Bathroom Exhaust Fan

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Craig Cannon

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Hopefully this is the correct forum.

In my master bath, there is a single exhaust fan installed and I'm guessing it original (house built in 92). I don't know the CFM, but it only has a 3" vent and based on the fan size I'm guessing 50 - 70 cfm. I know the fan is woefully undersized even though we do not have any mold/mildew issues.

The bathroom is 10 x 11 with a partial vaulted ceiling, so around 950 cu ft. The fan is over the toilet which is in a cubby hole, but no separate door (pics attached).

Options: I can install a new 110 CFM fan pretty easily, thought that is still borderline on minimum size. My concern is the 3" duct. Any idea if reducing a 110 fan to 3" will be problematic?

Second option would be to install a second fan over the tub/shower area. This makes the job larger and would require me to hire someone just to make sure it's done to code. I really hate to have to hire a contractor when I can replace the current fan myself, but think it may be the best option.

Thoughts?
 

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wwhitney

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If you use a fan like a Panasonic ECM fan, it will adjust the fan power to get the rated CFM, up to 0.375" w.c. static pressure from your duct. E.g.

https://na.panasonic.com/us/home-an...fans/whispergreenr-selecttm-fan-50-80-110-cfm

If you try to run 110 CFM through a 3" duct, that's an air speed of 2200 fpm, while I think 1000-1500 is a more common design (not sure). The static pressure loss will be 3.32" w.c. per 100 ft., per:

https://www.engineeringtoolbox.com/duct-friction-pressure-loss-d_444.html

So if you have only 10' of 3" duct with no fittings at all, and the terminations have 0' equivalent length, one of those Panasonic fans will actually do 110 CFM through a 3" duct. (This is assuming a rigid 3" duct). If you have have any fittings or a longer run, I expect it will be impossible to get 110 CFM through a 3" duct with commonly available bathroom exhaust fans.

Replacing the duct with a 4" or 5" rigid duct, or settling for a much lower CFM than the fan is rated for, are likely your only options.

Cheers, Wayne
 

jadnashua

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Is there attic above that room? I have put in a few Solatube ceiling light/fan assemblies that use a remote, inline fan...really neat, quiet, and it lets in the equivalent of about 300W incandescent light into the room from the roof. There are other brands, but Solatube is the original, largest, and I think they have the closest to daylight color temperature from their tubing. They’re made such that they can replace an existing light/ceiling fan assembly. Yes, it costs more, but in my case, my bathroom has no windows, and it took over a year to not reach for the light switch to turn it off when leaving the room! On a winter night where there’s snow on the roof, the streetlight glow lights the room enough to not need the light on at night on a piss run, or if the moon is out full.
 

Craig Cannon

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Thanks for the replies. The Solatube option is way more than I want to spend on this job, and the auto-adjustable Panasonic, while nice, is crazy expensive for a small exhaust fan.

I'm thinking the best option is to put in a larger fan and run new 4" duct to the soffit. The 3" runs to the soffit now and its only a bout 3 ft of length.

Anyone have any thoughts of connecting a 4" duct to a 3" soffit vent? While not ideal, any leaking air is still right at the soffit. I'm in Houston area, so any bit of humidity that escapes into the attic would be minimal and is no more than what is already in the air during peak summer.
 

Craig Cannon

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jadnashua

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Depending on the prevailing winds, discharging into a soffit may not be a great idea as the hot, moist air tends to cause condensation as it rises up to the horizontal surface. Going out a gable end or through the roof may be a better solution as it disperses better.

Panasonic doesn't make this switch, and you may find the OEM one for less (don't remember now who makes it for them), but I've also used this switch. Since it's lower down on the wall, it doesn't automatically turn on as soon as I'd like, but it can be added to any fan. You can manually turn it on, but it stays on for 30-minutes, or 30 after it senses the moisture level has dropped enough to prevent condensing. To check it after installation, just get up close and exhale into the grate...the fan should turn on. It's looking for moisture versus the relative humidity.
Panasonic FV-WCCS1-W Multi Function Bathroom Fan Switch, 5.5'' x 6.0'' x 10.5'', White - Wall Light Switches - Amazon.com
 
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