How to Replace a Thermostat and Wire It Correctly
Time required: 30-45 mins · Difficulty 2/5
Most thermostats run on safe low-voltage (24V) wiring, but always shut off power at the breaker or the furnace's service switch before disconnecting wires. If your system uses line-voltage (120V/240V) baseboard heat instead of standard low-voltage wiring, stop and call a licensed electrician — that setup carries a real shock hazard.
Est. DIY Parts: $25 - $40 · Pro Service: $150 - $250 · Est. Savings: $110 - $225
* Estimates based on regional/national contractor rate benchmarks. Actual part prices vary by brand and retailer.
Parts & Tools You'll Need
- Programmable / Smart Thermostat
- Wire Label Stickers
- Small Flathead/Phillips Screwdriver Set
- Level
Step-by-Step Manual
- Turn off power to the HVAC system Switch off the breaker for your furnace/AC, or flip the service switch near the unit, before touching any wires.
- Remove the old thermostat's cover and label wires Pop off the faceplate and take a photo of the wiring, then place a label sticker on each wire matching its terminal letter (R, C, W, Y, G, etc.) before disconnecting anything.
- Disconnect wires and remove the old baseplate Loosen each terminal screw, gently pull the wires free, and unscrew the old wall plate. Push the wire bundle slightly into the wall so it doesn't fall back inside.
- Mount the new baseplate level Feed the wire bundle through the new baseplate, use a level to confirm it's straight, and screw it into the wall (using drywall anchors if not hitting a stud).
- Connect each wire to its matching terminal Match each labeled wire to the identical terminal letter on the new thermostat and tighten the terminal screws firmly.
- Snap on the faceplate and restore power Attach the new thermostat's faceplate, restore power at the breaker, and run through a heat and cool cycle to confirm everything responds correctly.