How to Choose a Bathroom Exhaust Fan | Ask This Old House

Ask This Old House master electrician Heath Eastman discusses some new technology for bathroom vent fans.

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Cost: $100 and Up
Skill Level: Expert

Shopping List:

Steps for Choosing a Bathroom Exhaust Fan:
1. Bath fans help move air out of the bathroom to outdoors, in the process removing both odors and more importantly humidity. The fan should be on anytime somebody takes a bath or shower because when you have humidity building up in a bathroom, you invite mold and mildew to form.
2. Heath prefers using a 4-inch insulated flexible venting both to cut down on noise and to prevent condensation.
3. Heath emphasizes that bathrooms cannot vent into an attic. All that humidity will get trapped in the attic and cause mold to form. He suggests venting out a side wall whenever possible. If that’s not possible, go through the roof. Do not vent through a soffit.
4. Heath shows that most bath fans have the fan mounted right over the register, but there is an option where the fan is located elsewhere, often an attic or basement and is ducted to both the bathroom register and the vent to outside. He likes these because they allow for longer duct runs and can allow for proper venting where it might not always be the path of least resistance.
5. Heath also underscores the importance of actually operating the fan when needed and for the appropriate length of time, which can be controlled by timers.
a. The most basic form of control is a simple on/off switch, but that requires you to turn off the fan when you leave the room and it’s really better to run the fan for at least 30-40 minutes after a shower.
b. A decade or so ago, Heath would’ve used a rotary timer switch, which needs to be dialed when you enter the bathroom, and when the time runs out, the fan shuts off.
c. Nowadays, there are digital timers that do the same thing and look a little nicer, but you still have to turn them on to start them.
d. There are also humidistat switches that can go into the wall and can automatically turn on the fan when the humidity reaches a certain point and then turn it off when it dips below that point.
6. There are even newer fans that require constant power at the fan and use a switch leg for more control. It can be set to always be on at a low speed to keep air moving in a home.

Resources:
Heath recommends using an insulated flexible duct for venting bath fans both to eliminate condensation and noise, which are available at home centers. He prefers terminating these vents on a sidewall or through the roof, never into an attic or into a soffit.

Heath showed options for controlling a fan that included digital timers and humidistats. These are sold at home centers and electrical supply houses.

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How to Choose a Bathroom Exhaust Fan | Ask This Old House

[BradThePitts]

I actually prefer a relatively LOUD exhaust fan to cover up "bathroom sounds"

[thezfunk]

I had 5 fans all venting into the attic when we moved into our house built that was built in '79. I was not going to put 5 extra holes in my house so I found a whole house fan by Aldes. It has a central fan that mounts in the attic and registers with dampers in each bathroom/laundry area. Ran insulated flex duct to the main housing with the fan in the attic. When you hit the switch in the bathroom, it opens the damper, a read switch on the damper kicks on the fan in the attic. Almost completely silent and moves a ton of air. Most contractors I have shown it to have never seen anything like it.

[Fantechusa]

Thanks for choosing Fantech for this demonstration! We are happy to have served contractors like you for over 45 years. Keep up the good work!

[ytrewqwer]

Retired home inspector here, buyer beware. Ducting and venting placement, and a reliable fan that is cheap, easy to replace or repair is all home owners need. As far as noise, get good insulation, but don't get a fan to do that job or it will be major trouble some years later. The last two fans are only if you have pro maintenance such as common areas in condos or coops. The central blower option is fine, except the motor is usually in a place hard to get to unless you design easy access from blue prints. In homes with multiple fans and ducting, if one fails, you have the others, with central, if it fails, all goes, and often hard to replace. The last option is a DC blower fan with digital controls, a nightmare to repair, expensive to buy and install, but quiet and energy efficient. In a lightning storm, the whole thing can lock up, reset, never shut off or turn on, and requires a power down, if it didn't get damaged. Sometimes, it will just fail, or a module will fail, and hard to diagnose, is this worth quiet and energy savings? Only if you run the fan 24/7 which no one does except in buildings, or maybe mansions with full time maintenance; for us, you're better off just getting a cheap Broan 688 type and a timer, for $20 each, and it will last decades, I use both, installed in 1978 and still going.

IR or motion sensors? Humidity sensors? scheduled timers? I've not seen any make it past 10 years, way after warranty, and before it it may just not sense right. Unless you are disabled or in a retirement home with maintenance to adjust or repair these as needed, don't bother. Just run a timer for 2x the length you ever need, use 30 instead of 15min or a full hour if unsure. In winter time, when air is dry, the bathroom will dry up quickly and fan time may be nothing, to avoid heat loss to the vent just open the door, the fan is more helpful in summer or year round to vent odors and take the heat loss.

[halfdohm]

I have a Panasonic Whisper fan and I love it. Not cheap, but very quiet and efficient.

[kennylakits]

Great Video, this and a Tom Silva video on why not to use a soffit vent saved me from a dumb mistake. It makes sense once you think about it. I use Panasonic fans, seems the most quiet and most CFM with a 6 incg duct.

[MRYENNYENN]

*Well-made, delivered quickly, and easy to install with **Fastly.Cool** . I may be imagining it, but it seems quieter than the 14 year-old unit that I pulled out. Of course it was noisy at the end due to failed bearings, so hard to compare.*

[mcseforsale]

I have yet to see a video of someone retrofitting a fan into a bathroom with no fan or ceiling fixture, only an outlet next to a sink that's on the first floor of a 2 story house and venting it to the side of the house. THAT'S the video I want to see.

[PaulRudd]

I recognize that last fan! That's a Panasonic VKS2 fan with motion sensing and humidity monitors! Really excellent fans.

[NordicDan]

When my house was flipped (so would not have bought had I known at the time it had been flipped because literally EVERYTHING was done improperly with wrong hardware, incorrect wiring and major corners cut) they put in the bathroom exhaust fan and dumped blow-in insulation right over the top of it (and its exposed electrical hookup!) having it blow right into a pile of insulation. I finally got a vent in the roof and have it ducted to it, but what a nightmare.

[STXVIEC]

Most modern homes use the new lutron Maestro Timers for fans, as they look nice and run for whatever time you have it set to

[lucaspatterson]

They didn't even talk about the most important aspect of bathroom fans which is CFM.

[StinkyPony]

I use a Lutron OCC sensor switch for the fan. Turns on when you go in to cover "sounds". Turns off by itself after you leave.

[BrianDoesStuff]

It may be a good idea to let people know that you can run these fans to more than 1 bathroom and these fans can be used for kitchens also, very easy to install, i am talking about the fan that you can mount anywhere and just cut a small round hole in ceiling for the inlet, these are the best kind because the regular fans that have vent attached to the fan get gummed up with dust and moister causing them to go bad in a year or so.

[shilpi]

Can you recommend any exhaust that goes to attic as opposed to roof? Too much power/moisture may not be good for attic. Thanks

[oldtwinsna]

So they have auto timers that measure moisture. But the real question is if there are auto timers that have auto air quality sensors?

[jimbabbitt]

Where can I find the just the grill box to use with an in-line fan?

[mbahomeimprovement]

One of the biggest things this video neglected to mention was how powerful of a fan you need for the size of the bathroom you're trying to vent. Anywhere from 50 cfm up 150 cfm or more.

[torreslover]

What about flex duct decreasing airflow in duct? Also I don’t think you’re supposed to vent through the wall but I’m forgetting the reason... can you not add a damper?

[mavirek]

But my current bathroom fans go nowhere: they don’t vent anywhere; they are just a metal box with a fan; the only open end of the box is the side open into the room. What kind of fan system is this? The grill of each fan seems to have a space to insert a Glade plugin or something similar.