so you may have to change anyways?

Re: Bathroom exhaust vent tube getting condensation buildup.

Posted: Tue Dec 11, 2018 9:05 pm
by JohnDS
Thanks guys. The reason I went with 6" duct is that the fan actually has a 6" inlet and outlet on it. Also, yes, an inline damper is what I was looking for. It would close with gravity, and open with the suction of the air when the fan turns on, but they don't look all that promising. I gotta look around more.

I agree it is a long run. I guess I can alway go straight up out of bathroom, into attic, and directly out and up through roof(probably a 5ft run going straight up). Rigid makes more sense than what I have.

So if I did it with rigid and went straight up and out, insulated, and found a damper that works, do you think that would eliminate the problem of condensation?

Re: Bathroom exhaust vent tube getting condensation buildup.

Posted: Wed Dec 12, 2018 7:50 am
by Shannon
It should ,I also would just use insulated flex pipe for such a short run and create a small moisture trap (low spot) so if you get some slight moisture it will not drip out.