Bathroom vent fans, good or bad?

[Bob Glenn]

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?

[Kev Williams]

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--
414868

[Steve Eure]

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.

[Lisa Starr]

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.

[Edward Dyas]

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.

[Lee Schierer]

I believe most building codes call for bathroom fans to be vented outside (https://codes.iccsafe.org/content/IRC2018/chapter-15-exhaust-systems). 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.

[George Bokros]

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.

[Tom M King]

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: https://www.homedepot.com/p/Leviton-500-Watt-60-Minute-In-Wall-Digital-Timer-R62-6161T-1LW/202051145

[Pat Barry]

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.

[Gary Ragatz]

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.

[Mike Cutler]

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.

[roger wiegand]

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.

[Flamone LaChaud]

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.

[Bob Grier]

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.

[Mel Fulks]

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.

[Jon Nuckles]

As nearly everyone has said, yes to a fan vented to the outside. A big yes to a timer switch. Not only does it shut the fan off if you forget, it also runs the fan for a sufficient amount of time to remove the humidity even if you leave the bathroom quickly and turn off the light.

[mark kosse]

Here in SE TX venting into the attic doesn't add to the humidity. I really doubt it does anywhere.

[Andrew Seemann]

Here in SE TX venting into the attic doesn't add to the humidity. I really doubt it does anywhere.

I get mildew from air with a dew point of 80F condensing on walls at 70F in the bathroom when the teenager forgets to turn on the fan after showering. Now imagine venting air with a dew point of 80F onto a bunch of rafters with a surface temp of -10F

Rot and mold from condensation are sadly common around here, and the reason why there are those kinds of codes around venting, vapor barriers, make up air, etc. I imagine it would be different in the south of the country where indoor air is frequently dryer than outdoor air. We actually get both, dryer indoor air in the summer and much wetter air in the winter, but the difference in humidity and dew point is much more extreme in winter than summer.

[Bob Glenn]

I've seen mold in attics near the vent fan exit. Also had a mold problem in a house we bought to remodel. The previous owner shut off the power and basement filled with about foot of water before I could get possession and address the problem.

[Darcy Warner]

1250 cm, is that a 5hp exhaust fan?

[Andrew Hughes]

All bathrooms should have a fart fan.
That’s what we call them in roofing business they usually end with a t-top on the roof.

[Mike Cutler]

Here in SE TX venting into the attic doesn't add to the humidity. I really doubt it does anywhere.

Mark
Not so much humidity, but condensation.
Warm, moist, bathroom air, venting into an attic, that is below freezing, is going to be a problem after time. Might take twenty, or thirty years, but it will one day have to be remediated.
Thousands of examples in New England houses. ;)

[Brian Elfert]

Put the vent fan in. Put it on it's own circuit with a timer.


Why on a separate circuit? They don't draw anywhere close to 15 amps. You can connect to lighting circuit at the switch box without having the fan go off and on with the light. NEC requires a separate 20 amp circuit just for the receptacle that can't be used for the fan.

I have my bath fans on digital times that can do 10, 20, 30, or 60 minutes. I normally use 30 minutes for a shower.

[Ron Citerone]

As nearly everyone has said, yes to a fan vented to the outside. A big yes to a timer switch. Not only does it shut the fan off if you forget, it also runs the fan for a sufficient amount of time to remove the humidity even if you leave the bathroom quickly and turn off the light.


What Jon said. X2

[lowell holmes]

We have two bathrooms and two fans. They work.

[Perry Hilbert Jr]

Simple answer. My parent's bath room vent fan was operated by a rotary switch that turned the fan off after a few minutes. I think it could be turned all the way to 10 minutes and then shut off. House built in 1960 and still has the same switch on the wall. This one goes off after 30 minutes. https://www.1000bulbs.com/product/3305/ELEC-FD60MWC.html?gclid=EAIaIQobChMIrq707oah5AIVyAOGCh3 lRwueEAQYASABEgLzb_D_BwE

[Tom Stenzel]

My house has a full bathroom in the basement with a vent fan and no window. The upstairs bathroom has no fan but does have a window. Guess which bathroom has the mold problem? If you guessed 'upstairs bathroom' you would be right.

What really helps is the room has a pocket door separating the bathtub and the sit-upon from the rest of the room. The fan is about as small as they come but can clear the 5' X 6' space with no problem. It has a 30 minute timer on it, if it goes bad I might get a one hour timer. If the pocket door isn't closed 30 minutes doesn't cut it.

My brother built a house and decided to install a single fan for all the bathrooms and ran ducting to each bathroom. Each room had a timer that ran the shared fan. A few months after living there one of the rooms stopped getting any ventilation. He found the flexible ducting full of water. So he had his SIL hold a 5 gallon bucket under the bathroom vent while he lifted the ducting up to drain it.

It was the furthest room from the fan. There was a lot of duct. And a lot of water. The bucket wasn't nearly big enough.

After looking at all the options (insulation, smooth PVC) he ended up installing a dedicated fan for that room. The length of duct to the shared fan was too long. A direct path to an outside wall was was shorter. The remaining rooms didn't have a problem.

A friend was redoing the bathroom in his quad level house, pulled out the bathroom fan and found no duct hooked to it. None. The fan just blew into the space above the bathroom ceiling and below the floor of the bedroom above it. He showed that one to me and asked, "Where is the air going*?" His solution was to stop using the shower in that bathroom and use another one. Nice that he had the option.

-Tom

*He said other things but I'm trying to be nice to the mods.

[Brian Elfert]

My house has a full bathroom in the basement with a vent fan and no window. The upstairs bathroom has no fan but does have a window. Guess which bathroom has the mold problem? If you guessed 'upstairs bathroom' you would be right.


Most people don't want to open a window when it is hot or cold to ventilate the bathroom. Code was (maybe still is) to have either a window or a fan in a bathroom.

[Jim Falsetti]

We have a 1972 vintage house, plenty of air exchange (i.e, not an energy-efficient, air tight house). The bathroom fan, installed about 10-15 years ago, has a built-in humidity sensor, and you can set the fan switch to auto or manual, or off. So you can reduce humidity from a shower, or you can exhaust noxious smells. It vents outside, not into unheated attic space.

[Aaron Rosenthal]

Like everyone else yes to the fan, and ALWAYS to the outside.
I did have a cheapie Braun 50CFM, and tossed it when I went to a cage type Panasonic - quieter and double the CFM.

[Ole Anderson]

And use insulated vent pipe, whether rigid or flex, otherwise moisture will condense on a cold pipe in the attic and drip back down in the room.

[Lee Schierer]

And use insulated vent pipe, whether rigid or flex, otherwise moisture will condense on a cold pipe in the attic and drip back down in the room.

Sloping the pipe slightly toward the outside will prevent this. I don't think code allows flex piping anymore.

[Ole Anderson]

Sloping the pipe slightly toward the outside will prevent this. I don't think code allows flex piping anymore. Best solution, but difficult to do of you have to go through the roof.

[Jim Matthews]

Plus one on the Panasonic fans and routing outside the house envelope.

[Rollie Meyers]

In my rental wired the fan to come on with either the overhead or the light over the mirror, as it has been vacant for almost 2 years, do not know how well it will work but going to find out soon when it becomes occupied this weekend.

[Charlie Velasquez]

Sloping the pipe slightly toward the outside will prevent this. I don't think code allows flex piping anymore.

They make a condensate fitting that can be installed near the fan and under the attic insulation, or is easily insulated. Condensate collects and then flows downhill through a tube and out the eaves or soffit. Never see them in the US, but saw a number of them when in Europe.

[Andrew Seemann]

The old man ran some of that 3 inch flexible ducting for a bathroom vent though an uninsulated attic to a roof vent. After a winter, the fan didn't seem to vent well and it smelled a little musty. In spring we went up into the attic, and saw that it had sagged into a U shape which had filled completely with water that had condensed on the sides of the ducting. We ended up taking out the extra length and wiring it to a board to keep it straight from the fan to the roof. I would be surprised if it had not frozen during the winter as well.

[Brice Rogers]

In most of our bathrooms, I installed a timer. That eliminates the chance that it'll be running all day.

In our powder room off of the living room I had the opposite problem. My FIL would use the toilet, really stink up the place and NOT turn on the fan. Talking to him didn't do any good. Sometimes when we had company, and I noticed the smell from another room and I would have to go in and turn on the fan. But then another well-meaning guest would go in and shut it off.

So I ended up putting in a motion controlled fan switch. So if someone walks in, the fan will run for the next 15 minutes. It may be programmable but I've never bothered to change the setting. Interestingly, some of the solid state motion switches aren't compatible with a motor and won't work. I had to get a slightly better unit. My guests haven't figured out how to de-activate it (yet).

[Ole Anderson]

Interestingly, some of the solid state motion switches aren't compatible with a motor and won't work.

I recently bought motion detecting light switches for church. One restroom had a fan. At HD I noticed they had one motion detection room switch rated for light motor use. Seems to be working fine. Only problem I ran into is that they all require a ground (and neutral) wire in order to work and some of our restrooms had been remodeled by a contractor that wasn't apparently qualified as an electrician and just ran two wires to the light switches, no ground or neutral. That is one reason Michigan code now requires light switches to have a neutral and ground even if the neutral isn't being used initially. That one tripped me up on the rough electrical inspection for my son's basement project.

[Brian Elfert]

I recently bought motion detecting light switches for church. One restroom had a fan. At HD I noticed they had one motion detection room switch rated for light motor use. Seems to be working fine. Only problem I ran into is that they all require a ground (and neutral) wire in order to work and some of our restrooms had been remodeled by a contractor that wasn't apparently qualified as an electrician and just ran two wires to the light switches, no ground or neutral. That is one reason Michigan code now requires light switches to have a neutral and ground even if the neutral isn't being used initially. That one tripped me up on the rough electrical inspection for my son's basement project.

The neutral for switches thing is part of the national electrical code now. There was no reason in the past to run a neutral to a switch so the contractor didn't screw up there. (No ground is an issue.) Code requires a neutral now because of all the timers, smart switches, and the like that require a neutral.

[Andrew Pitonyak]

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.

Does this room contain a shower or bath? If you have lots of moisture that you dump in the attic, that can be a problem. There was mold in the attic of my house before we bought it and this was listed as the probable cause. it was mitigated before we actually bought the house.

If there is no shower or bath, probably less of a problem, but, venting warm air into a cold attic could cause issues I suppose.