Shut off the electrical power to your furnace by switching off the breaker at the home’s service panel (breaker box). Pull off the decorative ring or face from the old thermostat.
Unscrew the body of the thermostat from the base and carefully place the body in a safe location.
Disconnect each low-voltage wire from its screw terminal on the thermostat base, and label it with masking tape before moving to the next wire. On the tape label, write the letter or symbol next to the screw terminal the wire was attached to.
Tape the low-voltage cable to the wall to prevent it from slipping into the wall cavity. Remove the thermostat base from the wall.
Disassemble the new thermostat as needed. Thread the low-voltage cable through the new base, and mount the base to the wall.
Use hollow-wall anchors to secure the mounting screws if necessary.
Connect the low-voltage wires to the terminals on the new base, following the manufacturer’s wiring diagram. Install a backup battery into the thermostat body/cover, as applicable, then fit the body/cover over the base.
Program the thermostat as directed. Restore power to the furnace.
Things You Will Need
- Masking tape
- Mercury-type thermostats contain one or more glass vials filled with liquid mercury, a highly toxic material. Dispose of an old thermostat body at a local hazardous waste or mercury recycling site.