Thermostat hot wire became unconnected

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  #1  
Old 01-30-20, 04:01 PM
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Thermostat hot wire became unconnected

Hello everyone. I recently replaced my thermostat for my forced air heaters which has a 2 pole hookup (i.e. two hot wires, no neutral). It was working fine for a week but then stopped working. I checked the voltage at the two hot wires and one appears to be unconnected. I know this because there is 120 volts between one hot and the grounding wire and 0 volts between the second hot and grounding wire as well as 0 volts between the two hot wires. When I initially connected the thermostat the two hot wires were at 240 V between them. I checked the 2 pole circuit breaker and all outgoing wires are at 120 volts so the breaker is still good. So the only thing I can think of is a fault in the wire between the breaker and the hookup panel for the thermostat. Is there anything else you are aware of I can check has failed?

See attached picture for a shot of the wire in question.

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Thanks in advance!
 
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Old 01-30-20, 04:14 PM
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Welcome to the forums.

Yes....we see your wiring but it doesn't tell us much.
Are these 240v electric heaters ?
Do you have three cables in that box ?
It looks like three white wires connected in a splice.
That would mean one black wire is the hot/line and the other two go to heaters.

You should be able to connect all three black wires and the heaters should work.
 
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Old 01-30-20, 04:24 PM
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Hi and thanks for the reply. Two of the black wires are indeed the heater load wires and the other black one is the hot wire.

My issue is not the hookup as the thermostat was working correctly for a week. The issue is there is no voltage on the white hot wires. When I initially hooked it up the white wires were at 120V relative to ground and 240 relative to the hot black wire but now there is no voltage at all between the white wires and any other wire.
 
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Old 01-30-20, 04:31 PM
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If you don't have 240v between the white and black..... I'd start checking at the breaker.
If that wiring is spliced from the panel to that location.... there could be a problem there.
I have never seen wire go bad by itself.
 
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Old 01-30-20, 04:41 PM
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Right, I thought of that. In my original post I said I checked the 2 pole circuit breaker and all outgoing wires are at 120 volts so the breaker is still good. That's why I was suspicious of the wire, but as you say that seems very unlikely.

The really weird thing is that when I switch back to my old thermostat, which only used the hot, load, and grounding wires (the white ones were tied off) it doesn't work. I suppose the forced air heater could have gone bad but there are two of them connected in this circuit so it's unlikely they both went bad at once.
 
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Old 01-30-20, 04:52 PM
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When you are checking a 240v circuit..... you must see 240v. A connected load can cause 120v to be measured on each leg.

At the breaker..... you need to measure 240v on the white and black.
At the thermostat you need to measure 240v on the white and black.
 
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Old 01-30-20, 05:03 PM
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OK. When I go home I'll measure the outputs of the breaker relative to each other, the black and white wire. They should be 240V relative to each other at that point. Before I had only measured relative to ground. I'll let you know what I see. Thanks.
 
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Old 01-31-20, 12:43 PM
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OK. The outputs of the 2 pole breaker were at 240V relative to each other so that's fine.

The part I really don't understand is the old thermostat doesn't work now either even though it only used the black hot wire, the load wire, and the grounding wire. It didn't use the white hot wire at all.
 
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Old 01-31-20, 01:03 PM
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All circuits need two wires. With 120v it's a neutral and a hot. With 240v there are two hots.
In your application...... the white wire is one half of the circuit power. The black wire is the other half.
Your thermostat is only switching one half of the 240v supply.

Did you check from the white splice to the hot wire at the thermostat ?
 
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Old 01-31-20, 03:21 PM
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Yes. At the thermostat the white wire has no voltage at all even though it does at the breaker.
 
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Old 01-31-20, 04:40 PM
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Then there is something between that point and the panel. My bet would be on a junction box.
 
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Old 02-03-20, 04:23 PM
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OK. Would that junction box be at another outlet, like say for another thermostat hookup point, or would it be near the breaker box? Probably can't say without more info, I guess.
 
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Old 02-03-20, 05:34 PM
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If you panel is in an unfinished area..... the splice could be near there.
If there are/were other thermostat locations.... the wiring may pass thru there.
 
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Old 02-04-20, 03:41 PM
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OK. Your tip about the junction box helped me figure it out. Since there was no additional junction box I realized the only place it could be is at the fan hookup. I checked and sure enough the hookup at the fan wires nuts was poorly done and had come unconnected (though the wires were still in the wire nut).

Thanks for all the help!
 
 

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