How to Install a Line Voltage Thermostat for a Baseboard Heater


Hand turning a thermostat knob
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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 by separating it from the heater body. This provides more accurate temperature gauging, plus it makes it easier for users to access the thermostat.

Why Put the Thermostat on the Wall?

You can install a thermostat on the baseboard heater itself. Many heaters come with a kit that allows you to install the thermostat at one end of the heater body. While this type of installation is easiest since it does not involve opening up walls, it is considered to be inferior to installing the thermostat on the wall.

For one, thermostats situated in the lower 6 inches of a room are not accurately measuring temperature since cold air sinks. Wall thermostats allow you to situate your heater's sensor higher up, near the middle of the strata of heat layers, or about 48 inches high. This position is closer to where you are and it reflects your comfort.

Baseboard heater-mounted thermostats also entail bending down every time you want to change the temperature. Wall-mounted thermostats are easy to access.

If you have your walls open and no drywall has yet been installed, it is highly recommended that you install a wall line voltage thermostat. 

What Is a Line Voltage Thermostat?

Line voltage thermostats are simple mechanical devices that connect and disconnect the power going to your heater. These thermostats act much like a dam in a stream. When the dam is open, water flows. When the dam is closed, water stops. Line voltage thermostats mechanically interfere with the flow of electricity from the power lines to the baseboard heater. When there is no connection in the device, the heater cannot turn on or off. When the connection is restored, the heater can activate.

Line voltage thermostats have a basic temperature-sensing device so that they turn on or off according to a temperature range you have set. They are inexpensive and easy to install, but you especially need to make sure you've got the wiring correct.

Safety Considerations

Before you do anything, turn off the circuit and verify that no power is flowing with a voltage tester. Since this is a 240 V circuit, you will be turning off a double-pole circuit breaker, not a standard single-pole breaker. With your voltage tester, make certain that the circuit wiring is dead, with no current at all.

Even though these are not 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 will not do much harm to the device normally, but since this is a 240 V device that has so much juice flowing through it, you can easily damage the device or injure or kill yourself.

Project Metrics

  • Working Time: 60 minutes
  • Total Time: 75 minutes
  • Skill Level: Expert
  • Materials Cost: $20 to $40

Tools and Supplies You Will Need

  • Line voltage thermostat
  • Wire nuts
  • Wire stripper
  • Wire ripper
  • Manual screwdriver
  • Voltage tester

When to Call a Professional

Always feel free to call in a licensed electrician when installing a line voltage thermostat. Do this at any point during the installation process, whether for the entire project or just for part of it.

Attach a Wire Nut to Ground and White Wires

Wire nut on ground wire
Lee Wallender

There are two ways to approach the attachment of the ground wire to the thermostat:

No Ground for the 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, does it have a green or bare wire coming out of it? If so, this is the wire that connects your device to the ground, and this is crucial for safety. Wire-nut the two ground wires, adding a third ground wire. That third ground will attach to the device (or you will use the wire already on the device). If the thermostat's electrical box is metal, it also must be grounded, with a pigtail wire or with a grounded conduit path. 

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

Wire-Nut One Hot Wire to Bypass the Thermostat

Cap red wires to bypass thermostat
Lee Wallender

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

You can choose either wire to continue onward.

Attach the Line and Load Black Wire To Thermostat

Attach line and load black wire to thermostat
Lee Wallender

Attach the other hot wire to your device. One end of your hot wire is a line wire, which means that it comes off of the power lines and carries power. The other end of the hot wire is a load wire, meaning that it flows to your baseboard heater.

Push the Green, White, and Red Wires Into the Box

Push green, white, and red wires into the box
Lee Wallender

The wiring connection part of the project is finished. Carefully shove the green, white, and red wires back into the box. The farther back you can shove the wires, the better. This will give you more room to fit the body of the thermostat.

Line up the Screws on the Box

Line up screws on box
Lee Wallender

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 are not correctly folded in. Try pulling out the green, white, and red wires and folding them back into the box.

Screw the Thermostat Into the Box

Screw thermostat into box
Lee Wallender

If you are 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 tip: Use a manual screwdriver, as this gives you better control.

Replace the Thermostat Faceplate

Replace thermostat face plate
Lee Wallender

Before you test your thermostat, install the faceplate. Even though a properly installed thermostat minus a faceplate should not have any powered-up exposed sections, you need to be extremely careful with high voltages. Even 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.