Proper bathroom fan ventilation question

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Mayfield571

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Good Morning,

I have an older bathroom in a space with a thick concrete ceiling so a ceiling mounted bath fan is out. The bathroom is 12’6” x 9’6” with 8’ ceiling. Double vanity, shower, soak tub, & toilet. I am trying to properly vent this room. I think two 70cfm slim wall-mounted bath fans that vent down and through the crawl to the exterior wall will be sufficient. Can I connect both fans to the same switch to turn on simultaneously and can they be vented on the same main duct under the house? I was thinking I’d run 3” duct down into the crawl and then increase to 4” and combine them. I don’t want a direct vent bc the exterior wall is on the front of the house and would not look great. Any suggestions?
 

Dana

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When operated intermittently it needs to deliver at least 8 air exchanges per hour, but if operated continuously a 20cfm fan would meet code in most locations. (Not sure if it's different SC), which might be the better option.

Venting humid bathroom exhaust into a crawlspace is going to increase the mold risk by quite a bit. Ducting it to another location other than the front of the house for venting it to the outdoors would be preferable, even if it takes a long run of 6" to get the static pressures low enough to deliver the 100cfm+ solution.
 

Jadnashua

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Pushing hot/humid air down isn't as efficient as horizontally or vertically. Then, depending on the season and temperatures, that humid air might condense in the crawl space creating other issues. You'd want to use insulated, smooth, solid ducts and be careful about the slope. Switching them both with one switch is fine. Better might be a timer or humidity activated switch so that they will turn on if you forget, and stay on long enough to do what they need to, but not be accidentally left on forever.

The elevation where they exhaust to the outside might be an issue, but you probably don't have to worry about snow depth, at least very often.

Depending on the effective length (this includes the actual length plus the effect of any elbows), upping the diameter can make a huge difference in the efficiency. It also tends to be quieter.
 

WorthFlorida

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Just cannot vent into the crawl space, period. Is this an interior room with no exterior wall?
 

WorthFlorida

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This picture below was from my parents home. Built 1953, moved in 1957 and this was March 2017 (it was just sold last month). Sometime in the 1980's my dad added a bathroom exhaust fan and placed the vent near the soffit without any direct outside exhaust. Just snugged it up to the aluminum slotted soffit covering the wood. The fascia was completely gone. There was more damage to the right of the picture. As you can see the vent stack for the bathroom.

This shows what happens when not exhausting the bathroom vent properly. Placing the vent to the front wall of your home through the crawl space will be low to the ground and should not be noticeable. Placing a plant or two in front of it will hide it.


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