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I am trying to replace my old 240v line-voltage thermostat with a modern version. The problem is that after wiring the new version in like the old it doesn't work. I have (2) 12-2 wires coming out of the box and they are both hot. Any advice?

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Can you include a picture or a diagram of what is in the box? In the US, in order to get 220V there will be two hot wires. But you should also have other wires going out to the heater (I assume that is what the thermostat is controlling). – auujay Jan 19 '12 at 19:40
How did you determine both wires are hot? – Steven Jan 19 '12 at 20:27
can you access the wiring connecting to the heater? – jberger Jan 19 '12 at 21:29

Simple "line voltage" T stat wiring. One wire is L1 (120V potential) from source, one wire is L2(120v potential)source connected to heating strips, then the switch, then L1 source. When the switch closes the 240V circuit is completed. If you have 240V when checking both wires together, and only two wires on switch, there is something wrong with the new fancy switch. If you have three wires coming from the switch then you need a neutral that you probably don't have.

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Both lines could "look" hot if you are measuring to ground. But one could be coming from the low-resistance heater, and your high-impedance voltmeter can't even tell it's there.

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What's coming out of the box is two 12/2 wires which, like you mentioned, doesn't make sense for a 220v heater. I used a volt-meter and checked each line individually. Also I used a voltage detector and it alerted on both. – Rujaba Jan 29 '12 at 19:18
Yes I can access the wiring connection to the heater. – Rujaba Jan 29 '12 at 19:22

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