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Old 11-02-2007, 08:56 AM   #1
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Complex challenge with bath exhaust fan - need help with solution!

Don't know if this is an electric or HVAC question, but here is my problem ...

we have a 60's brick ranch house that we are remodeling ... one story with a full basement ... it has 2 small baths that stack on top of each other, both on the back wall of the house ... the exterior wall on the top bath is brick veneer on wood frame and the the exterior wall on the lower bath is 10" concrete block ... now, the issue we have ...

neither bath has an exhaust fan ... I don't want to just exhaust the upper bath into the attic space and going through the brick wall to the outside would be a pain; however, going into the attic and piping to the soffit (we have a fairly wide overhang) seems like a reasonable solution ... but, what about ...

the lower bath? ... the only options I can come up with to vent to the outside is to go through the concrete block (ugh!), go into the space between the two floors and then through the brick wall above (not easy either), or lastly ...

go up and through the upstairs bath and tie into the exhaust pipe in the attic (in other words, share the same exit pipe with the upstairs bath) ... this could be done fairly easily since both baths are currently torn down to the studs ... my question is ...

can or should this be done? ... I've never seen it done before ... it's a long way for the exhaust to go up and out from the lower bath to the soffit on the upper floor ...

any ideas or suggestions about how to solve this exhaust challenge? ...

if my idea makes sense, how should I size each of the exhaust fans? again, both baths are small (about 6' x 8').

Thanks in advance, Bill


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Old 11-02-2007, 10:10 AM   #2
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If this question was posed in Australia, there would be a similar response as to what I am about to deliver & that is, exhaust both rooms through their respective external walls. It's not the answer that you wanted but it is by far the easiest solution.

Your other suggested methods require running ducts through floors etc & having fans in "difficult to maintain" locations. A fan in a wall might not be so attractive but it is very effective & easily maintained.


Switchboard design engineer & Licensed Electrician (Australia).
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Old 11-02-2007, 10:23 AM   #3
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Consider a multiport remote inline fan. If there is a way to run a duct inside one of the interior walls, you should be able to vent both bathrooms with one unit.
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Old 11-02-2007, 11:17 AM   #4
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Wow, that's a lot of CFM's with that particular multi-port unit. To calculate how much CFM's you need in a bathroom go to

which takes into consideration the type of pipe you plan on using, size, elbows, etc. Just a warning if you do link the two together and go up, the problem with blowing moisture "up" is that it condenses and once the fan shuts off will drip down and settle at the lowest point, which will most likely be the lowest 90 degree turn. It's up to you how you handle that water pooling there, you don't want that water leaking out and dripping in the wall cavity (so make sure that lower section is water proof). You probably want to make sure it's not coming back and dripping through the lower bathroom fan opening either (so make sure the vent from the lower bathroom runs downhill before turning 90 up). It's up to you how you handle condensation dripping down the pipe and pooling at the lowest point, just want to make you aware. I wouldn't use metal vent pipe you piece together in sections for the lowest section, particularly a metal variable 90 which are far from waterproof at the turn up.

Last edited by Piedmont; 11-02-2007 at 11:22 AM.
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Old 11-02-2007, 11:36 AM   #5
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Building a shaft or ...

chase in the upper bath to enclose the duct running from the lower bath is not an issue ... in fact, we could use the back of a deep linen closet we will be building to do this ... however, the comments in regard to condensation running back down the duct and pooling or exiting the vent in the downstairs bath is troubling. With that exception, the configuration I have suggested seems to be an easy solution. I could put the vent in the lower bath in the corner of the ceiling and run the duct straight up through the upstairs bath (through a chase) and into the attic ... the only turn would be from the attic to the soffit vent, which would only be about one foot (both baths are on an outside wall.

Additional thoughts?

Thanks for the help!
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Old 11-02-2007, 12:11 PM   #6
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I don't think you can join the two vent systems, unless they're on the same exhaust fan. If you have a fan on in the downstairs bath, it will blow wet air into the upstairs bath, and viseversa.
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Old 11-02-2007, 02:52 PM   #7
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If you got the cash, bill, the remote fan is by far the best solution. It is the new "in" way to exaust fans in all the big pimp Mcmansions ATL has. One of the reasons is the fact that the fan motor is remote, usually in the attic. The first time I wired one of these I had to climb up and confirm the thing was on. It has strong suction and is SILENT. The fart fan in my house sounds like a Cessna.

As far as the condensation problems, I can't speak to that.


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