Electrical – Baseboard heater thermostat doesn’t shut off the heat

electricalheater

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.

Best Answer

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.

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