How to Replace a Thermostat Safely (Step by Step)

Updated 2026-09-12

1. Turn off power to the furnace/air handler at its switch or breaker (and the AC condenser breaker). Confirm the old thermostat screen is dead.
2. Pull off the faceplate and take a clear photo of the terminals and wires. Our tool can read it for you.
3. Check for line voltage (thick wires, wire nuts, 120/240 V labels). If yes, stop and read this guide.
4. Label each wire by its terminal letter with the stickers in the new thermostat’s box. Ignore short jumper wires.
5. Disconnect wires and remove the old base. Wrap the cable around a pencil so it can’t fall into the wall.
6. Mount the new base (use the trim plate to cover old holes) and connect wires as shown in your wiring result. Tug each gently.
7. Cap unused wires individually. Never let bare ends touch.
8. Restore power and follow the app setup: equipment type, stages, O/B setting, aux heat.
9. Test each mode: heat, cool, and fan on. For heat pumps, check that O/B isn’t reversed.

If a breaker trips, a fuse blows, or equipment runs when it shouldn’t, turn power off and call an HVAC technician.