Bath Fans Exhaust Venting to Exterior

brianmiller · 12-09-2012, 02:52 PM

Are multiple vents allowed through one opening?

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**Rick Cantrell** · 12-09-2012, 02:57 PM

Originally Posted by brianmiller

Are multiple vents allowed through one opening?

I believe it would be OK, IF it cannot blow air back into the other room.
But I suspect that it will blow back, so most likely NO.

**Lon Henderson** · 12-09-2012, 03:51 PM

I see that all the time, but several of the municipalities around here require each vent to have its own dedicated penetration through the roof.

Inspectapedia says, "We avoid venting bath fans up through the roof both to avoid an extra roof penetration (and leak risk) and to avoid condensate leaks into the bathroom ceiling. At above-right we show an ugly bathroom exhaust vent installation through the roof surface using a laundry dryer sidewall vent cover. Not only were the roof shingles torn up and sealed again to leave a leak-risk around the vent penetration of the roof surface, but because this roof exhaust vent was installed on a home in a snow-climate, in winter with snow cover on the roof the vent is likely to be blocked."

**Jim Luttrall** · 12-09-2012, 04:04 PM

No prohibition I know of, very common here in new construction.
Each fan will typically have an integral damper to prevent blow back.

**Eric Barker** · 12-10-2012, 07:43 AM

I report based upon my opinion which is that this is not a good practice and not an installation I would want in my home. I really don't care if it's "allowed."

**Lon Henderson** · 12-10-2012, 08:02 AM

Originally Posted by Eric Barker

I report based upon my opinion which is that this is not a good practice and not an installation I would want in my home. I really don't care if it's "allowed."

So, do you say things like, "the xyz item is installed compliant to building codes, but I don't like it and recommend doing it differently?"

**Scott Patterson** · 12-10-2012, 09:01 AM

Originally Posted by Lon Henderson

So, do you say things like, "the xyz item is installed compliant to building codes, but I don't like it and recommend doing it differently?"

What not say something like this: The ____ is installed in an unconventional manner and unconventional things tend to perform in unconventional ways!

**Jerry Peck** · 12-10-2012, 04:33 PM

Not allowed as shown, however, if they had installed a wye fitting and connected each to the inlets of the wye, then that ... would still not be allowed.

Exhaust area required to exhaust air so that the exhaust air cannot be blown back into the home, and that will happen, even with a wye fitting, and even with dampers in the fan discharge outlets air will be blown back into the exhaust fan.

Ted Menelly · 12-10-2012, 05:57 PM

I think the likelihood of air blowing back is about as likely as me becoming a millionaire in the next month. What little may try will be thwarted by the damper as they do close pretty tight. I doubt anymore air will blow back than what tries to blow back on a windy day. It is blowing up into the roof vent and that would be the line of least resistance and 99 999999 should blow outside.

However. I would say to my client that they should consider adding an additional roof vent for each ceiling vent as it is the way it should be. In saying that I do not know of a city inspector that would fail an inspection with that situation around here..

JMHO

**Jerry Peck** · 12-10-2012, 07:23 PM

Originally Posted by Ted Menelly

I think the likelihood of air blowing back is about as likely as me becoming a millionaire in the next month. What little may try will be thwarted by the damper as they do close pretty tight. I doubt anymore air will blow back than what tries to blow back on a windy day.

"I doubt anymore air will blow back than what tries to blow back on a windy day."

You could not have picked a better analogy, here is why - I have seen the following many times:

New homes in new developments with undeveloped lots, which means sand and dirt blowing around in the air. The owner asks what can be done about all the dirt blowing into the bathrooms, so I go to look and there, beneath the bath fan is a dirty/sandy spot, and dirt/sand on the towels, etc., including actually having been there to see the dirt/sand blow in.

Ted, you are absolutely correct that two fans is like a windy day. Glad you brought that up.