How to Replace a Single Pole Line Voltage Thermostat With a Double Pole

Turn off the circuit breaker that controls the baseboard heater. Verify the power is off by bringing the no-contact voltage tester close to the thermostat. If the light stays off, the power is off.

Remove the screws that hold the thermostat to the wiring box. Pull the thermostat off the wall. Note the wire connections. The single pole thermostat has two wires connected to it. The other two wires in the box are joined with a wiring nut.

Note which wire on the single-pole thermostat is marked "Line" or "L1" and which is marked "Load" or "T1." Place a piece of tape on the wire coming into the box that is attached to the "Line" wire. The wires come into the box in two cables.

Of the two wires that enter the box and are connected together with a wire nut, one comes into the box inside the cable with the wire that connects to the "Line" wire on the thermostat. Place a piece of tape on that wire also, as they are both "Line" wires.

Remove the wiring nuts from the wires and separate the wires. Pair one incoming "Line" wire, marked with tape, with the "Line 1" or "L1" wire on the double-pole thermostat. Put the two bare wire ends into a wire nut and turn the wire nut clockwise to tighten it.

Pair the second incoming "Line" wire with the "Line 2" or "L2" wire on the new thermostat. Put the wire ends into a wire nut and screw it on tight.

Connect each of the two remaining wires in the box to one of the remaining wires from the thermostat with wire nuts. The wires on the thermostat are labeled "Load" or "T1" or "T2."

Push the wires back into the box and place the thermostat against the box. Install the two supplied screws to hold the thermostat to the wiring box. Turn the circuit breaker back on and test the installation by setting the thermostat higher than the current room temperature. The baseboard heater will turn on.