I have a baseboard heater in an older cabin and i want to replace the thermostat. Its a two wire thermostat and inside the box there are two insulated wires (one black and one white) along with a bare ground. Testing the voltages across these wires I have 220 volts from white to black, which I expected. But when I check black to ground I get 220 volts, while white to ground has zero volts. This surprised me because I thought the two power legs should each have 110 to ground.

There are two heaters connected to the circuit, each with its own thermostat. Both have the same condition of 220 volts to ground on one leg and zero volts on the other. They both worked well until last winter then this one wouldn't turn off, even with the thermostat right down as far as it would go. That's why I looked into replacing the thermostat.

Any thoughts or advice would be most welcome.


I just went underneath into the crawl space and unhooked the wires coming up to the thermostat. I guess I didn't explain it very well before because that was a switch loop from the heater that was causing the problem. I haven't turned the power back on but just checking the resistance it seems to have fixed the problem because now the two conductors and the bare wire at all boxes are isolated, where before the white and the ground were connected. Haven't checked the run to the heater yet.

I can't get back to it for a day or two because I'm going to change it to a 4 wire thermostat and have to wait to get it. I'll make sure to check as I go along in case its a burned wire or? somewhere in the system.

Thanks very much for all of your thoughts. I knew something wasn't right. and also that it was dangerous.

I'll post back once I have the new thermostat and have it hooked up.


  • Do the thermostats switch both white and black, or just the black? – ThreePhaseEel Aug 12 '18 at 22:13
  • Also, where on this planet are you? – ThreePhaseEel Aug 12 '18 at 22:16
  • the thermostat bridges across the white to black, I'm in BC canada – user89827 Aug 12 '18 at 22:43
  • A two wire line voltage thermostat for a typical 240V heater is a switch that opens one leg of the power. The white isn't neutral or the opposite leg. Black is constant hot, white is switched hot (on the same leg). – batsplatsterson Aug 12 '18 at 23:30
  • @batsplatsterson is that specifically how it’s done in BC Canada? That’s not how it’s s supposed to be done where I am.... – Tyson Aug 12 '18 at 23:36

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