Install 240 V Line Voltage Thermostat For a Baseboard Heater

  • 01 of 09


    Hand turning a thermostat knob
    John Lamb/Getty Images

    Electric baseboard heaters are good for "spot-heating" areas of your house that central HVAC does not reach.

    If you are going to put in a baseboard heater, make the thermostat set-up the best you can.

    On the Wall or the Baseboard?

    You can install a thermostat on the heater itself, but this means bending down every time you want to adjust the heat. Plus, a thermostat situated in the lower 6 inches of your room isn't accurately measuring temperature, since cold air sinks.

    Wall thermostats allow you to situate your heater's "brain" near the middle of the strata of heat layers, or about 48" high. This position is closer to where you are and reflects your comfort. The simplest thermostat is called a line-voltage thermostat.

    On-Off Switch: What Could Be Easier?

    If you have your walls open and drywall down, install a wall thermostat. Line voltage thermostats are extremely simple mechanical devices. All they do is connect and disconnect the power going to your heater.

    The only way they are "smart" is that they have a basic temperature-sensing device so that they turn on or off according to a temperature range you have set. They are cheap and easy to install, but you especially need to make sure you've got the wiring correct.

    Wire It Right!

    Even though these aren't overly sensitive chip-enabled programmable devices, you can still break them if you wire them wrong. Incorrectly wiring up a 120 V switch, outlet, or even GFCI won't do much harm to the device normally, but since this is a 240 V device that has so much juice flowing through it, you could easily fry the device--or worse, fry yourself--if you do it wrong.

    Is the Circuit Off? 

    Before you do anything, turn off the circuit and verify that no power is flowing. Since this is a 240 V circuit, you're turning off 2 circuit breakers, not just one. With your voltage tester, make utterly certain that this area is dead--no current at all.

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  • 02 of 09

    Wiring Diagram 240 V Line Voltage Thermostat

    Wiring diagram 240 V line voltage thermostat

    Let's start with the end result. Each number below corresponds to a number on the wiring diagram:

    1. Thermostat.
    2. Neutrals, wire-nutted to bypass thermostat and continue.
    3. Green (ground), wire-nutted to bypass thermostat and continue.
    4. One of the two hot wires, wire-nutted to bypass thermostat and continue.
    5. "Line" end of one of the two hot wires. Power is flowing into the device.
    6. "Load" end of one of the two hot wires. Power is flowing out of the device.
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  • 03 of 09

    Attach Wire Nut To Ground and White Wires

    Wire nut on ground wire

    There are two ways to approach ground:

    • No Ground For Thermostat: Devices such as a Cadet Mechanical Thermostat are plastic and thus do not require grounding. You will wire nut the ground wire coming in with the ground wire leaving the box, bypassing the device entirely.
    • Grounded Thermostat: Does your thermostat have a green ground screw on it, or similarly, a green or bare wire coming out of it? If so, this is the wire that connects your device to the ground--important for safety. Wirenut the two ground wires, adding a third ground wire. That third ground will attach to the device (or you'll use the wire already on the device).

    If you have a white neutral wire, connect it so that the "in" and "out" wires continue onward, bypassing the device. Alternatively, since you don't need the neutral for your 240 V device, you can cap it off entirely in the box.

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  • 04 of 09

    Wirenut One Hot Wire to Bypass Thermostat

    Cap red wires to bypass thermostat

    Two hot wires, each carrying power, enter your box. Typically, one is black, and the other is red.

    The important thing to remember is that only one hot wire enters the thermostat; the other is wire nutted to bypass the device and continue onward.

    If you try to connect both hot wires to the thermostat, the circuit breaker will shut off, or you may experience a little electrical "flash" explosion. 

    It doesn't matter which color wire you pick to continue onward. I just chose the red wire.

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  • 05 of 09

    Attach Line and Load Black Wire To Thermostat

    Attach line and load black wire to thermostat

    Attach the other hot wire (in this case, black) to your device.

    One end of your hot wire is a line, which means it carries power. The other end of the hot wire is load, meaning it flows to your baseboard.

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  • 06 of 09

    Push Green, White, and Red Wires Into Box

    Push green, white, and red wires into the box

    You're done with the wiring part of this project. Carefully shove the green, white, and red wires back into the box.

    The farther back, the better, as this will give you more room to insert the thermostat.

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  • 07 of 09

    Line Up Screws on Box

    Line up screws on box

    By turning the thermostat to the side, you can see where the screws line up with the electrical box.

    If you have to force the thermostat even to get the screws to touch the box, the wires aren't correctly folded in. Try pulling out the green, white, and red wires and folding them back into the box.

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  • 08 of 09

    Screw Thermostat Into Box

    Screw thermostat into box

    If you're having a hard time screwing the thermostat into the box, it could be either that the wires inside the box are preventing the device from moving inward or something much simpler.

    The simple solution, at least with metal boxes, is to jiggle the screw side-to-side to get it to seat properly. If the screw is seated properly, it should easily turn in.

    Another "trick" is to use a manual screwdriver, as this gives you better control.

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  • 09 of 09

    Replace Thermostat Faceplate

    Replace thermostat face plate

    Before you test your thermostat, replace the faceplate. Even though a properly installed thermostat minus faceplate shouldn't have any powered-up exposed sections, you can't be too careful with high voltages (an errant screwdriver can touch hot areas in the box).

    Flip on the circuit breaker and turn up the thermostat until it clicks. The baseboard heater should start heating up.