Proper bath fan exhaust in new contruction

11 years ago

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.

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Comments (3)

audiomixer
11 years ago
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
mike_home
11 years ago
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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Thanks for the suggestions. Text communication being what it is, I may have been misunderstood on a couple of points. The reason for not wanting to vent near the porch or under windows has nothing to do with appearance from the street, but rather what I thought was a more obvious objection to moisture and odor wafting in through open windows during the warmer months, or making the porch a less enticing place to relax or entertain when the bath is in use. I confess to being a bit fussy about details, so if I did go through the roof, I wanted the vent to be in the same line as the other two bath vents, which are a couple feet apart in the same shingle row, thus the offset in the attic. Another Q/A site suggests that the worst two places to vent a bath is 1) into the attic, even if under a ridge vent, and 2) under a vented soffit. The site "expert" instead recommends going through the roof, though the bath in this case is on the highest floor. The two existing baths have vented through the roof for many years with no apparent issues. The runs are relatively short, the duct is insulated and the vents themselves have dampers. Enough warmth seems to escape to keep snow clear around the vents. The clothes dryer also vents through the roof as the alternative would be to vent directly out to the front porch, which would create a nice steamy fog on cool days, not to mention the moisture rising up through the porch ceiling vents. Again, no apparent issues after years of use. Having said all of this, I appreciate the concern about running a duct from the basement to the roof. If I can find a path to an acceptable point along the rimjoist, I'll take it.
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Happy to help. I've had a bit of this work done myself, in two different houses. In one, I had a Fantech BFRK 100 installed. There was an existing bathroom exhaust fan that worked poorly but I didn't want to put in a new grill. With the BFRK, you take out the guts of the old fan, keep the housing in place, and then connect ducting that runs first to the Fantech unit located a few feet away up in the attic and then from it to the roof passthrough. I had that done by an HVAC contractor who was on site replacing HVAC equipment. I'm not sure they would have done the work otherwise, it was like a favor to me. The second one was the replacement of a 20 year old exhaust fan with light in a WC closet that was noisy and ineffective. I had that done by an electrician who also was doing other work. He needed to go into the attic to remove the old unit, connect the new one to the framing, put in a new duct and connect the power. It made a huge difference. That one also vents through a roof pass-through, important because we use it to vent the bathroom of moisture from showers and baths. Good luck and keep your BS detector handy. If other exhaust vents terminate in the attic, it's time to fix those too. Each duct run needs its own roof passthrough unless you're using a Y setup with one fan at the base of the Y for two locations. Be careful if so, those can be tricky if the duct runs from the rooms to the base of the Y aren't of equal length. As far as duct run distances go, the unit will specify the maximum distance. For my most recent job, the ceiling unit with light that was 110 CFM, I want to say the spec for the duct run was something like 60 feet. I wrestled with putting a wall vent on a gable end but decided to stick with the existing through-roof vent.
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weedmeister
11 years ago
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.

Proper bath fan exhaust in new contruction

Comments (3)

audiomixer

mike_home

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