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I have an electric heater, connected to a resistive load thermostat (240V). Yesterday we found the thermostat apparently totally dead (display off, etc). I've checked the breaker, of course. (There is no battery in this type of thermostat.)

The thermostat is wired like this:

enter image description here

I have a decent multimeter. How can I verify that this is indeed a problem with the thermostat and not possibly with something else (ex. power supply, wiring to heater, or the heater itself). If it's relevant, the heater is a simple convection electric heater with no controls.

Thank you!

UPDATE: Finally, it turns out that the (certified!) electrician completely botched the wiring the heater itself. When I opened up that junction, the wire just popped right out. I connected it properly and securely, and the problem appears to be resolved.

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  • Can you find a brand/model of the thermostat?
    – DoxyLover
    Oct 3, 2020 at 18:32
  • Yea, it's the Stelpro - ST302P. Here's the manual. orbit.org/tmp/Stelpro-ST302P.pdf It appears to be a glued-together box without any way of looking inside or doing anything.
    – logidelic
    Oct 3, 2020 at 18:36
  • Ok, I also cannot find any reference to it having a battery.
    – DoxyLover
    Oct 3, 2020 at 18:41

2 Answers 2

Reset to default
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I would test the system as follows:

  1. Turn of power at the breaker/fuse box
  2. Disconnect the two wirenuts going to the wires coming from the wall.
  3. Directly connect together those two wires, using a wirenut.
  4. Restore power at the breaker/fuse box.

If the heater comes on at full power, then the problem must be the thermostat. If the heater doesn't come on or at low power, then the problem is elsewhere. For example, if the heater element has failed open, then no power can flow and the thermostat would not get power.

After testing, be sure to disconnect power at the breaker/fuse box. Do not try to use the breaker as a switch as most are not rated to be operated many times.

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  • First of all thank you for the elegant answer to my question. I tried this and the heater did not come on. I guess the problem is elsewhere. I will test the power source coming from the breaker, but how do I test the heater? Do simple electric convection heaters really fail? This one is only a bit over a year old... Seems surprising...
    – logidelic
    Oct 3, 2020 at 19:40
  • @logidelic check for voltage between the two non-ground junctions in the box where the thermostat was installed
    – ThreePhaseEel
    Oct 3, 2020 at 19:43
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    I see 240V coming from the source. I also tested the circuit including the heater load, and it still says 240V! Wouldn't that mean the heater should have come on when I did the above experiment? I'm confused... Edit: I now understand. The electrician had botched the wiring to the heater. It was intermittent. Problem now solved. Thank you!
    – logidelic
    Oct 4, 2020 at 12:44
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Check to see that you indeed have power to the tstat where the tstat is located. If so, it's most likely a bad tstat, not much else to do other than replace it. Anybody else have an idea?

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  • Thanks for this, I will try. Naive question: What setting should the mutimeter be on? If I was just checking a normal line, I know how to measure the AC voltage, but since there is a load/resistence (i.e. the heater) between the terminals, I'm not sure how to measure...
    – logidelic
    Oct 3, 2020 at 15:26

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