How to Replace a Thermostat and Wire It Correctly

Why You Might Need to Replace the Thermostat

According to our research, there are three common reasons to replace a thermostat:

1. HVAC system won't start or stop: Broken thermostat wiring or terminals can prevent the HVAC components from receiving the signal to start. A miscalibrated thermostat may misread the temperature and fail to start the system when it should or start it when it shouldn't.
2. Hot and cold swings: A faulty thermostat can cause heating and cooling cycles to run too long or too short, leading to temperature fluctuations.
3. Upgrading to a programmable or WiFi thermostat: You may want the convenience and potential energy savings of a programmable thermostat or a WiFi thermostat like the Nest, Ecobee, or Honeywell Lyric.

Tools & Specs to Keep in Mind

The tool list for replacing a thermostat is short:

• Narrow flat screwdriver
• Phillips screwdriver
• Vise-grip pliers
• Drill or driver
• 3/16" or 1/4" drill bit
• 12" to 24" level
• Cellphone with camera or masking tape and a marker

The new control must match your system's capabilities. If you don't know these, pull the cover off to check the thermostat wiring on the old unit. Yellow wires are typically for AC, and white wires are for the furnace. The number of wires indicates whether the system is single-stage, two-stage, or variable-capacity.

Replacing a Thermostat in 14 Steps

1. Turn off the power
2. Remove the thermostat cover
3. Open the new thermostat package
4. Take a picture and label the wires
5. Remove the wiring from the old unit
6. Remove the baseplate from the wall
7. Attach vise grip pliers to the wires
8. Mark the new baseplate holes
9. Install the new baseplate
10. Attach the wires to the new thermostat
11. Install batteries in the thermostat
12. Mount the thermostat on the baseplate
13. Program the new thermostat
14. Turn on the power, and test your HVAC system

Caution Notes

While there is an industry standard regarding the color of the wires used for each function, it is surprising how often the standard isn't followed. This is why labeling and taking a picture of the thermostat wiring on the old unit are essential. When installing the new unit, connect the wires to the same terminal on the new unit they were connected to on the old thermostat, regardless of color.

If your HVAC system doesn't work properly once the new thermostat is installed, make sure each wire is secure in its terminal. If problems persist, turn off the system and get assistance from a local HVAC professional.

Save Money on Thermostat Wiring

Replacing a thermostat is DIY-friendly. According to our research, hiring an HVAC technician for the work will cost $75 or more. By following this guide and taking your time, you can save that money and keep it in your pocket.