How to Wire a Line-Voltage Thermostat for a Baseboard Heater

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Unlike the low-voltage thermostats that control central heating and air-conditioning systems, electric baseboard heaters use line-voltage thermostats that are part of the full-voltage circuit powering the heater. The thermostat can be mounted directly on the heater itself, or it can be mounted on the wall so that the circuit wiring passes through the thermostat first on the way to the baseboard heater. A new line-voltage thermostat will need to be connected as part of the installation of the new baseboard heater. They also can wear out and require replacement. Wherever it is located, connecting the line-voltage thermostat for an electric baseboard heater is a fairly easy task.

How Line-Voltage Thermostats Work

A line-voltage thermostat for a baseboard heater is basically just a variable switch that is installed along the electrical circuit that runs from the circuit breaker panel to the baseboard heater. It operates much the same way as a dimmer switch providing variable control to a light fixture. Basic line-voltage thermostats use simple dial controls, but there are also more sophisticated electronic programmable models. Although programmable thermostats offer more control options, the wiring is done exactly the same way as for basic dial thermostats.

A line-voltage thermostat can control a single baseboard heater or several heaters wired together. The thermostat works by sensing the temperature of the room and controlling the amount of current that passes through the wires to the heater. Most baseboard heating systems use 240-volt circuits, but 120-volt heaters are also available; they are often used to provide supplemental heat in individual rooms.

Single-Pole vs. Double-Pole Thermostats

Line-voltage thermostats for 240-volt heaters come in two types: single-pole and double-pole. Single-pole thermostats are sometimes known as "two-wire," while double-pole thermostats are called "four-wire." Some thermostats can be wired in either fashion.

In single-pole thermostats, only one of the two hot wires entering the thermostat box is attached to the thermostat; the other wire bypasses the thermostat box and continues on to the baseboard heater. This means that the baseboard will always have current flowing into it.

With double-pole thermostats, both hot wires entering the thermostat box are attached to the thermostat. Double-pole thermostats offer a true "OFF" function since the current is completely interrupted to the baseboard heater when the thermostat is off.

Make sure to choose the type of thermostat specified by the baseboard heater. Most will specify either a single-pole or double-pole thermostat, though some may allow either type.

Preliminaries

This project discusses only the electrical connection for the thermostat itself; it assumes that the electrical cable for the line-voltage thermostat has already been routed from the circuit breaker panel to the thermostat location on the wall, and from the thermostat location to the baseboard heater. For wall-mounted installation, we're also assuming that an electrical box has been installed in the proper location in the wall.

These preliminaries will be already be handled if you are replacing an existing thermostat, but a new heater installation will involve running new cable, installing a new electrical box, and mounting a baseboard heater on the wall. Running new circuits for electric baseboard heaters is normally done by a professional electrician since it requires specialized skills and involves working at the main circuit breaker panel, where there is a distinct danger of shock. Such work should be done by DIYers only if they have substantial knowledge and experience. However, connecting a line-voltage thermostat is a fairly simple job that you can easily do.

Tools and Supplies You Will Need

  • Line-voltage thermostat to match the voltage of the baseboard heater
  • Wire connectors (wire nuts)
  • Wire strippers
  • Screwdriver

Line-voltage thermostats come in many forms, and it is critical to choose one that matches the specifications of the baseboard heater.

Instructions: 240-Volt Double-Pole Thermostat

Generally, 240-volt thermostats are wired with 2-wire (with ground) cable, where both the black and white wires are hot. The thermostat has a total of four wire leads: two marked LINE that connect to the circuit feed wires, and two marked LOAD that connect to the circuit wires leading onward to the heater. The leads are often red and black in color—a color coding that indicates that all wires carry hot current.

Make Sure the Power Is Off

The cable delivering power to the thermostat box must be shut off before you connect the thermostat. Make sure the circuit breaker controlling the wires is shut off, and use a non-contact circuit tester to test for current at the thermostat box.

Read the Thermostat Schematic

Unpackage the thermostat and carefully read the instructions and the wiring schematic. In particular, make sure you identify the wire leads marked LINE, which will be attached to the circuit wires that feed current from the power source, and those marked LOAD, which will carry power onward to the baseboard heater.

Strip Wires

If necessary, strip about 3/4 inch of insulation from each insulated circuit wire entering the thermostat's electrical box, using wire strippers. This usually involves two supply wires from the power source and two wires passing onward to the heater. Or, if the thermostat is controlling two baseboard heaters from the same location, there will be additional outgoing wires leading to the other heater.

Make Ground Wire Connections

The bare copper wires from each cable entering the box need to be joined together with a wire nut, and the metal box also needs to be grounded with a "pigtail" wire that links to the circuit grounding wires. Typically this is done with a short green or bare copper wire attached to the metal box with a green grounding screw, with the other end of the pigtail joined to the circuit grounding wires with a wire nut. The thermostat itself usually does not have any grounding connection. If it does have a green grounding lead, join this to the other circuit grounding wires.

Make LINE Connections

Now, use wire nuts to connect the two feed wires entering the box from the power source to the wire leads on the thermostat marked LINE. In most 240-volt wiring situations, the circuit wires will be black and white; it doesn't make any difference which wire lead gets attached to which feed wire since both are hot.

Make LOAD Connections

Use wire nuts to connect the outdoing circuit wires leading to the baseboard heater(s) to the wire leads marked LOAD on the thermostat. If the thermostat is controlling a single heater, each lead will be attached to a single wire. If the thermostat is controlling two or more thermostats from the same location, then each lead will be attached to multiple outgoing wires.

Complete the Installation

Tuck the wires into the electrical box and secure the thermostat with the mounting screws. Complete the wire connections on the baseboard heater, if this has not already been done. Turn on the circuit breaker and test the heater and thermostat for proper operation.

Instructions: 240-Volt Single-Pole Thermostat

Wiring for a 240-volt single-pole thermostat is similar to that for a double-pole, but in this case one of the hot wires will simply bypass the thermostat and pass onward to the baseboard heater.

Make Sure Power Is Off

The cable delivering power to the thermostat box must be shut off before you connect the thermostat. Make sure the circuit breaker controlling the wires is shut off, and use a non-contact circuit tester to test for current at the thermostat box.

Read the Thermostat Schematic

Unpackage the thermostat and carefully read the instructions and the wiring schematic. In particular, make sure you identify the wire lead marked LINE, which will be attached to a circuit wire that feed current from the power source, and the one marked LOAD, which will carry power onward to the baseboard heater.

Make the Ground Connections

The bare copper wires from each cable entering the box need to be joined together with a wire nut, and the metal box also needs to be grounded with a "pigtail" wire that links to the circuit grounding wires. Typically this is done with a short green or bare copper wire attached to the metal box with a green grounding screw, with the other end of the pigtail joined to the circuit grounding wires with a wire nut. The thermostat itself usually does not have any grounding connection. If it does have a green grounding lead, join this to the other circuit grounding wires.

Join the LINE Connection

Connect the black circuit feed wire to the wire lead on the thermostat marked LINE, using a wire nut.

Join the LOAD Connection

Connect the wire lead on the thermostat marked LOAD to the black outdoing wire(s) leading to the heater(s), using a wire nut.

Complete the Bypass Connection

Use a wire nut to connect the white feed wire to the white outgoing wire(s). This is also a hot wire, but in this case, it bypasses the thermostat and runs directly to the heater(s).

Complete the Installation

Tuck the wires into the electrical box and secure the thermostat with the mounting screws. Complete the wire connections on the baseboard heater, if this has not already been done. Turn on the circuit breaker and test the heater and thermostat for proper operation.

Tips

  • Remember that in a 240-volt circuit, both the black and white wires are hot; there is no separate neutral wire, as there is with a 120-volt circuit. Whether the thermostat is being wired as a single-pole or double-pole, both the black and white wires are hot in any 240-volt circuit.
  • If you are wiring a 120-volt baseboard heater, however, the white wire in the circuit will be a true neutral wire. This white wire will not be connected to the thermostat and will connect to the white neutral wire passing on to the baseboard heater.