# Two Bath Fans Sharing One Vent?



## 71SoCal (Jan 9, 2015)

I'm adding bathroom fans to two back-to-back bathrooms on a single story house. Can they share one T-Top going out of the roof? Each fan has a 6" flexible duct, so I'm thinking they'd connect to a "Y", then exit the roof.

Thanks for any info,
-Mike


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## sunkist (Apr 27, 2012)

mike call the ahj they are the one to make the call


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## pappagor (Jan 29, 2008)

no you need to run 2 lines and vents


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## paul100 (Dec 7, 2008)

If you want to do both bathrooms on one vent then get a remote fan. I think a company called fantech makes them. You will use just one fan but that will connect to both bathrooms. Only draw back is that both rooms will be vented at same time.


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## avenge (Sep 25, 2008)

Is there a reason not to add a separate vent? Doesn't take much to add a roof vent and it would be proper practice.


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## rselectric1 (Sep 20, 2009)

I have no idea on the code that would apply in your area. But air takes the path of least resistance. Much of the air vented from one bath would simply end up in the other in most cases, and then you have solved nothing.

Why not just eliminate that possibility and vent them separately as we have to here for no questions about that?

Those plastic air check valves if they survive the installation usually don't amount to much protection against this.


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## griz (Nov 26, 2009)

paul100 said:


> If you want to do both bathrooms on one vent then get a remote fan. I think a company called fantech makes them. You will use just one fan but that will connect to both bathrooms. Only draw back is that both rooms will be vented at same time.


I have a Fantech remote fan venting two bathrooms.

It works great with absolutely no problems.


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## Calidecks (Nov 19, 2011)

griz said:


> I have a Fantech remote fan venting two bathrooms.
> 
> It works great with absolutely no problems.


Your fart fan is a "smart fart fan?"


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## griz (Nov 26, 2009)

Californiadecks said:


> Your fart fan is a "smart fart fan?"


Yup, you can set it to turn on depending on the density of the odor...


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## griz (Nov 26, 2009)

griz said:


> Yup, you can set it to turn on depending on the density of the odor...


Just kidding.

Fan unit is up in the attic. VERY quiet. 

I had to add lights to it so we would remember to turn it off.

Each bathroom has a switch. Works great.


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## rselectric1 (Sep 20, 2009)

griz said:


> Yup, you can set it to turn on depending on the density of the odor...


I am so glad they haven't invented smelavision yet.


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## Calidecks (Nov 19, 2011)

griz said:


> Just kidding.
> 
> Fan unit is up in the attic. VERY quiet.
> 
> ...


Damn Griz, "nock" that chit off. You had me going there for a minute.


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## m1911 (Feb 24, 2009)

griz said:


> Just kidding.
> 
> Fan unit is up in the attic. VERY quiet.
> 
> ...



I put an inline fan in the attic for a neighbor about 15 years ago. Back-to-back baths with timer switches in both. I seem to recall it was a Panasonic unit with insulated flex duct.


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## flashheatingand (May 3, 2008)

rselectric1 said:


> I have no idea on the code that would apply in your area. But air takes the path of least resistance. Much of the air vented from one bath would simply end up in the other in most cases, and then you have solved nothing.
> 
> Why not just eliminate that possibility and vent them separately as we have to here for no questions about that?
> 
> Those plastic air check valves if they survive the installation usually don't amount to much protection against this.


One could vent it in a wye to take care of that.


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## Anthill (Mar 23, 2013)

flashheatingand said:


> One could vent it in a wye to take care of that.


we're talking positive pressure. I don't think a wye would do the trick


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## TimNJ (Sep 7, 2005)

If one fan is on the pressure will shut the flapper on the other fan.


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## builditguy (Nov 10, 2013)

One less penetration in the roof.


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## TNTRenovate (Aug 19, 2010)

TimNJ said:


> If one fan is on the pressure will shut the flapper on the other fan.


What happens when you need to run both fans? 

Don't put two fans on the same line. You'll either have to only use one at a time, have them working against each other or have air forced from one bathroom to the other.


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## griz (Nov 26, 2009)

With three kids both fans were often run at the same time.

One bathroom was landlocked...

Neither of the mirrors in the bathrooms would fog up significantly with this fan.


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## builditguy (Nov 10, 2013)

I really don't want to cause any controversy on this or any other subject, but I really don't see the problem with 2 ducts leading to 1 fan with 1 duct exiting the building.

I think Griz mentioned Fantech. This may be the company I'm thinking of. I see the bathroom fan advertised alot. The fan is in the attic. 2 bathroooms lead to the fan, then exit through one pipe, out the roof. Of course there is some issue with the volume of air from 2 ducts going into 1 duct, but I'm sure some engineer has figured that part out. Whether it is a larger exhaust duct or the fact that 2 - 4" ducts are overkill for a fan that can't produce enough volume for that 4" duct.

Isn't this the type of system almost every hotel has? You never see a bath exhaust fan. Only a grill with continuous suction. (obviously not every single hotel ever, but for the most part anyway)

I say, it can be done.


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## TNTRenovate (Aug 19, 2010)

hdavis said:


> OK, so what's the required performance? At least X CFM pulled from the bath and vented out of the building, no matter the state of the individual fans. This isn't too complicated, and is very doable either with an in-line or two fan units, as long as you have back draft preventers (even the hoaky built in ones).
> 
> From the engineering side, back pressure off of 2 units feeding a 6" duct will be less than a single unit feeding a 4" duct, so you aren't going to burn out the fan units.
> 
> ...


This still doesn't address operating both fans at the same time. The one fan could prevent the other fan from operating properly.


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## superseal (Feb 4, 2009)

TNTSERVICES said:


> This still doesn't address operating both fans at the same time. The one fan could prevent the other fan from operating properly.


If you upsize the wye and vent termination at 50%...e.g. two 4" exhaust extending to 6" wye and vent, I don't see how operating performance would be affected. The idea would be to connect the wye at the vent exhaust hood and run your duct from there. I can however, sense a scenario of a high wind events or negative pressure events which pull odors from room to room...possibly that is. 

I know at my house on windy days when the fan is running, that damper can get slammed shut with back pressure and will actually slow the fan blades down. Pretty good strong unit too.


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## TNTRenovate (Aug 19, 2010)

superseal said:


> If you upsize the wye and vent termination at 50%...e.g. two 4" exhaust extending to 6" wye and vent, I don't see how operating performance would be affected. The idea would be to connect the wye at the vent exhaust hood and run your duct from there. I can however, sense a scenario of a high wind events or negative pressure events which pull odors from room to room...possibly that is.
> 
> I know at my house on windy days when the fan is running, that damper can get slammed shut with back pressure and will actually slow the fan blades down. Pretty good strong unit too.


Don't get me wrong I'm not saying you guys are wrong, just that I am not convinced based on anyone saying "I can't imagine" or "I don't see how". I know that 5" line is 1.5 times larger than 4" and 6" is 1.5 times larger than a 5" line.

In my head I can see how a wye connection at the hood shouldn't pose any problems, but I'm one that would need to see it work and know that I wasn't going to cause issues down the road.

Unfortunately there isn't any written code that I can find on the subject. I also know that every inspection I have seen required each fan to have dedicated line.


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## Tinstaafl (Jan 6, 2008)

TNTSERVICES said:


> In my head I can see how a wye connection at the hood shouldn't pose any problems, but I'm one that would need to see it work and know that I wasn't going to cause issues down the road.


How far down the road? One I did five years ago is still going strong, with no issues at all. Will it last another 30? Doubtful. I doubt one of those fans would last 30 years anyway.

There are theoretical inefficiencies in just about everything we do. It's up to us to have a clue when they're large enough to matter.


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## TNTRenovate (Aug 19, 2010)

Tinstaafl said:


> How far down the road? One I did five years ago is still going strong, with no issues at all. Will it last another 30? Doubtful. I doubt one of those fans would last 30 years anyway.
> 
> There are theoretical inefficiencies in just about everything we do. It's up to us to have a clue when they're large enough to matter.


Tins, that is what I am after, the clue. So far it's just trust me, I have done it before. I have never bought into that. My gut tells me that a wye at the hood might be okay, but anything beyond that could be a potential source of issues.

I'll never tell a customer, with questions, not to worry I know a guy online who said he did it this way and he had no issues with it.


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## hdavis (Feb 14, 2012)

TNTSERVICES said:


> This still doesn't address operating both fans at the same time. The one fan could prevent the other fan from operating properly.


Re-read, it does.:thumbsup:


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## hdavis (Feb 14, 2012)

Tinstaafl said:


> How far down the road? One I did five years ago is still going strong, with no issues at all. Will it last another 30? Doubtful. I doubt one of those fans would last 30 years anyway.


I just got through changing out one that was done exactly like we've been talking. Made it over 25 years with no problems.


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## TNTRenovate (Aug 19, 2010)

hdavis said:


> Re-read, it does.:thumbsup:


Not if an additional dampener is used. But go back and read my later posts and you will see that I agree that a wye at the hood might be okay.


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## hdavis (Feb 14, 2012)

TNTSERVICES said:


> Not if an additional dampener is used. But go back and read my later posts and you will see that I agree that a wye at the hood might be okay.


Let me see if I have this right. 2 fan units, both with their cheap dampers, then a damper at the outlet (side vent). And another damper is being added somewhere?


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## TNTRenovate (Aug 19, 2010)

hdavis said:


> Let me see if I have this right. 2 fan units, both with their cheap dampers, then a damper at the outlet (side vent). And another damper is being added somewhere?


I have no idea what you are talking about. It was mentioned that one could add a damper to the line to help with back draft.


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## hdavis (Feb 14, 2012)

TNTSERVICES said:


> I have no idea what you are talking about. It was mentioned that one could add a damper to the line to help with back draft.


So what's the proposed hardware setup? Standard is a damper at the fan unit (usually part of the fan unit) and at or near the building exit. You don't need more than that.


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## superseal (Feb 4, 2009)

TNTSERVICES said:


> I have no idea what you are talking about. It was mentioned that one could add a damper to the line to help with back draft.


But if the hood has a damper, then you're potentially having three dampers in one exhaust duct which would be akin two having to many ells on a sump pump discharge... now we're talking about potential decrease in performance and motor life I would assume.

Then again, I tend to assume a lot :laughing:


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## MarkJames (Nov 25, 2012)

hdavis said:


> So what's the proposed hardware setup? Standard is a damper at the fan unit (usually part of the fan unit) and at or near the building exit. You don't need more than that.


Proposed is two dedicated lines only. Don't make trouble.


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## MarkJames (Nov 25, 2012)

superseal said:


> But if the hood has a damper, then you're potentially having three dampers in one exhaust duct which would be akin two having to many ells on a sump pump discharge... now we're talking about potential decrease in performance and motor life I would assume.
> 
> Then again, I tend to assume a lot :laughing:


I sense a PhD thesis.


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## TNTRenovate (Aug 19, 2010)

superseal said:


> But if the hood has a damper, then you're potentially having three dampers in one exhaust duct which would be akin two having to many ells on a sump pump discharge... now we're talking about potential decrease in performance and motor life I would assume.
> 
> Then again, I tend to assume a lot :laughing:


And that was my point, not my suggestion. I don't think adding dampers to the line solves anything and will add to the reduction in the performance and effectiveness of the unit.


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## TNTRenovate (Aug 19, 2010)

hdavis said:


> So what's the proposed hardware setup? Standard is a damper at the fan unit (usually part of the fan unit) and at or near the building exit. You don't need more than that.


I have only recommended one setup, one direct line for each fan.

My comment about the damper was to illustrate my scenario had not been addressed. If an additional damper is used then one fan will cause the use of the other fan to be reduced or rendered useless.


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## hdavis (Feb 14, 2012)

TNTSERVICES said:


> I have only recommended one setup, one direct line for each fan.
> 
> My comment about the damper was to illustrate my scenario had not been addressed. If an additional damper is used then one fan will cause the use of the other fan to be reduced or rendered useless.


You haven't said where the additional damper is put in your scenario. That's what I was trying to find out - where is the additional damper being put?


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## TNTRenovate (Aug 19, 2010)

hdavis said:


> You haven't said where the additional damper is put in your scenario. That's what I was trying to find out - where is the additional damper being put?


Like I said it was mentioned by someone else. I have no idea where they where putting it.


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## hdavis (Feb 14, 2012)

TNTSERVICES said:


> Like I said it was mentioned by someone else. I have no idea where they where putting it.


OK, I'm with you now.


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## Anthill (Mar 23, 2013)

OK let's just say that 2 lines are better than one unless you go with the inline fan system. But, in a pinch, if circumstances do not allow that, it would work to wye the two together at the termination though performance may be compromised in some situations.


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## hdavis (Feb 14, 2012)

What's better may not be obvious, and it depends on who is deciding what's better. 

In the OP's situation where he's bringing the vents(s) up through the roof, I'd think that a single roof penetration would be better than two. IMO, the primary purpose of a bath fan is to protect the building from a moisture source, and every one here knows roof leak do happen around roof penetrations. It's a significant risk.

To me, this is a simple engineering problem. You know what the design exit CFM needs to be, you know how long the run(s) is going to be. You don't have a measured leakage past the fan's damper, but even for a sloppy damper it's going to be less than 3/4"" hole equivalent. Compare that to a 3" hole for the typical fan's exhaust - maybe 6% of the exhaust flow is going to leak past a really bad damper worst case teed together (I haven't heard anyone does this).

For a given usage scenario, fan life is mainly driven by who made the motor. The rest of it is by far secondary to the extent that 's a hypothetical discussion. 

Personally, I've never seen a problem due to running a single vent line with two fans on it. I've seen lots of problems due to improperly sloped lines, not adequately sealing the duct to the fan exhaust, not having a damper as the vent exits the building, and the occasional critter filling the duct with grass and other stuff.


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## TNTRenovate (Aug 19, 2010)

That's good for made up numbers, but is that actually true.

We've already heard about two fans issues that others have experienced.


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## hdavis (Feb 14, 2012)

TNTSERVICES said:


> That's good for made up numbers, but is that actually true.


I haven't seen a fan out of the box or decades later that is any where near that bad, so, yeah. It would have to have a 1/16" gap all the way around the flapper in the closed position to be that bad.


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## cedarboarder (Mar 30, 2015)

if you use both fans hooked up to one vent they would just be fighting each other.


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