How to Attach Wiring of Your Heater Thermostat to Your Furnace

  • 3-5 hours
  • Advanced
  • 100-300
What You'll Need
Drywall anchors
Masking tape

Learning how to wire your heater thermostat to your furnace will help you save on professional electrician charges. If you are careful and follow the instructions, you can save a great deal of money by wiring yourself.

Step 1 – Electric Off

Always turn off the electrical power to the home thermostat at the power panel or circuit breaker box. Once you’ve located the correct circuit breaker and have switched it off, place a piece of masking tape over the switch so no one will turn it back on during your replacement installation process.

Step 2 – Disconnect Wiring

Remove the electric power supply wiring from the back of the thermostat. Be careful not to allow the wires to fall back into the wall. Wrap these around a pencil to keep them from being able to go through the entry/exit hole. If the wires are not color coded to the new thermostat, mark each one noting which terminal to which they were attached. Remove the mounting plate.

Step 3 -Wires to the Furnace

There will be several wires that lead to different locations, especially if the thermostat operates a dual heating/cooling control. The wires for connection to the furnace are coded as follows:

  • W

This is the main wire that connects to the furnace whether the heating source is a gas, oil or electric furnace. The “W” stands for white. To replace the white power wire from the heater thermostat to the furnace, wrap the new one intertwining it with the end of the old one that you have disconnected from the screw terminal on the thermostat, Secure the two together. At the other end of the new white wire, wrap it around a pencil to prevent the wire from falling into the wall. At the furnace end, disconnect the old white wire and carefully pull it through the wall until the new wire emerges attached to the old. Remove the old wire and attach the new to the furnace terminal for the white wire.

  • W2

This is wiring for a two-stage set-up which is necessary for gas furnaces that have low fire and high fire ignitions. This power wire is necessary to complete a two-stage firing of the furnace. Many heat pumps also employ a staging process requiring a W2 terminal as well. This wire may be brown-colored but could vary based on the choice from the original installer. Repeat the replacement process for the W wiring to complete connection for any W2 wiring if necessary.

Step 4 – Attach Thermostat Ends

Once you have connected the new wire or wires to the furnace terminals, return to the thermostat and remove the wire from the pencil. Attach the W wire(s) to the proper terminal on the thermostat.

Step 5 – Connect Wires

When connecting the wires, make sure the other thermostat wires connected to the air conditioner and fan blower are also properly attached. The green wire is connected to the terminal marked “G” or the same color. This operates the furnace fan blower. The “Y” is for the yellow wire that connects to the air conditioner.

Step 6 – Place on Mounting Plate

Replace the mounting plate by sliding on the wall stud choosing a different hole location. Do not use the old holes because they may be loose now and will present a hazard. If your thermostat was located on a wall without a stud, make sure to use drywall anchors to secure it properly.

Things to Consider

Check to make sure the house wiring to the thermostat is smaller than those connected to the house electrical outlets. If the wires to the thermostat are large like outlet wires, you will need to have these replaced from the power source to the thermostat location. This will need to be accomplished by a licensed electrician.