# Can bathroom exhaust and dryer share same outside duct?



## fabrk8r

Bath exhaust should always be separate from any other duct, as should dryer vents. 

All the code books I've ever seen say that too.

The reason it's not a safe thing to do is because of both sanitation issues and fire safety issues.


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## spaceman spif

Plus if your backflow prevent ever failed, your clothes would come out smelling like crap.


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## NitrNate

don't bathroom exhaust vents vent through the roof, while dryer vents vent out the wall on the side? at least that is how my house is set up. all my bathroom vents run up through the attic and out the roof.


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## Rambo

benze said:


> Hi,
> 
> I have a dryer sharing the same wall as my basement bathroom that exhausts directly to the outside through the foundation wall using a 4" duct. Pipe run is about 8'. My bathroom, however, doesn't have a ceiling fan, so it accumulates a lot of moisture.
> 
> Can I tee off the dryer line and connect a bathroom exhaust pipe to the same line to avoid having 2 outlets 2 feet from each other on the outside? I would put in backflow preventers on both lines to avoid the dryer from pumping air into the shower, and vice versa.
> 
> If it is not recommended b/c it is a 4" outside duct, if I were to increase that to 5", would it be acceptable? Or must the exahust lines for bathroom fan and dryer be separate? Is there any code restrictions on this?
> 
> I live in Montreal, Quebec, Canada, if that makes any difference (code wise or other).
> 
> Thanks!
> 
> Eric


 
thats a big fat NEGATIVE.....


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## bernieb

I've seen an awful lot of bathroom exhast fans just exit into the ceiling proper, between ceiling /floor joists.


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## beenthere

Code prohibits them sharing the same duct. Or vent hood.


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## Marty S.

NitroNate said:


> don't bathroom exhaust vents vent through the roof, while dryer vents vent out the wall on the side? at least that is how my house is set up. all my bathroom vents run up through the attic and out the roof.


 Depends on the house lay out. I wouldn't run basement bath fans out the roof or first floor bath vents on a two story house. My slab on grade house has the laundry area on an interior section so out the roof with the drier vent is the only option.


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## Scuba_Dave

bernieb said:


> I've seen an awful lot of bathroom exhast fans just exit into the ceiling proper, between ceiling /floor joists.


Which does not meet code & a good way to cause mold if there is a shower in the bathroom


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## benze

Scuba_Dave said:


> Which does not meet code & a good way to cause mold if there is a shower in the bathjoom


How bad is it to vent a powder-room exhaust straight into the attic? Am living in Montreal, Quebec, Canada, so I am concerned about condensation in winter months; when ventillation being evacuated from the powder room is warm coming into contact with the cold air in the attic.

Do I have to worry about that? Putting in a roof vent would be quite ugly as this powder room is in the front of the house, the vent would be quite visible from the front door.

Thanks,

Eric


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## Scuba_Dave

Venting warm air into the attic in the winter is not a good idea
Every code I have seen people refer to indicates exhaust MUST exit the building


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## beenthere

benze said:


> How bad is it to vent a powder-room exhaust straight into the attic? Am living in Montreal, Quebec, Canada, so I am concerned about condensation in winter months; when ventillation being evacuated from the powder room is warm coming into contact with the cold air in the attic.
> 
> Do I have to worry about that? Putting in a roof vent would be quite ugly as this powder room is in the front of the house, the vent would be quite visible from the front door.
> 
> Thanks,
> 
> Eric


You'll end up with lots of condensate in the attic, and soak your insulation, and grow mold.


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## hvaclover

LOLOLOLOL...this thread has me rolling on the floor!!!

I have a neighbor who vented a basement bathroom into a 4" tee off his water heater flue pipe. Been like that for twenty years. The guy's wife and mine are best friends. i don't hang with the guy but take care of his ac.
I was there this summer and he still has the bathroom fan duct going to the water heater flue.:laughing:

Some people never listen


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## benze

beenthere said:


> You'll end up with lots of condensate in the attic, and soak your insulation, and grow mold.


That's pretty much what I had figured too, but was kinda hoping someone would tell me otherwise.

Another option I have is to exhaust it over my front porch. I have a large overhang in front (about 1-2 feet high and about 5 feet deep) that has no insulation and soffit venting. I could just tie the end of the duct work so that it exhausts in that space, but I am concerned about ice buildup in a space that wasn't designed for it.

I'm sure that it is a terrible option too, but just on the chance that it might not be as bad as I think, I'm hoping to get some feedback....

Thanks,

Eric


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## fabrk8r

Exhaust it outside through an appropriate vent hood. Do not terminate the exhaust inside the structure, anywhere, at all. All the reasons why it isn't a good idea to terminate bathroom exhaust inside the structure have been discussed in previous posts.


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## benze

*sigh* That's kind of what I had expected / figured as well. Probably one of the reasons a fan was never installed in the first place.

Back to the drawing board; will have to figure out another way of doing this, I guess....

Thanks again,

Eric


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## Michael Thomas

Why you don't want to vent bathroom exhaust fans or clothes dryers into an attic:












I have seen many such examples of sheathing damage and mold growth in attics as a result of such "venting".

Best practice if you are in a cold climate is to vent through solid metal wall metal pipe, insulated to help control condensation:












- http://www.rd.com/19559/article19559.html

Some installers recommend the use of spray foam insulation instead.

If you're using flexible pipe up make certain that there no compressed bends, or dips were water condensing out of the bath or dryer exhaust can accumulate, and with either type of vent pipe that no other bathroom or other exhaust fans are venting through the same cap:











Do not know about Canada, but if the codes are based on the International Residential Code you can not connect a positively pressurized portion of a bathroom exhaust with another bathroom exhaust or any other exhaust:



_M1507.1 Where toilet rooms and bathrooms are mechanically ventilated, the ventilation equipment shall be installed in accordance with this section._
_
_
_ M1506.2 Recirculation of air. Exhaust air from bathrooms and toilet rooms shall not be recirculated within a residence or to another dwelling unit and shall be exhausted directly to the outdoors. Exhaust air from bathrooms and toilet rooms shall not discharge into an attic, crawl space or other areas inside the building. _
 

Thus combined vents from standard exhaust fans are prohibited, the reasoning is that air from one exhaust will back up into other rooms and/or appliances on the combined line - the "flappers" at exhaust fan housings are not air tight. (There are exception for engineered systems in multi-unit buildings, but they don't apply in your case.)


Finally, the instructions for every dryer I've seen (except for condensing units) includes something similar to the following:


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## hvaclover

Very good presentaion.


Your post is bound to have given all the Pros and Diys alike alot of food for thought. I'm sure us old salts of the hvac biz picked up at least ont new tip form your post:thumbsup::yes:


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## harrydnyc

Picking this very old thread back up because there exists a similar issue at my house. We have a full bathroom and laundry room (right next to each other) in our finished basement. One day, I noticed that the vent for the dryer was not venting outside at all. It was basically venting into a drop-down ceiling. I called a handy man, and he noticed that the reason the dryer wasn't venting outside was because the vent was being used for the bathroom. Not knowing any better, I asked him if we could use a splitter to vent both of them through the side of the house using the same exterior vent. So, that's what he did. Once the weather started getting cooler, I am noticing that the ceiling of the basement is building up a ton of condensation (to the point where it is dripping to the floor). I THINK this is happening when we are using the dryer. So, based on reading this thread, I realize that using the same exterior vent for both the bathroom and the dryer is not up to code, and I plan to have that resolved. My question is: does it make sense that this is what is causing the condensation on my ceiling in the bathroom, or do you think that I have a separate issue as well?


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## chandler48

Yeah, it's a 10 year old thread, but it is good you used the search feature to find information. Basically what you have is back pressure through your common venting and that back pressure is causing the moist air from the dryer to back up into the bathroom vent where it is possibly sent throughout the basement. Yes, correct it as it can cause definite problems.


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