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I am trying to install the referenced thermostat for my baseboard heat in one room. I did follow along this post

2 wires, both hot, for 240v baseboard thermostat

After color coding my wires that post does not say what to do with the hot black wire.

I’m creating a new post because if the different thermostat.

This is the box. The white and black are on the left are hot. enter image description here

This s the back of the new thermostat enter image description here

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    How were the wires connected when you started? Did you take a "before picture? were they connected to nothing previously?
    – Ecnerwal
    Commented Oct 8, 2023 at 18:31
  • What brand/model is the new thermostat? What do the instructions say to do?
    – FreeMan
    Commented Oct 8, 2023 at 22:57

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Either this location controls two heaters, or this location controls one heater and sends unswitched power to another box.

Since you appear to be stuck with a cheesy one-contact thermostat (color me an old-fashioned type that likes both hots shut off for a 240V device not powered on) one side of the line is unswitched.

So, apply red or black marking to the 3 whites, since they are Not Neutrals, and join them together. That's one side of the line, at 120V to ground, unswitched, because the thermostat is lame. That's an opinion. Code allows it, but I consider a Wicked Dumb Idea.

The black line hot (the other hot, that's not white, also 120V to ground and 240V to whites) goes to one thermostat contact; or, to one thermostat contact and one other black.

The other thermostat contact goes to either both of the other blacks, or one of the other blacks.

If two heaters are controlled here, line to thermostat, both other blacks to other side of thermostat. If one heater is controlled here, line to one black and thermostat, other side of thermostat to one black. If you have a multimeter you can generally find a heater by looking for 6-25 ohms between one black and the whites (all with the breaker shut off, of course.) Or you can wire it up and see if the heater or heaters are controlled by the thermostat, or running all the time, and correct the wiring after shutting the breaker off again.

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