How often do home thermostats fail?
March 27, 2010 12:53 PM   Subscribe

My friend's thermostat failed. He came home and his house was 120 degrees. How does this happen? How common is something like this? Could this have been prevented?

I have never heard of anything like this happening, so now am wondering how this can happen and how it can be prevented. Obviously, mechanical devices fail, but I'm surprised there wasn't a mechanism in place to shut the heater down or something.
posted by hazyspring to Home & Garden (10 answers total)
Thermostats are generally pretty simple. If it's hotter than you set it, the switch goes off. If it's colder, the switch goes on. Somehow his got stuck on.
posted by octothorpe at 1:13 PM on March 27, 2010

Happened to me once about thirty years ago. The walls were actually hot when we got home, but I don't think it was 120 degrees. Back then we had floor furnaces in an older home. We replaced the thermostat, and never had another problem.
posted by raisingsand at 1:15 PM on March 27, 2010

Happened to me too, in a room with an electric heater in the floor. Thermostat failed and the room became incredibly hot. The problem was apparently caused by the dodgy wiring job the previous owner had done--the thermostat was connected directly to a 30-amp 240-volt circuit and got comprehensively fried. We didn't take it apart to find out why it had stuck on though.
posted by Logophiliac at 1:42 PM on March 27, 2010

I used to live in an apartment with a dying furnace, that had to be repaired several times in the two years I was there. One day, after the furnace had been fixed, I woke up in a sweat with the furnace going full blast. Turned out that the repairman had disassembled my thermostat and put it back together wrong.

My thermostat was incredible simple, because it allowed no user adjustment. If you opened it up you could see it contained a small mercury vial thermometer with a metal band on it. A detachable metal clip held the vial in place by the band. The thermometer was marked 68°. I guess the landlord could choose a different target temperature by swapping in a different thermometer vial.

When the temperature reached 68°F the mercury would rise to the level of the band, thus completing an electric circuit than ran through the clip and back into the thermostat. The instructions inside the cover helpfully explained that just removing the thermometer would cause the furnace to get stuck open, while replacing it with a piece of metal to complete the circuit would force the furnace off. I guess they were trying to discourage tenants from tampering with it?

Unfortunately the system was not completely foolproof. The heating repair technician managed to put the thermometer back in upside-down, so the clip was just holding onto the glass, and could never complete the circuit. If your friend's thermostat worked similarly then any failure of that electrical circuit would cause the furnace to get stuck open. The thermometer could leak or clip holding it could slip, or some internal wiring could fail for whatever reason. The design just elegantly simple, but not robust at all.
posted by serathen at 2:00 PM on March 27, 2010 [1 favorite]

"I have never heard of anything like this happening, so now am wondering how this can happen and how it can be prevented. Obviously, mechanical devices fail, but I'm surprised there wasn't a mechanism in place to shut the heater down or something. "

Furnaces are basically self limiting, they can only heat the air so much compared to the flows out of the building. This kind of failure mode could be prevented by putting two thermostats in series but that increases the possibility of twice as many failures in the other direction, something that is infinitely worse than the experienced failure. So control circuits are made as simple as possible.

This kind of failure is pretty common with cheaper programmable thermostats.
posted by Mitheral at 2:11 PM on March 27, 2010 [1 favorite]

Happened to me last week -- modern digital thermostat pulled out a bit from the wall and something shorted out. House got to 85 degrees before I fixed it.
posted by miyabo at 4:18 PM on March 27, 2010

Yeah, this happened to me a couple of years ago. I've got a programmable thermostat, and I guess the battery died while it was heating. I came home and the house was really, really hot. I replaced the battery, cycled it on/off once or twice, and it turned off the furnace.

It comes down to poor engineering. The "loss of power" failure mode doesn't cut off the signal to the furnace, which is plain old dumb. Now I replace the batteries once per year, which is definitely overkill but better than taking the risk. That's the only easy way to prevent it from happening again.

Now that I think about it, you could maybe add an old-style mechanical bimetal strip-type thermostat inline with the existing one, with a setting a few degrees above what you would usually set the programmable one. That way, you've got an upper limit on the temp.
posted by Simon Barclay at 5:10 PM on March 27, 2010

Simon Barclay writes "The 'loss of power' failure mode doesn't cut off the signal to the furnace, which is plain old dumb."

This is the correct failure mode. Imagine you go away for the weekend in the dead of winter and the stat fails. Do you want your furnace to stay on or off for 72 hours when it's -30 outside?
posted by Mitheral at 6:53 PM on March 27, 2010 [2 favorites]

This happened to someone in a building I rented a condo in; new owner of another unit had the thermostat stick when he wasn't home, and the persons in the unit above got so hot they thought there was a fire and called the fire department. It happens.
posted by davejay at 9:05 PM on March 27, 2010

I just moved into a new place and installed a new programmable thermostat with my dad. Except that we didn't check the HE - HG switch, so it was set to Heat for Gas and not for Electric. There's a fair chance we had the wrong switch for Heat Pump vs. Standard as well.

We only tested to ensure that the thermostat would turn the heater ON. Not that it would trigger it to shut off.

I'm in Arizona, so I go most nights without heat, but three weeks ago, it was getting pretty damn chilly in the morning, so I set the thermostat to kick in at 70 degrees. I woke up sweating about 6 hours later and wandered out into the hall to notice the air blowing and the temperature at 87 degrees, despite the set temperature still sitting at 70. Whoops.

Put another way, the thermostat IS the device to shut off the heater. :-)
posted by disillusioned at 12:51 AM on March 28, 2010

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