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Thread: Bathroom vent fans, good or bad?

  1. #1
    Join Date
    May 2007
    Newburgh, Indiana

    Bathroom vent fans, good or bad?

    The son is having some work done on his house and wants a bathroom vent fan installed. It is a vaulted ceiling in the midwest, so I'm guessing there is not much room to route a vent pipe unless It goes right out through the roof. I am also guessing, that there is not a vent in there now, but still in code because of a window.

    Bath vents, usually around 1250 CFM, will empty a room of conditioned air in short order The make up air has to come from some place and that is outside. So we are pumping heated air out to replaced with cold air in winter and the opposite in the summer. If the vent pipe is not installed correctly, there will be water build up in pipe which will go racid. What's that funny smell?

    If there is not a vent installed now, I'm sure the fan will just be wired to come on with the light. Not a problen until one of the kids goes off to school and forgets to turn the light off. Furnace runs all day and now it gets expensive.

    What say you? Other than code which was probably introduced in NYC then adopted by everyone else, has anyone thought this through?
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  2. #2
    Bathroom vent fans per Home Depot ads only move around 50 CFM-- Harbor Freight's 'green' dust collection blower is rated at 660 CFM, and I've had 4 of them running in my house/workshop for going on 20 years, many days all 4 of them for 15+ hours a day, and I honestly don't notice any difference in air temps or how often the furnace runs- or doesn't, when they're running or off. Although the power bill might suffer...

    And in this same house in the master bath (no window) is the original 53 year old vent fan, that just vents into the attic space. I've been up in the attic a few times in the past few years wiring new lights & stuff, and there's no signs of ill effects, like mold or mildew or any weird smells. And if you DON'T run the fan during or after a nice hot shower, it's like a swamp in there. Running the fan makes a big difference.

    As for wiring it up separately without much trouble, just get a double 1-gang switch--
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  3. #3
    Join Date
    Mar 2016
    Elmodel, Ga.
    The vent fan in our bathroom doesn't use duct piping. It just vents into the attic area. I haven't noticed any adverse affect on our heating/cooling bill. BTW, it only pulls 60 cfm's.

  4. #4
    Join Date
    Nov 2013
    Waterford, PA
    Yes, he should have vent installed. It is very important that it be vented outside. If the room has a window, it should be near an exterior wall and could be routed thru the wall. No-do not allow it to be attached to the lighting circuit. Code in many places do not allow that and it precludes using a timer type switch. He could always look at a remote unit that only has the grill in the bathroom and allows the fan portion to mounted somewhere else. The remote units are very quiet, but do need timers as you can hear them to remember to shut them off. Depending on the proximity of the 2 baths, some can even share a fan unit. We have FanTech brand and they were simple to install and work flawlessly. As for CFM, look on google. I used a chart that looked at the number of cubic feet in the room to determine the fan size.

  5. #5
    A bath fan can be very helpful. It would remove excessive damp air that can cause the paint and wood in the room to deteriorate. If he is taking the sheetrock off the ceiling the air could be vented out to the soffit with either flexible hose or PVC pipe. I would rather not cut through the roof unless the roofing is being replaced. I've seen too many roof leaks from a retro cut through a roof.

  6. #6
    Join Date
    Feb 2003
    McKean, PA
    I believe most building codes call for bathroom fans to be vented outside. Having them vent directly into an attic space is asking for mold to develop in the attic spaces. This is particularly true in northern areas where it gets cold in the winter months. The moisture being pulled out of the bathroom will condense on the underside of the roofing material and mold will grow. It is best to use 4" pvc pipe for the vent pipe and to slope it toward the outside. If you have vented soffets you must put in non-vented sections several places either side of the vent opening to prevent warm moist air from going into the attic space.

    If the building has a high efficiency gas water heater and/or a high efficiency gas furnace it is a good idea to connect the air intake tube to the outside. Some contractors will save a few dollars on installation and leave this air intake open to the room where the appliance is. There is potential in a tight house for the kitchen and bath fans to pull combustion products back into the house if these air inlets are not vented outside.
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  7. #7
    Join Date
    Jan 2009
    N.E, Ohio
    Quote Originally Posted by Steve Eure View Post
    The vent fan in our bathroom doesn't use duct piping. It just vents into the attic area. I haven't noticed any adverse affect on our heating/cooling bill. BTW, it only pulls 60 cfm's.
    May not be an issue in GA but up in IN that moisture will condense on the inside of the roof and that is not good.

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  8. #8
    Join Date
    Feb 2014
    Lake Gaston, Henrico, NC
    For about as long as they have been available, I use the double switches that have a regular toggle lever on one side, and a row of buttons on the other side that are different length timed cycles. The push buttons get used for the fan, that can be run something like 5,10, 15 minutes, and continuous on. That way it doesn't have to be remembered to go back, and turn the fan off.

    there are many variables, but the ones I've been using look something like this on one side-could be used for light, and fan, but then fan gets used every time, whether it's needed, or not:
    Last edited by Tom M King; 08-23-2019 at 7:58 AM.

  9. #9
    Join Date
    Aug 2010
    I can think of two reasons for venting a bathroom. First is to get rid of moist air from the shower in humid months so that moist air doesn't go into the home which adds to the AC load or discomfort. The second has to do with noxious odors obv. Vent it outside, not into the attic.

  10. #10
    Join Date
    Aug 2018
    Michigan, USA
    Definitely go for the fan, whether it's required by code or not. Our previous home, built ca. 1967, had no fans in the bathrooms. In the full baths, we were always fighting mold/mildew in and around the shower/bath. We finally had fans installed, and it made a world of difference. One was vented through the roof, because that bath was not near a gable-end wall. The other was vented through the gable end. They were wired with the light and fan on the same switch (I think that's allowed if there is another light fixture in the room) - but I would put them on separate switches if you can.

    Re: pulling in outside air -

    In our old home, I didn't detect any increase in our heating/cooling costs after we had the fans installed. Our new home, built in 2016, has fans in the baths that are set to automatically run periodically throughout the day, for the specific purpose of bringing in fresh (outdoor) air. Our builder claims that the house is so airtight that, without the fans, there would be a concern about the indoor air getting stale.

  11. #11
    Join Date
    Feb 2003
    Griswold Connecticut
    Put the vent fan in. Put it on it's own circuit with a timer.
    I can't speak to the codes in your area, but for new construction in the NE, the vent fan has to vent outside now.
    They used to be allowed to vent into the attic space, and then soffett vents were all the rage, but now they have to vent directly to the outside.
    I've not seen one rated for 1250cfm. I think mine are in the 50-70cfm.
    "The first thing you need to know, will likely be the last thing you learn." (Unknown)

  12. #12
    Join Date
    Sep 2013
    Wayland, MA
    Definitely want to vent outside. Mold in the attic is a big, expensive error. If he's improving the house presumably that will include better air sealing, with that correct venting of warm moist air becomes progressively important. MUA is an issue in very tight (ie Passiv Haus standard) houses where mechanical ventilation is typically used but hard to imagine that it is a problem in any older American house. Bath fans run 50 to maybe 120 cfm; for 1250 cfm it would definitely be a concern, but that is the size of the biggest kitchen hood fans with 10-12 inch ducting. No way you want a fan that big in your bathroom! As with dust collection, smooth walled pipe gives much better results than flex hose.

    Two different electrical inspectors I've ha had no issue with the fan being on the lighting circuit (though switched separately), I've not actually seen it done any other way. I found humidity sensing switches (Honeywell, I think) that turn on automatically when the humidity hits a certain point and then off a set period later. They have worked quite well for us. You can also just hit the switch for a timed period. Occupancy sensing switches are also available, but I liked the system that only ran when necessary.

    You didn't ask, but Panasonic makes the best (quietest) bath fans.

  13. #13
    Join Date
    Nov 2017
    Winston Salem, NC
    Since he has a vaulted ceiling, that doesn't automatically mean vent through the roof, you/he can do a faux feature that acts like a bulkhead that hides the pipe from the vent to the wall, where it can go to the outside. But, yes - yes to the bathroom fan. Spent too may years cleaning mould from the ceiling/walls in mine that doesn't have a fan.

  14. #14
    Join Date
    Nov 2011
    Tacoma, WA
    My 2 cents. Add small bathroom fan vented to the outside. A 50 to 70 cfm fan with smooth duct that has insulation wrapped around it if it passes through an unheated area would be sufficient to take care of shower or bath unless the bathroom is really cold. Have the fan on dedicated switch (maybe timer too) and I would put it on a receptacle circuit if that is available and if not then on a light circuit rather than run a new circuit.

    If for some reason (code) you have to run makeup air then that sucks. I don't think it is worth the money to add the makeup air unless the house is supper tight. Even then I would probably try to have it tested to see if pressure change is low enough to not use makeup air.

    If you use a larger fan then makeup air is a good idea and might be a safety consideration if you have combustion appliances for hot water heater or forced air furnace. If you do use a large fan like you are talking and you do have forced air heat then I would connect the makeup air to the return air at the furnace. I would connect the pressure switch that would be needed in the bathroom exhaust duct to the the makeup air baffle and to the fan relay in the furnace so the furnace fan comes on at same time as return air baffle opens. That way the makeup air will go through the house filter system and also be somewhat warmer than if piped directly from outside into the bathroom.

    If code requires you to use heat recovery system on the makeup air for the bathroom fan then I guess that is that. I don't know anything about heat recovery on makeup air.

  15. #15
    We have a story and half house, upstair bath was vented to attic. I installed a small clothes dryer type vent to side.
    That was after we replaced a rather messy looking cedar shake roof that gave more than adequate air vents.

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