How-to guide

How to Replace a Thermostat Safely (Step by Step)

By Ismail Farooq · Updated 2026-10-29

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.

Before you buy the new thermostat

Take the faceplate off the old thermostat first and note which terminals have wires. That tells you whether you have a C wire, a heat pump, two stages or a dual transformer, and so which thermostats will work and whether you need an adapter. The wiring tool does this check for five popular models. If you find thick wires and wire nuts, you have a line-voltage system and need a different kind of thermostat.

What you’ll need

• A small screwdriver (some thermostats include one).
• A drill and bit if the new base doesn’t line up with the old holes; wall anchors are usually in the box.
• The wire labels from the new thermostat’s box, or masking tape and a pen.
• Optional: a multimeter to check for 24 V between R and C, and a trim plate or paint to cover the old outline.

Old mercury thermostats

If the old thermostat has a small glass bulb with a silver bead inside, it contains mercury. Don’t put it in the trash: mercury thermostats are treated as hazardous waste in many places. Many heating supply stores and local hazardous waste sites take them; in the US, the Thermostat Recycling Corporation runs a collection program.

After it’s installed

Go through the equipment settings carefully (heat type, stages, O/B, fan control) and check each mode works. Our checklist of settings to check after installing covers the ones that most often cause trouble, and the troubleshooting guide helps if something doesn’t work.