How to Remove a Thermostat. Sometimes a thermostat breaks or you are ready to upgrade to a new one with more energy-saving features. Before you can install a new thermostat, you must remove the old one. This job requires only a few minutes (and possibly a screwdriver) to complete.
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Cut the furnace's power source before beginning. Check for a power switch near your furnace or turn off the electrical power supply at the circuit breaker.
Turn off the gas supply if you have a gas furnace. Look around the furnace for a black pipe. This pipe is the gas line. Check for a valve located near the pipe. Turn off the gas supply by moving the valve so it's perpendicular to the gas line.
Take off the thermostat's old cover by unscrewing the screws that attach the cover to the wall. Some covers don't have screws and simply pull off.
Unscrew the mounting plate and notice the wires.
Place a small piece of masking tape on each wire. Use a felt-tip marker to mark the masking tape according to the wire's screw terminal location. Usually, the wires are marked with letters.
Disconnect the low-voltage wires from the base plate by loosening the screws and then pulling the wires away.
Locate the mounting screws for the base plate. Unscrew them and take off the base plate. Guide the base plate over the wires carefully.