How to Run Wire From a Unit to a Thermostat

Turn off the power to the heating or cooling unit at the switch or the circuit breaker before beginning any work to prevent shock or damage to the transformer. Use a meter to test the unit's power to ensure it is off because circuit breakers may have been mislabeled or safety switches connected improperly.

Attach the wires to the heating or cooling unit. Older conventional gas units can use the gas valve to connect the thermostat, and the thermostat wires would connect to the Th and TR screws. Similar setups can be found on fireplaces that do not have a control board for wiring. Older furnaces that will use cooling as well will require a device that allows wires from both to enter and control both units separately.

Drill a hole in the wall for the wires in the location you would like the thermostat.

Thread a steel fishing tape through the new hole in the wall toward the heating or cooling unit. Attach the wire running from the unit to the fishing tape and gently pull the fishing tape back through the hole in the wall.

Connect the wires to the thermostat. There are typically four wires for controlling the heating, cooling and the fan. Red is used for power entering and can be separate for heating and cooling, or shared if the terminal contains a jumper between the RH and the RC terminal. The yellow wire is for the cooling and connects to the Y terminal. The green wire is usually designated for the fan or blower. The white wire control the heating and connects the W terminal. While many manufacturers use this common wire coding, some systems are more complex or the manufacturer may change to colors. If you are not certain about the color-coding scheme, contact the manufacturer or a licensed HVAC professional for clarification.