# Can I use one roof vent for 2 bathroom exhausts fans?



## P550 (Apr 11, 2009)

I have two upstairs bathrooms but neither have exhaust fans. The only existing vent in the roof is for the clothes dryer, which is all the way down in the garage. My idea is to vent the clothes dryer into the garage (there is a hole in the wall I can use, that is not an issue) and then use the existing roof vent for the bathrooms.

Since there is only one roof vent, my question is, can I use it for two different bathroom fans? I can install a 2-1 coupler duct on the existing vent and then connect the two individual fan ducts to it.

Will this work, or will the air extracted from one of the bathrooms go back into the other one, if one fan is on but not the other?

(BTW installing a second vent is not economically feasible.)

Thanks in advance!


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## Bonzai (Oct 29, 2010)

You said you would vent the dryer "into" the garage ... Did you mean to say you would vent it through the garage wall? You never want to vent in to any internal space or too close to a window or under a deck.

As regards the bathroom vents; if budget is really tight then why not just vent to an outside wall? No changing the dryer vent and probably less ducting needed in total. The wall mounted vents are pretty inexpensive. Around here it's not permitted to do what you are suggesting.


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## Michael Thomas (Jan 27, 2008)

Combined positive pressure bath vents are prohibited:

I_RC 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. *_

Bold is mine.

Thus combined vents from standard exhaust fans are prohibited, reasoning is that air from one bathroom or kitchen will back up into other rooms 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.)

However... you _could_ combine the two baths on one exhaust line_ if _ you use a remote fan which _pulls_ air out of the bathrooms, for example one of the Fantech products











as air pulled _toward _the fan will not back-flow from one bathroom to the other.


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## P550 (Apr 11, 2009)

Thanks for the replies. I have another option:

Is it possible to install the vents on the roof underhang, where the sofit vents are? That way the bathroom exhaust would go outside, and I can have one for each bath fan. The vents would be far enough from the sofits so they don't draw in any exhaust back into the attic.


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## P550 (Apr 11, 2009)

Michael Thomas said:


> However... you _could_ combine the two baths on one exhaust line_ if _ you use a remote fan which _pulls_ air out of the bathrooms, for example one of the Fantech products as air pulled _toward _the fan will not back-flow from one bathroom to the other.


I like this idea, just concerned about the cost of that particular fan. Also, how difficult would it be to rig the bathroom switches so they both activate the same fan? Or would I have to hire an electrician?


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## P550 (Apr 11, 2009)

Bonzai said:


> You said you would vent the dryer "into" the garage ... Did you mean to say you would vent it through the garage wall? You never want to vent in to any internal space or too close to a window or under a deck.
> 
> As regards the bathroom vents; if budget is really tight then why not just vent to an outside wall? No changing the dryer vent and probably less ducting needed in total. The wall mounted vents are pretty inexpensive. Around here it's not permitted to do what you are suggesting.


Thanks for the reply, but the dryer is not a problem, like I said in the initial post it would vent outside. Opening the walls to install new bath vents is a good idea, but it would probably increase cost, since my house is cement construction, and I may have to end up getting a county permit.


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## WillK (Aug 29, 2010)

Okay, first an exhaust vent next to a soffit vent is a no-no as the humid air will go into the attic where you don't want it.

Second... not to say 2 exhaust going out 1 vent is a good idea, but most bathroom exhaust fans have a flapper that blocks air from wind from coming in, so this would leak some but you wouldn't get as much coming in as if it was wide open.

But I think the 1 fan pulling sounds best. I'd suggest wiring it with 3 way switches, same as you would a light on a stairway. I'm not sure it'd cost less than putting in 2 wall vents though. Or adding a roof vent.


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## P550 (Apr 11, 2009)

WillK said:


> Okay, first an exhaust vent next to a soffit vent is a no-no as the humid air will go into the attic where you don't want it.
> 
> Second... not to say 2 exhaust going out 1 vent is a good idea, but most bathroom exhaust fans have a flapper that blocks air from wind from coming in, so this would leak some but you wouldn't get as much coming in as if it was wide open.
> 
> But I think the 1 fan pulling sounds best. I'd suggest wiring it with 3 way switches, same as you would a light on a stairway. I'm not sure it'd cost less than putting in 2 wall vents though. Or adding a roof vent.


Like I mentioned, the vents would be far enough from the sofits so they won't draw in any exhaust back into the attic, probably a good 7-8 feet away. Besides, the bathroom fan will run only ocassionally, so as long as most of the exhaust goes outside that's fine with me.

About the three-way switches, wouldn't activating one or the other while the fan is on turn it off? At least that's how the 3-way switch I have in my staircase works.

So if bathroom A turns the fan on, if someone in bathroom B turns that switch on wouldn't that then turn the fan off? There must be a way of having both switches work, but that they don't interfere with each other....

p.s. found this....is this the way to connect a single fan with two different switches?

*Switches in Parallel*









If several on-off switches are connected in parallel only one needs to be closed (on) to complete the circuit. The diagram shows a simple circuit with two switches connected in parallel to control a lamp. Switch S1 *OR* Switch S2 (or both of them) must be closed to light the lamp.


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## vre332 (Nov 7, 2010)

I just wanted to add,... does the bathroom need to be vented outside? How about venting in the attic instead


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## Scuba_Dave (Jan 16, 2009)

vre332 said:


> I just wanted to add,... does the bathroom need to be vented outside? How about venting in the attic instead


No, you never vent into an attic


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## WillK (Aug 29, 2010)

I think between 3-way and parallel switches it would be a matter of preference, either should be allowed. I'd pose this: either way, someone in either bathroom can turn the system on or off and when on both bathrooms are vented. If somebody leaves the system on with a parallel switch setup, you might have to go to both bathrooms to turn it off if both of them leave the switch on. With 3-way you can turn it off from either bathroom.

But if you wanted to use a timer or humidistat in parallel, that I think would make sense.

When I think about how to set up a bathroom fan, I tend to think of how often I've gone upstairs at the end of the day to find I forgot to turn off the fan in the morning, meaning I left it on all day.


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## P550 (Apr 11, 2009)

WillK said:


> I think between 3-way and parallel switches it would be a matter of preference, either should be allowed. I'd pose this: either way, someone in either bathroom can turn the system on or off and when on both bathrooms are vented. If somebody leaves the system on with a parallel switch setup, you might have to go to both bathrooms to turn it off if both of them leave the switch on. With 3-way you can turn it off from either bathroom.
> 
> But if you wanted to use a timer or humidistat in parallel, that I think would make sense.
> 
> When I think about how to set up a bathroom fan, I tend to think of how often I've gone upstairs at the end of the day to find I forgot to turn off the fan in the morning, meaning I left it on all day.


Parallel would be the way for me. I'd put the switches next to the light switch, _unless I find a single switch with two independent connections_ so that one action closes the two separate circuits? Do they make something like that? Can't imagine they don't. That way the fan would always be activated when the light is turned on or off.

p.s. I meant two _pairs_ of independent connections in a single switch...


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## WillK (Aug 29, 2010)

What you're looking for is a double pole single throw switch, they exist for electronic project applications but I'm not so sure about that being common for household applications... A 240 circuit breaker is kind of like a DPST switch, but not intended for turning on and off things daily.

This is sounding like determination to do something convoluted... I've been there, are you sure this is what you want to do?

I'll point out that seperately switched lights and exhaust is appropriate... you only need the light when you're in the bathroom. The fan should be on for at least 15 minutes after you're done to ensure the moisture is removed.


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## P550 (Apr 11, 2009)

WillK said:


> What you're looking for is a double pole single throw switch, they exist for electronic project applications but I'm not so sure about that being common for household applications... A 240 circuit breaker is kind of like a DPST switch, but not intended for turning on and off things daily.
> 
> This is sounding like determination to do something convoluted... I've been there, are you sure this is what you want to do?
> 
> I'll point out that seperately switched lights and exhaust is appropriate... you only need the light when you're in the bathroom. The fan should be on for at least 15 minutes after you're done to ensure the moisture is removed.


Good points, and I think you're right, I'm taking the complex route. Best to keep it simple. Separate switches will do fine. Thanks.


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## michaelcherr (Nov 10, 2010)

If it were me, I'd use two timers. Wired like that diagram.
I know I've turned off my fan prematurely just because of how many times I've forgotten and left it on for hours.
Another benifit of this type of fan setup is it will be quieter.(from what I'm told)


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## Termite (Apr 13, 2008)

Venting bath fans into a ventilated attic was legal under the 2000 IRC, but the 2003 IRC changed that, requiring that the fans vent directly outdoors. Putting the vent pipe by a roof vent is not "directly outdoors", and an actual penetration of the wall or roof is necessary to meet code. 

There is nothing in the code that prohibits interconnecting bath fans into a common or shared vent. Code's a minimum standard, and I agree that it is less desirable than individual venting.

Another consideration is condensation of the vent in a cold attic. Warm air will cause the vent to sweat, sometimes causing water to run down the pipe and drip on insulation, celings, etc. Insulating bath fan vents with flex duct isn't a bad idea.


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## Michael Thomas (Jan 27, 2008)

thekctermite said:


> There is nothing in the code that prohibits interconnecting bath fans into a common or shared vent. Code's a minimum standard, and I agree that it is less desirable than individual venting.


Your jurisdiction interprets IRC M1506.1 in combination with M1507.1 as allowing the interconnection of two standard residential positive pressure bathroom exhaust ducts?

Not trying to start a fight, just surprised...


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## Michael Thomas (Jan 27, 2008)

In addition to everything else, most times I see a bathroom fan's exhaust duct discharging into a powered attic ventilator (PAV) and the installation is more than a few years old, the moist exhaust has rusted out the ventilator's motor.


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## ccnvchris (Oct 29, 2010)

How big is the existing roof jack [diameter]? What size would a fan like the Fantech type suggested require? It could very well be less expensive to just install another 4" roof jack.That would simplify your wiring.


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## MLMIB (Aug 12, 2010)

*3 bathrooms exhausting out one vent*

I have an old house with 3 bathrooms that need vents and since 2 are on the same floor next to each other and the third is right below them I'd like to minimize the roof or house penetrations. I know you have to watch out for back flow to meet the reg.

IRC 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.​
but I'm curious, if I put an in-line fan on the exhaust just before the vent to make sure that the air pressure keeps the moist air going out of the house, would this meet code?

and if so, how would I wire it? I've put a diagram in to show my thoughts. They'd all share the same circuit. Red would be the original circuit, blue would be the feed after going through the fan and I'd like to rig it so that the in-line fan turned on after one of the three bathroom fans turns on but I'm not 100% sure how to wire this. Can I set up a diode in-line so that they can all three feed the in-line fan but they don't feed each other?

Thanks for the help!


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