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I have a 2-pole thermostat that I want to install to replace the old mercury thermostat. From the wall there is a green, white, and red wire. The thermostat has 2 black and two red. If you cannot see the labels in the picture below, the wires from the new thermostat are labeled as: L1 L2 T1 T2

I understand if I have to use this thermostat as a single pole (if I cannot use it as a 2-pole). I know the green is ground. It looks as though the red goes to the power, and the white goes to the heater. My question is, what wires do I attach to the wall wires?

  • Do you have both heat and cooling? This would change my answer slightly but in that case a 2 pole stat is not normally the correct type to use.
    – Ed Beal
    Commented Feb 15, 2021 at 21:25
  • Can you post photos of the inside of the box the thermostat mounts to please? Also, what make and model is this thermostat you're trying to use? Commented Feb 16, 2021 at 0:00

2 Answers 2


Your thermostat provides nowhere to attach ground, so you won't be attaching it to anything, unless the junction box is metal, in which case it must be attached to the metal box.

The white wire is being used as a hot, and needs to be re-marked by wrapping it with black tape.

NEC 110.3(B) says you must install the item according to its labeling and instructions. That means you must read them :)

You have the heater version of a "switch loop" and it can only support 1-pole thermostats. That's not necessarily failure. However, each 2-pole thermostat treats that situation differently. Some have instructions on how to wire it. (use one pole? wire both in series? IDK, check the instructions). Others say not to do it. Others can't do it, e.g. almost any thermostat with onboard smarts that needs power.


You have a line voltage thermostat it may work in place of a mercury but it may not if the contacts don’t have a low enough resistance. Put L1 and T1 to the wires that went to the Mercury thermostat (normally red white).

The reason this may not work is most gas valves are millivolt and used mercury thermostats very low current. If going to an electric furnace the mercury thermostat was just the most accurate back in the day but did not use line voltage where most base board and ceiling cable heat used the heavier line voltage thermostats.

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