Fan timer switch only works when adjacent light switch is off

Question

I have a bathroom ceiling fan with a light. The fan is controlled by a mechanical dial timer switch and the light is controlled by an on-off switch in the same wall box. The mechanical dial has been sticking, so I decided to replace it with this digital timer switch with minute buttons:

https://www.amazon.com/gp/product/B007PZAZXU/

Inside the wall box, the in/out wires are both black, but the old timer switch was marked Line and Load, so I used those to determine which wire was which and connected Line to black and Load to red on the new timer switch. I also connected the ground wire in the box to both the green and white on the new timer switch, according to its instructions.

Now the new timer works as expected when the light switch is off. But when I turn the light on, the timer switch seems to power down -- its LEDs slowly dim and turn off, and its buttons don't do anything. When I turn the light back off, the timer switch goes through a restart sequence of blinking LEDs, then it works normally again.

In case the original timer was wired backwards, I reversed the wires connected to the new timer's red and black, but then it didn't turn on at all. I also tried reversing the wires connected to the light switch and that reversed the function of the timer, so it only worked when the light was on instead of only working when the light was off.

I don't understand why the light switch would affect the fan timer since they are independent functions on the fan unit and were working independently with the old timer. Maybe there is some kind of interference with the wiring inside the box? There is a lot of extra wire in there and it's packed in pretty tightly, but even with both switches pulled out and dangling by their wires, the same behavior occurs.

Inside the wall box, packed into the back, is also a lot of white wire with various twist cap connections between them. I hoped this was other room wiring that is being joined here inside the box and doesn't affect the fan and light; I thought I could just swap out one timer switch for another and not mess with the rest. But maybe this is more complicated than I thought.

Does anyone have an idea of what to do, before I give up and call an electrician?

Here's the wiring diagram from the instructions; I used option 2:

Since some of the comments have questioned the instructions regarding the neutral wire, I searched the product reviews and found this: "It uses the ground as neutral (!) to power the electronics inside the switch. That's pretty bizarre even if the current involved is minuscule." Given that, it does make sense that you would connect a neutral wire to the timer if available, or connect the ground wire to both the neutral and ground on the timer if not. Since the timer does function correctly with the light off, that part seems correct, but what happens when the light is turned on that apparently removes power from the timer?

Update and solution

As suggested by manassehkatz, I pulled everything out to see what was happening. At that point I remembered that the light switch not only controls the fan light but also a light above the sink. The switch box also seems to be acting as a junction box for an outgoing romex wire, probably leading to a half bathroom directly above this room. So that explains all the wires packed into there: one pair coming in from the basement, and four pairs going out to the fan (via the timer switch), the fan light (via the light switch), the sink light (also via the light switch) and the upstairs room (connected directly to the incoming wires).

Anyway, this also made it clear that the neutrals were indeed in there, all bundled together as manassehkatz described. I connected the timer switch to that bundle instead of the ground and then everything worked as expected.

So the problem is solved, but I'm still curious why this change made a difference, since the timer switch is supposed to work when the neutral wire isn't available in the box, and turning on the light shouldn't affect it. 🤷

I did take some pictures, but I couldn't untangle the black wires enough to make them easy to trace without running a finger along them.

Answer 1

Not a full answer, but a bit more than comments, and hopefully the start of a full answer:

Ground vs. Neutral

In general, ground and neutral are to be kept separate in all switches, receptacles, appliances, etc. However, there is an exception for smart switches, timers, etc. provided:

• If the device can use neutral, and neutral is available, it uses neutral.

In other words: Lazy connection of ground to save 1 minute of messing with the neutral bundle doesn't count.

• The device is designed to use very little current.

Unfortunately, the data sheet I found confirmed the neutral/ground issue but did not state the current used in operation. Probably because it was so little as to be insignificant from an overall power consumption standpoint. My guess is something like 10mA - meaning if you used it an hour a day you'd use less than 1 kWh per year. I expect the standby (waiting to be turned on) usage would be far less than that. This is important for two safety-related reasons: if installed on a GFCI-protected circuit, which is likely the case in a bathroom (even though not generally required for fans and lights) then too much current use over ground instead of neutral will trip the GFCI; and you want to keep the actual current flowing over ground wires to a minimum for general safety reasons.

• The device must be designed and tested to meet UL (or similar) requirements.

It is trivial to design a device to put way too much current on ground. It is much harder to design a device that makes functional use of ground while keeping current use very low.

This device meets the specs. But you didn't follow directions, because you have neutral available. Which brings us to the second item:

Neutral in Switch Boxes

New switch installations are, with certain limited exceptions, required to have neutral in each switch box. That makes it easy to install smart switches, add receptacles next to switches and do lots of other things. However, in older houses (under previous versions of the NEC), a switch loop was allowed with hot and switched hot but without neutral. (I have some in my 1950s house.) A switch loop is a neat trick - it lets you use two wires to connect a switch instead of needing three wires. That means in a typical house you could get away with using 2-wire cable for almost every 120V connection. Hot/neutral for receptacles. Hot/switched hot for switches. The only exception is 3-way switches, which need hot or switched hot or neutral plus two travelers. Using 2-wire cable almost everywhere saves builders money. Until the recent proliferation of smart switches, timers (some older timers were purely mechanical, some electromechanical timers actually needed neutral), motion sensors, humidity sensors, etc. there was really no downside. In fact, if neutral is not needed and simply capped for future use then it makes no difference at all to have it (except for future use).

It is therefore quite common to open up a switch box to install a fancy new switch and find no neutral. It is even possible to find no ground, but that is far less common and has more solutions than no neutral. So an exception was made to legally allow certain types of switches to make use of ground when no neutral is available.

However, in many cases, particularly in bathrooms where a circuit may come in to power lights (sometimes multiple sets), fan, receptacles (either in the same box or elsewhere in the bathroom), etc. there will be a bundle of neutrals, and no problematic switch loops. Basically hot and neutral come in and split up to the various devices. However, if some of those devices are not in the same box then instead of chaining from one device to the next, all the neutrals get put together in one big wire nut (or even two if there are enough neutrals). Each switch (old "dumb" type) only needs hot (line, incoming) and switched hot (load, outgoing) but doesn't need neutral.

For example, with incoming power, two switches, outgoing power to a receptacle in another box will result in 4 white neutrals all connected together. The solution to the neutral problem in this case is actually really simple: Open up the 4-neutral wire nut, add a fifth white neutral wire and connect that wire to the new switch. Note that in some cases, either due to capacity or just to make sure it is in good condition, replacing the wire nut is advisable.

That still leaves the original problem: Why is the timer dependent on another switch?

My hunch is that there is something out of place. To start with, there was an "extra" ground wire. There is no such thing. Which means it was either never connected properly or came loose over time. Which means there could be other wires that were never connected properly. Which really raises questions about the status of all wires in the box except the nice bundle of white neutrals.

Fixing the core problem will require figuring out every wire in the box. More specifically:

• Unless there is an actual true switch loop, all whites should be neutral and all connected together. So that part is easy to verify.
• All grounds should be connected together and/or to the box if the box is metal.
• Identify the incoming hot (almost definitely black). Trace where it goes and, wire by wire, identify each hot, switched hot and traveler (if there are any 3-way switches).

We can help with this if you provide clear pictures showing the wires as they are currently connected. (Do not disconnect wires from anything yet - that makes the job harder.) Pull all switches/receptacles/etc. out of the box as far as you can safely do so, and take pictures showing where each wire goes into each device.

Answer 2

From a comment on the question:

I don't know how to identify it among the tangle of white wires packed into the back

That "tangle of white wires" is the neutral entering the box from "upstream" and heading off "downstream" somewhere.

If you trace your "Line" black wire, you'll see that it enters a cable^* that also has a white wire in it that goes into that tangle. That sucker is your neutral and your switch neutral needs to attach to those. You can simply add the neutral from the switch to that bundle of white wires.

You do, however, need to be aware of wire nut sizing and make sure that you're not exceeding the capacity of the existing nut. If you are, you can pull some of the wires from the bundle, put in pig-tail (a 6" piece of white insulated wire) into the existing bundle, then jump to a new wire nut where the pig-tail is connected to the white from the switch.

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^*It's possible that you have individual wires run in conduit. If that's the case, you'll need to determine if all the neutral wires in the box are connected. If so, you're good to go. If not, take a picture and ask a new question on how to identify which neutral bundle goes with each hot wire.