Thermostats are usually trouble-free pieces of equipment, but they can fail just like any other electrical device. Fortunately, replacement is an easy task as long as you pay careful attention to what you are doing. Below is a step-by-step guide to replacing your air-conditioning or heating system's thermostat:

1. Disconnect all electrical power - While thermostats rarely use connections with more than 24 volts of electricity, it is always safest to turn off the power supply. In addition, you don't want to risk the system abruptly turning on and off during the replacement, as this can cause needless wear and tear. Turning off the power is an easy matter, as you can do this at your home's breaker box. Simply look for the appropriate breaker switch that connects to the air-conditioning and heating system and flip it into the "off" position.

2. Remove the thermostat - Most thermostats use a base plate that connects to the wall and wiring and is covered by a separate control unit. Removing the old thermostat is usually straightforward; simply grab the control unit and slide it upward to free it from the base plate. Once the control unit has been removed, you will be able to view the wiring and connections to the base plate.

3. Understand the connections - Thermostats will contain a variable number of wired connections depending upon what type of system you have installed. You will notice the connections are labeled and that color-coded wiring is also standard. Understanding the purpose behind each wire can be helpful for future troubleshooting. Below is more information about the specific connections and wire colors and what they mean.

  • B (Blue) - This terminal connects the thermostat to the system's common power supply.

  • G (Green) - This terminal is connected to the system's blower, and the wire carries signals from the thermostat to turn it on and off.

  • RC (Red) - This connection links the thermostat to the system's cooling power supply.

  • RH (Red) - This is a connection between the thermostat and heating power supply.

  • W/O - This particular connection is with the control for the system's cooling function and integrates the blower and condenser into a single operation.

  • Y (Yellow) - This connection links to the control of the heating function and also integrates the blower into its operation.

4. Take a photo of the connections - Before you remove any of the wires from their respective terminals, be sure to take a photograph of the connections to clearly show which wires are connected to each. Though wires ideally match in color and letter designations, there are times when they do not correspond, and taking a quick snapshot will ensure that you match everything correctly while installing the new thermostat.

5. Remove the old base plate - Once you have taken a photograph, you can safely remove the base plate from the wall. Disconnect wires from the appropriate terminals and then remove the base plate by taking out the screws holding it in place. Be careful that you do not accidentally push the wires back into the wall, or else you will be forced to retrieve them. To prevent this from happening, tape loose wire ends to the wall to hold them in place.

6. Install the new base plate - Untape the wires from the wall and slip them through the opening in the new base plate. Next, line up the new base plate on the wall and attach it using screws and anchors, if the base plate is to be attached directly to the drywall.

7. Reconnect the wires - Once the new base plate has been attached and is firmly in position, unscrew the wire terminals and connect the wires using the photo you took in step four as a guide. You may need to strip away a small length of insulation from the ends of the wires, if the wire ends are bent or close to breaking. Verify the connections are properly made and fit snugly before moving to the next step.

8. Attach the control unit - The last step before restoring power to the system and testing it is to attach the new control unit. In most cases, the control unit will slide down on top of the base plate to make its connections, but be sure to consult the installation instructions for details.

9. Restore power and test the system - Turn the main power supply to the system on at the breaker panel, and then turn on the thermostat. Set the temperature to an appropriate level to verify that the system is operating correctly.