My heater won’t shut off. Neither lowering the thermostat to a low temperature nor turning the switch to “off” shuts the damn thing off. I have to go to the breaker panel and cut off the breaker to keep my house from getting too hot.
What’s going on, and how much will it cost to fix?
TIA
We’re talking about an electric baseboard heater here? Do you hear a click when the thermostat is turned to the ‘off’ position?
I’ve replace blower motors and control modules on my furnace but I don’t feel comfortable walking you through it. It was trial and error for me to tweak the settings. For the professionals who may respond it would help if you specified if you are talking about a forced heating unit and if so what type of fuel (oil, natural gas, propane).
Just as an FYI, the cost of the parts were low enough that it made sense to replace them. If it requires a professional it would still be cheaper than replacing the furnace.
I’m afraid I can’t help with the particulars, but I will pop in to say: Don’t delay fixing this! When I was in 4th grade, we lost almost everything to a house fire - or a house smoke, more accurately. There was very little fire damage, but lots and lots of smoke damage. It was because the switch that shuts off the furnace when it’s too hot (not the wall thermostat, but some switch inside the actual furnace) didn’t flip, and so it just kept on going until a fire broke out. Thank goodness that was one of the less than 20 days ever that my mother was substituting at my school. Instead of walking home that day, I waited for a ride from her, so I wasn’t home alone when that happened.
No matter how expensive it is (and IIRC, it wasn’t an expensive part), it’s cheaper than replacing everything you own and being out of your house for two months while they clean and replace your walls and floors. 
It’s a forced-air electric furnace.
Yes, I hear the click.
I suspect your t-stat isn’t functioning. If that is the cause, replacing it yourself, which should be pretty easy, might cost $30-50 USD, depending on features, perhaps more.
That should be easy to test… all the thermostat does to turn on the furnace is connect two wires with a relay. If you disconnect those wires and the furnace stops, it’s the thermostat. If it keeps going, it’s something else.
Oh, and uh… if you happen to mount a new thermostat and are drilling holes in the wall, make sure you tape the wires out of the way, 'cause if they touch the drill casing then it could, I’m pretty sure, short out with some sparks and then you have to go find a replacement fuse at 1am before your house freezes. just saying
Funny, that doesn’t sound like a hypothesis.
It’s all there in the schematics. BTW, did you know that Lennox furnaces use the same fuses as your car? I think that’s fantastic.
Well, then the Tstat box really should have had a warning label not to attempt the project after midnight, unless, ya know, you have spare car fuses.
Then there will be a circuit breaker in the main electrical box for this furnace. Turning that breaker off will shut off the furnace until you can get it fixed.
I once had the exact opposite problem. The furnace wouldn’t turn on. Turned out to be a weak battery in the thermostat. There was enough power left to operate the thermostat itself but not the controls on the furnace. Check the battery first. It may save you a lot of grief. Good luck.
BTW - My Trane furnace also uses automobile fuses.
It’s hard to give a less simplistic answer without having some more info, and I hope your problem has been resolved by now.
Batteries are often used with programmable t-stats in the event that the (often) 24vac control power goes from line loss. Also why you see auto-style fuses.
How many terminated wires attach to the t-stat? Is it a heat pump? They qualify as “forced-air electric” furnaces. Separate fan?
I wasn't concerned about your safety- if there is no, or malfunctioning, high-limit control, your house breaker(s) would have over amped and tripped.