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Thread: how to change a thermostat

  1. #1

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    how to change a thermostat

    If you are replacing your thermostat because there is an issue with your air conditioning or heating system there is a good chance that the thermostat is not the issue. Most air conditioning and heating issues are not related to the thermostat.

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    Call a HVAC technician to install your thermostat if you do not have experience with air conditioning and heating systems.

    Read the instructions that came with your new thermostat before installing the thermostat.

    Turn off the power supply to the thermostat. Typically there is a transformer in the air handling unit that powers the thermostat with 24 volts A/C power. The air handing unit is usually in the attic or a closet. The power to the thermostat should be off if you turn the breaker off to the air handling unit, furnace and the outside condensing unit. On some older systems there may be two separate power sources providing power to the thermostat. If there are separate wires on terminals RC and RH terminals on your thermostat your thermostat has two separate power sources.

    Remove the cover from the thermostat or remove the thermostat from the sub base so that the thermostat terminals are visible. Use an electrical meter to test for power on the R terminal. Place meters leads on the R and common terminals and check for 24vac. If there is no power on the R terminal proceed to the next step. If you do not have an electrical meter you could purchase an inexpensive multi meter from radio shack or at a home improvement store.

    Most new thermostats come with wire labels. Install the wire labels before you remove the wires from the old thermostat. If you do not have wire labels make note of were the wires are attached on the old thermostat. Check our thermostat wiring diagram thread that will show how a typical thermostat is wired. Depending on who originally installed your thermostat they could have use any wiring coloring.

    Remove the wires from the old thermostat.

    Remove the thermostat from the wall. Secure the wires so that they cannot fall back into the wall.

    Seal the area around the thermostat wires where they come through the wall. This will prevent any drafts in the wall from affecting the thermostat.

    Install the new thermostat sub base, if there is one. You may need to drill new holes for the new thermostat.

    Match each labeled wire with the same letter on the new thermostat terminal, and connect them accordingly. Connect these wires as directed by the instructions that come with your new thermostat.

    Install batteries, if needed.

    Mount the new thermostat to the sub base.

    Restore power and program the new thermostat as directed.

  2. #2
    HVAC Thermostat Specialist

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    Other helpful tips:

    Take photos of how old thermostat is wired before removing it. If there is plenty of extra wire cut the thermostat cable next to the old backplate, leaving the original thermostat connections intact. Either of these will give you a "wiring diagram" if something doesn't work right with the new thermostat.

    Be sure to secure thermostat wire so it doesn't fall back into the wall !!!

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