Replacing existing single pole bathroom fan switch

[Wen72]

I finally had a chance today to test the voltage. Here is the result:

white/bare: 0 V
white/black: 125 V
black/bare: 125 V

I also tested the red wire:

red/bare: 125 V
white/red: 125 V

So from my test, both black wire and red wire carry 125 V when the breaker is on. And for the switch, red wire carries electricity. When the switch is on, it passes to white wire which makes the fan on.

Thanks,
Wen

 

[wwhitney]

So the voltages on the various colors are as typical for the color, and your fan really is controlled by a switched neutral. Bizarre. And since the humidistat also makes the fan run, that portion of the circuit must provide it's own switched neutral.

[With the switch turned off (or the red wire disconnected) you are reading 125V (relative to bare/white) because it is connected through the fan to the black wire; no current is flowing, so there is no voltage drop across the fan. When the switch is turned on, the red wire will read 0V (relative to bare/white) because the 125V will drop across the fan, making the fan run. You can verify this if you feel like it, but it has to be true, based on the voltage measurements so far.]

If you want to gain access to the fan junction box, you could rewire that junction box, along with the fan switch and the humidistat, to do the more normal thing of switching the hots. Then you could install an electronic timer as per normal. Otherwise, you have to work around the fact the neutral is switched in the fan switch box. You can use a mechanical timer or a relay, as discussed earlier.

Another option that should work, but feels quite wrong, is to get an electronic timer with Bare/White/Black/Red leads (I'm capitalizing to distinguish the timer leads from the wires in the switch box) and hook it up Bare to bare, White to black, Black to white, Red to red. That should still power the timer electronics properly, and should make the Red wire into a switched neutral instead of a switched hot. I don't see any way in which this is any more dangerous that your existing manual switch, but I'm a little hesitant to recommend it due to not being 100% sure I know what I don't know.

P.S. Also still unexplained is how wiring an electronic timer as usual would cause the fan to run at low speed all the time.

Cheers, Wayne

 

[Sponsor]

 

[wwhitney]

PPS Another conceivable option would be to use an in wall electronic timer that effectively has a relay built in, if they exist. I.e. it would have 4 wires (plus ground), say Black, White, and (2) Reds. Then you could power it as normal (Black to black, White to white) and have it act like your manual switch: Red to red and Red to White (for the (2) Reds). However, I don't know if such a product exists.

Cheers, Wayne

 

[Wen72]

If I understand you correctly, the evidence we have so far kind of suggesting that somehow the black and white wires were switched. How? We will never know unless I get to the fan junction box then we can be 100% sure.

I am thinking to go buy a new electronic timer today and try to wire it Bare to bare, White to black, Black to white, Red to red. Let's see if it will work. Will keep you posted.

P.S. Also still unexplained is how wiring an electronic timer as usual would cause the fan to run at low speed all the time.

It was more than two years ago when I bought the electronic timer. I tried to wire it, didn't work and returned. One thing is that I might not have the correct memory of what happened exactly at that time.. I could be wrong to say that the low fan was running all the time. Maybe it was that when I turned on the switch, the low fan got turned on. And when the switch was off, everything was off. But one thing for sure, I wired it bare to bare, black to black, white to white and red to red as I wouldn't have guess any other way to wire it. I just remembered all I got was the low fan running.

Thanks,
Wen

 

[wwhitney]

If I understand you correctly, the evidence we have so far kind of suggesting that somehow the black and white wires were switched.

No, that's not what I'm saying. That would have been black-bare = 0V ; white-bare = 125V ; white-black = 125V.

There is one other thing that you could try. Namely, we infer that the 3-conductor cable leaving the fan switch box (white, black, red) goes to the fan junction box. Call those wires W1, B1, and R1. And that circuit just has one other cable entering the fan switch box, call its 2 wires W2, B2.

You could temporarily configure the connections as W1, R1, and W2 under one wire nut, B1 capped and B2 capped. Then turn the breaker on. See if the fan is off and everything else on the circuit works correctly (furnace?), in particular that the dehumidistat still turns the fan on at low speed.

If so, then you can move the switch from being on the white wires to being on the black wires as normal. However, since it wasn't done that way, I assume there was a reason, and this test will fail--something in the circuit won't work as intended. You really need to test everything to be sure there isn't an unexpected problem caused by this.

In the lucky event that it does work, then you could wire an electronic timer with leads W, R, B along with a white jumper (short white wire) WJ as: W1, R1, W, WJ under one wire nut; B2 and B under another ; B1 and R under another. That would be totally normal except for the R1 wire.

Cheers, Wayne