So my programmable thermostat exploded

Installed a Lux brand programmable (not “smart”) thermostat last year. Worked fine until I got home today and attempted to adjust the temperature. It refused to turn on the heat, so I checked the batteries and tried to reset it a couple of times. Then I checked the relevant circuit breaker and found it had tripped. Upon resetting it I heard a whump. Went back to the living room and found the unit charred black.

No further fire, so I made sure the breaker was off again and pulled it out of the wall. The wires don’t seem to have been damaged, but what should I do before replacing it with a new thermostat of some kind?

Also, what are some other decent programmable - but not “smart” - thermostats I could purchase?

For my smart thermostats I usually* install a simple ‘dumb’ one, mechanical if I have one, as a backup.

  • Usually because it’s all but one zone, however the house should not freeze if that one goes out as the other zones should give enough heat.

So while their are plenty of options, consider a backup.

Hold on a second…
What breaker tripped?

The one routing power to the thermostat / baseboard heat for that room. I reset it and poof.

OK, I still need more information.
Generally, thermostats are run off of 24v, from a transformer in the HVAC. The only ones that are controlling 120V that I know of are old radiant heating systems. Is this what you have? What is the make and model of the thermostat.

This is electric baseboard heating, not an HVAC system. I have a condo, and one of the regular household breakers controls the electricity going to the thermostat in the living room which in turn controls three baseboards (all in the same room). Two other rooms have their own thermostats and baseboards and were not affected.

The thermostat was a Lux ELV4. Looking it up, I found several bad reviews including photos of scorched walls very much like mine. I’ve sent a message to the company, for all the good that will do.

It would be interesting to take it apart and see what failed.
Generally, a breaker won’t trip instantly unless there is a dead short. I would guess that either a wire came loose, or a screw fell across the supply terminals.

I’m really curious too. A low voltage supply being driven by a step down transformer, I’d think, wouldn’t be able to trip a breaker on the high side.

The only way I could see that (if someone didn’t mention similar reviews) would be if there was a problem with the transformer.

This sounds exactly like something bigclivedotcom would be interested in. You might shoot him and email. Maybe he’ll even foot the bill to get it shipped to Wales.

But remember—the OP said this thermostat controlled a baseboard heater. Aren’t those often controlled by line-voltage thermostats? A short at line voltage would account for everything the OP described, wouldn’t it?

I hadn’t encountered line-voltage thermostats until I moved from WI to the Pacific Northwest a few years ago. I love many things about my new home, but mine-voltage thermostats are not among them.

I have a Lux thermostat controlling a pair of 240v baseboard heaters. It is not programmable but is certainly a line voltage unit. I can see incorrect voltage frying this unit, but given the proper voltage is 240v it seems unlikely.

I’ve never had any issues with Honeywell’s products, mechanical, programmable or smart. They aren’t the fanciest but they do the job.

I just wired up my old thermostat, the kind with a dial that would look at home in the 1950s. Reset the breaker, turned it on and it works. So I have heat again, but this is a temporary fix. Still not sure what caused the problem.

If it’s running on mains voltage, then there’s not really anything to odd about it. By that I mean, I don’t know why it happened to this specific device, but anything running on 120 or 240, right from the breaker has the ability to blow the breaker and/or heat up enough to scorch walls or start on fire.
A simple short, some arcing or a welded contact and all kinds of odd things can happen.

Just as an example if a light switch stopped working and you pulled it apart to find the guts have melted or scorched, it’s certainly not a good thing, but it’s not out of the ordinary for it to happen.

OP, it wouldn’t hurt to check the connections at the heater as well. A bad/loose connection there could cause it to pull more amps at the t-stat. The extra amps can cause plastic components to melt which results in things touching that shouldn’t be touching.

If the stat is working on line voltage and the new stat is a 24 Volt stat that may be your problem

My programmable thermostat caught on fire a few years back after I turned it up because I was just about to take a shower. Fortunately, I hadn’t started to shower when it caught on fire.

First (and only) time I ever had to use an fire extinguisher.