How to wire a bathroom fan to go on with light switch?

April 28, 2023 5:26 AM

How can I wire a bathroom fan to be “always on” when the light is on?

My teens are trashing a bathroom by steaming it up until everything molds. They take unreasonably long showers and do not turn on the fan. A better parent would somehow convince them to stop doing this, but I give up. How do I wire the fan in the bathroom to always go on when they have the lights on? Right now, the light and the fan are on separate “rocker” switches right next to each other in the same box. The fan is a separate fixture - it is not a fan with lights. The lights are recessed cans. Is there some simple way to wire them/fix the switches together? I have done simple DIY stuff with light switches and fixtures, and could replace the switches if that helps.

posted by Mid to Home & Garden (24 answers total)

Best answer: This is pretty easy. You'll need to combine the wires for the fan and the light onto one switch. Most switches have both press-in connections and screw terminals, so you can just add the fan wires to the connectors on the light switch the light isn't using. Then remove the inactive switch, and replace the double gang switch plate with a blank+switch plate.

It is, however, possible that there's more going on in the switch junction box than you suspect, and it may involve moving things out of the way, cramped quarters, or doing some extra connections. The only way to know is to open it up and look.

Never do any electrical work without turning off the breaker and making sure the power's off.

If this sounds too complex, described like that, just call an electrician to do it for you. It's really fast/easy work and they likely won't charge you a whole lot.
posted by seanmpuckett at 5:30 AM on April 28, 2023 [3 favorites]

Response by poster: Thanks, I was vaguely thinking about that - but is there any concern about double-loading a switch?
posted by Mid at 5:33 AM on April 28, 2023

Only if the fan and lights draw more than 15 amps combined. If they're on the same circuit, they aren't.
posted by Kyol at 5:37 AM on April 28, 2023 [2 favorites]

It'll depend on whether you want the fan to continue operating after the light is turned off. Fans with a timer (generally a good idea) will require slightly more complicated wiring. There's also the question of whether to install an isolation switch for the fan. I'd tend to go to an electrician.
posted by pipeski at 5:39 AM on April 28, 2023 [2 favorites]

Best answer: Another option worth considering is a motion sensor. Lutron sells a motion-sensor switch that's a drop-in replacement for a regular lights switch, and has a timeout you can set to 30 minutes, so it will just start automatically when someone walks in and turn itself off later. Drama-free home automation like this is kind of great.
posted by mhoye at 6:15 AM on April 28, 2023 [7 favorites]

Best answer: You should consider replacing the fan's rocker switch with a motion-sensing switch. That way the fan automatically goes on when someone is in the bathroom, and then will stay on for an adjustable amount of time. If you simply wire the fan to the light, you won't get the benefit of the fan continuing to run for a while after someone finishes their shower.
posted by Dip Flash at 6:16 AM on April 28, 2023 [2 favorites]

We have a humidity-detecting switch for our fan. It can be set to turn on automatically when the moisture hits a certain level.

(I'll note that the one we have turned out to be a bit too sensitive and we ended up just putting it on a timer, but our shower's only used by two adults so we don't have the issues you have. And, a different model may work better than the one we got.)
posted by synecdoche at 6:42 AM on April 28, 2023 [2 favorites]

The humidity detecting switch or a humidity detecting fan is the real way to solve this problem.
posted by rockindata at 7:11 AM on April 28, 2023

I was just in the Depot looking at fans yesterday. I'd suggest replacing the fan for a couple of reasons:

-- You should get a fan that is "sized" larger than the room - like, just get the most powerful fan.
-- As above, several models now come with humidity sensors.
-- One NuTone/Broad model came with a UV light whose putative job is to kill mold. UV lights do this in some contexts, but usually only in safety-protected ones (ie labs, or with sensors to not expose humans to the UV). The NuTone is a ~400 nm LED, which is just on the boundary of true ultraviolet and visible; and I doubt it's -really- powerful enough to kill, but may be worth a shot.
posted by Dashy at 7:14 AM on April 28, 2023 [1 favorite]

Best answer: This is a common problem. If the fan currently is controlled by one wall switch, replacing it with a humidity sensor switch seems like a good option.
posted by Glomar response at 7:23 AM on April 28, 2023

Try seanmpuckett's idea before you do anything else. It's going to be easy, is safe, and may do the trick in and of itself without spending much, if anything. After that, try the humidity sensor switch.

Replacing a fan can be easy, but there's a stack of "ifs" and "buts" to it. If it's a standard size and if it's a standard depth and if it was mounted to the guidelines, it will be easy to replace. (And to be fair, most are.) But maybe 10% of them aren't and that won't be fun at all.
posted by DirtyOldTown at 7:25 AM on April 28, 2023 [1 favorite]

Best answer: I'd replace the switch with a timer switch before I rewired anything. That's a 5 minute job, rewiring two switches is more like an hour, even for an electrician.
posted by The_Vegetables at 7:30 AM on April 28, 2023

If you simply wire the fan to the light, you won't get the benefit of the fan continuing to run for a while after someone finishes their shower

Or, alternatively, the fan will always be running—I don’t get the impression these are kids who will conscientiously turn the lights off when leaving the bathroom.
posted by staggernation at 7:49 AM on April 28, 2023

Response by poster: I love the idea of a timer switch or a humidity switch, but I am pretty sure there is no neutral in the box...
posted by Mid at 8:17 AM on April 28, 2023

Best answer: Check out the Lutron Maestro models, which don't require a neutral (but do require a Ground).

The benefit of an occupancy sensor / motion sensor over a humidity sensor is that it also works for bathroom smells. Lutron Maestro Instruction PDF
posted by soylent00FF00 at 8:43 AM on April 28, 2023 [2 favorites]

Oh, there's definitely a neutral in the box. All 120V circuits have a hot (supply) and neutral (return). Perhaps you meant that the ground wire is missing.

Although, I may be missing the context here, as I see no prior mention of "neutral" above. Anyway, the advanced switch solutions above should work with whatever wiring you have in there. It'll just take a little while to tease apart the tangle and figure out what to connect.
posted by intermod at 1:38 PM on April 28, 2023

I must correct the above comment. Sometimes, switch boxes don't have a neutral. Sometimes the neutral is on the other side of the load; or the hot is on the other side of the load (and you don't have a hot). All you have is a switch. This sucks but can absolutely be the case if the house was (re)wired by an idiot. That's why you always need to test whether your wires are hot even if you've shut the breaker you think supplies them is off. Some dumb-ass may have put a nail in something, or bridged a circuit, or, or, or. So yeah, per my other comments on electrical asks, before working on house wiring, get a contact-free AC electrical field detector. They're about 10 bucks and can save your life.
posted by seanmpuckett at 1:57 PM on April 28, 2023 [4 favorites]

Humidity switches will likely require neutral, but you can get timer switches that don't require them, both mechanical and electronic. (Older homes were not required by code to wire neutral to all switch boxes, so not all homes have them. There is a neutral path back to the breaker/fuse still, it just might not go back to the switch.)

Seconding to get an AC non-contact voltage detector (like this) if you want to do any electrical work. Don't trust the breaker by itself.
posted by Aleyn at 2:29 PM on April 28, 2023

Sometimes, switch boxes don't have a neutral.

If that's the case for your switch box, you'll find three wires in it: there will be an always-on supply hot that connects to both of the switches, plus a switched hot for the fan and a switched hot for the light, each connected to its own switch on the other end from the shared supply hot.

If that's how your box is wired, and you just jumper the two switched hots together, then turning on either switch will turn on both the light and the fan. If you're happy with that then you don't need to remove any switches or change the switch plate.
posted by flabdablet at 4:18 PM on April 28, 2023

That same three-wire scheme and the same jumpering modification would also work if seanmpuckett's hypothetical idiot has indeed wired the switches in on the neutral side of the loads instead of the hot side.

If there are more than three wires to the switch box and one of them is not clearly an unused neutral and/or ground and you don't see an existing connection that's common to two of the switches, do not blindly add a jumper on their other side. Because if that's what you have, then the hypothetical idiot might have put your lights and your fan on different breakers, or even put the light switch on the hot side of the lights and the fan switch on the neutral side of the fan, or vice versa, and you do not want to be messing with wiring as stupid as that.

Also adding my endorsement for acquiring a non-contact liveness tester.
posted by flabdablet at 4:31 PM on April 28, 2023

I don't know if it's possible for your bathroom, but in mine there are vanity lights in a little enclosure above the mirror. I put a bulb plug adapter like this in one, bulb goes on it, and now there's an outlet up there. I just have it attached to a mini fan that blows towards the vent.

May not work with recessed cans but it depends on the way it's set up. You could run a small to medium vent fan or get creative with a disassembled floor fan. Probably not the greatest advice but it is super cheap to test out and doesn't require any electrical work at all.
posted by BlackLeotardFront at 6:23 PM on April 28, 2023

Nothing to add about the switch - but find out from someone about products/cleaners/tools to deal with the mold and assign the teens to do the work while you watch and _don't_ help. I found, years ago when I had teens, that they paid more attention to such things when it was _their_ problem to solve
posted by TimHare at 7:14 PM on April 28, 2023 [2 favorites]

Minor point, but my household has stopped installing humidity-sensor fans because the sensors fail much faster than the fans and haven’t been easy (or possible) to replace. So at least check for that.

I like the motion sensor + lag time to turn off. The lights will be on extra, but I suspect decent LED lights use less power than dealing with even a moderate mold problem.
posted by clew at 8:44 AM on April 29, 2023 [1 favorite]

Response by poster: Just installed the Lutron timer switch. It turns out there is a neutral - I was mistaken about that. Seems like a good solution, thanks!
posted by Mid at 12:26 PM on April 30, 2023 [1 favorite]