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My living room is heated by two electric baseboard heaters controlled by a single thermostat. Earlier this week, I noticed that the room was much warmer than it should be, and figured that the thermostat had gotten stuck again*. After turning the dial failed to produce the "click" of the switch turning off, and dialing the temperature all the way down didn't cool the room, I assumed the thermostat had failed and replaced it. However, the new thermostat shows what appears to be the same behavior: when the thermostat turns off, the heaters don't turn all the way off.

  1. Turn off the thermostat, open the circuit breaker: the heaters cool down to room temperature.
  2. Close the circuit breaker: the heaters stay at room temperature.
  3. Set the thermostat to 70F: heaters warm to around 200-250F.
  4. Room reaches 70F: thermostat turns off (I can hear the click), one of the heaters cools to around 110F while the other cools to around 160F.
  5. Turn the dial all the way down to "off": heaters stay the same temperature.

Both the old thermostat and the new one are simple bimetallic-strip switches. I tested the old one with a multimeter, and it switches between infinite resistance when off, and zero resistance when on.

Any idea what's going on, and how I could troubleshoot/fix it?

* The switch in the old thermostat occasionally got stuck, not turning off until much warmer than the setpoint.

  • If he had tried to switch 240V at 4A++ with a 24-volt thermostat, I don't think he'd be here discussing it :) These thermostats fail because they cost $15 and are switching 22 amps. – Harper - Reinstate Monica Feb 21 '16 at 15:45
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First, are you really sure they are still powered? Heaters have a fair amount of "thermal inertia" - they take awhile to warm up, and they also give off heat for some minutes after they're shut off. Obviously, the sure way to tell is to pop the service covers off and test with preferably non-contact testers, ideally a clamp-on ammeter. Although at some point you'll want to measure actual voltage.

One way to measure is your house's electric meter, as it has a spinning disc or similar digital readout that will show rate of power consumption. A stopwatch and a bit of research will tell you exactly what your whole house is drawing. To measure the heater, shut off all other large loads.

Here's a little secret about resistive heaters that I use intentionally sometimes. If you apply 120V to a 240V heater, you get 25% of the heat. So, if you're still getting (roughly 1/4) of the heat an hour after you "shut the thermostat off", there's a good chance a heater is seeing 120V instead of 240V. The very serious question is WHY? Because whatever's doing that could burn down your house.

  • I'm not certain they're still powered in the sense that I've checked with a multimeter, but thermal inertia shouldn't keep the heaters warm for 12+ hours after shutoff. – Mark Feb 21 '16 at 20:03
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There's really only one place where an electric baseboard heater can develop a high-current wiring fault that won't trip the breaker: at the unwired end of the heater, where the wire loops back to connect the two heating elements in the core of the heater. A fault here (say, a short to ground) will pass through a heating element, restricting the total current that can flow.

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40 years of cyclic thermal expansion wore through the insulation, resulting in a short circuit forming when the heater warmed up and brought the current-carrying wires in contact with the body of the heater. Since it's a 240-volt heater on a single-pole thermostat, one leg of the circuit is live even when the thermostat is off, providing 120 volts of heating.

  • I have the same problem with a family members heaters. Can a short that is described in one heater cause all 3 that are on a thermostat to stay on? – user67273 Mar 10 '17 at 1:15
  • Yes, it can. I had two heaters connected to the same thermostat, and a single short circuit was keeping both of them warm. – Mark Mar 10 '17 at 2:01
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As has been stated these old analogue thermostats function using a bimetallic strip, this is two dissimilar metals bonded together either straight or in the form of a coil. These thermostats generally use a mercury switch, mercury in a little glass tube with two electrodes at one end.... When the temperature is above the set temp the bimetallic strip or coil expands and moves the mercury switch to a physical position where the mercury cannot contact the terminals therefore no current flows. When the temperature is below the set temperature the mercury in the tube is moved so it a contact position by the bimetallic strip or coil contracting and power flows. Most of these heaters use a single contact/single throw so as noted in a 230vac circuit only one side is cut off but its cheaper for industry why double contact and single throw was not used. Finally the lowest position in most thermostats is somewhat higher than freezing for obvious reasons. Two solutions apply 1) Buy digital thermostats that actually cut power flow at minimum or have an off position 2) Turn off the relative breaker supplying the baseboard heater. A caution applies to always turn off the main breaker or particular breaker and then test for power, never assume power is off as this is a deadly mistake.

I have actually pulled someone off a 230vac wire who was leaning against his city water supply, water pipes are excellent grounds and he was just lucky I was there and acted quickly or he would be dead. Make no mistake with electricity it can easily kill or in the least do lots of neural damage. I might add a suggestion that everyone should take a CPR course in the least and First Responder Course would be even better training like this saves lives.......

  • As I mentioned in the question, I've rather thoroughly eliminated the possibility that the thermostat was at fault: the problem persists after replacing the thermostat, and the old thermostat, tested in isolation, works properly. – Mark Mar 9 '17 at 20:44
  • Where you have the thermostat what is the usual temperature there? – Plinker Mar 9 '17 at 21:18

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