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Proper bath fan exhaust in new contruction

Posted by jr3andb (My Page) on
Sun, Feb 17, 13 at 11:44

Building new home. Please excuse my laymen terms. We are at punch stage and 7 bath fans are exhausted out one roof exhaust cap or pipe or whatever you call it. Any problems with this?

#'s of fans as follows:
2 master toilet closets
1 humidity sensing fan in main master bath area for 2 sinks, air jet tub and shower.
2 for sink vanities adjacent to bath 2 with separate door separating from main bath (hollywood or jack and jill layout) (needed per code)
1 humidity sensing fan in main bath two area for shower and tub.
1 Powder bath

Additionally, I upgraded the fans and they call for 6inch ducting and as far as I can tell they took them down to 3 or 4 inch ducting and the runs are really long to the 1 roof pipe.

Please advise. I am doing punch walk with builder and I have mentioned my concern about this many times since they were first installed and it is yet to be resolved.


Follow-Up Postings:

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RE: Proper bath fan exhaust in new contruction

Personally I wouldn't do it. I'd have a external vent for each fan.

It's really up to the local code in your area. Most of the time it may be permitted if (and only if) there is a integrated damper on each fan to prevent backdraft. And, some municipal codes require that no two fans could operate at the same time.

In the cases where code permits multiple exhaust vents into one duct, then they are the type where there is a central fan (sucking) not several fans (pushing). Plus, there are dampers that must block the bathroom vents that aren't being used. Example link below.

You also can't or shouldn't terminate in the attic, inside at the soffit vents or inside into a roof vent. In other words it should be hooked to its own dedicated roof jack/vent.

Also the duct work from the fan housing to the roof jack should be insulated to prevent condensation from warm vent air and cold duct. This is commonly overlooked and is a big deal in cold climates.

Here's a bit of the code:

The IRC is pretty clear on this: (underlining and bold are mine)
- M1501.1 Outdoor discharge. The air removed by every mechanical exhaust system shall be discharged to the outdoors. Air shall not be exhausted into an attic, soffit, ridge vent or crawl space.

That does not leave any room for any air to be pushed back into the living space by any other mechanical exhaust system.

Then the IRC adds this:
- M1507.1 General. 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.

That clearly states that exhaust air from the bathroom/toilet room fan shall be exhausted ... directly ... to the outdoors.

Based on the above two code sections, unless the installation instructions specifically state that two exhaust fans may be connected to a single duct, then two exhaust fans may not be connected to a single duct. Unless, of course, permitted by local code.

Here is a link that might be useful: Ventilator example

This post was edited by audiomixer on Sun, Feb 17, 13 at 22:46


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RE: Proper bath fan exhaust in new contruction

I think you have a number of problems with you installation. You have 7 fans with 6 inch exhausts. Who did the calculation for the fan sizing? That is a lot of air flow when they are all operating. I would be worried about causing a negative air pressure condition in the house.

I don't see why you would need more than one fan per bathroom. If the bathroom is large then size the fan properly. If you have closed toilet area then install vents so air can flow to the one fan in the bathroom.

Each fan needs its own exhaust to the outside. The vent should be consistent and insulated up to the exhaust point.

What do you think will happen when you turn on 6 of the 7 fans? Air will be exhaust out the fan that is not operating. You need to fix this.


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RE: Proper bath fan exhaust in new contruction

If I understand, the 7 pipes are not spliced together. Rather the ends all meet under the cap. The pipe ends are not connected to each other.
/. .\
. . .
\. ./

My house has two like this. The exhaust flow of one is not connected to the other. It blows out the cap/cover. The cap is not a duct.


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