March 3, 2022

How to replace a heating and air conditioning system thermostat

By Lyle Weischwill
How to replace a heating and air conditioning system thermostat

This step-by-step repair guide explains how to replace the control thermostat for a furnace or central air conditioner. Located on the wall of your home near the HVAC unit, the control thermostat displays the settings and sends signals to the electronic control board to activate components when you push the buttons. If the display won't light up or if the thermostat won't respond when you select a button, replace the thermostat with the manufacturer-approved HVAC part.

The steps in this repair guide will help you replace the control thermostat for Kenmore, Carrier, Lennox and Trane HVAC systems.

Quick links
Tools required
Screwdriver
Work gloves
Repair difficulty
Time required
30 minutes or less
Repair difficulty
Time required
30 minutes or less

Instructions

  1. 01.

    Disconnect electrical power

    Shut off the house circuit breaker for the HVAC unit to disconnect electrical power.

    PHOTO: Shut off the house circuit breaker.

    PHOTO: Shut off the house circuit breaker.

  2. 02.

    Remove the existing control thermostat

    Pry off or remove the existing thermostat's cover plate.

    Remove the screws from the thermostat body and pull the thermostat out from the wall to see where the wires are connected.

    Take a digital photo or note which terminals the wires are connected to on the existing thermostat.

    Disconnect the thermostat wires.

  3. 03.

    Install the new thermostat base and connect the wires

    Follow directions in the installation guide for installing the new thermostat to the wall. You will typically attach the new thermostat to the wall using mounting screws provided with the new control thermostat.

    Connect the wires to the new thermostat according to the directions provided in the thermostat's installation guide.

    You will typically connect these wires to the new thermostat terminals:

    • Red--24 volt power

    • Yellow--Signal to start cooling

    • White--Signal to start heating

    • Green--Signal to start the circulation fan

    • Blue--Common ground

    Attach the new thermostat base to the wall and install the control unit to the base.

  4. 04.

    Restore electrical power

    Reset the house circuit breaker to restore electrical power.

Warning: Undertaking repairs to appliances can be hazardous. Use the proper tools and safety equipment noted in the guide and follow all instructions. Do not proceed until you are confident that you understand all of the steps and are capable of completing the repair. Some repairs should only be performed by a qualified technician.

Symptoms common to all heating & cooling combined units

Choose a symptom to see related heating and air conditioning system repairs.

Dirty air filter, clogged condenser coils, low refrigerant charge, faulty compressor
Thermostat problem, clogged air filter, burner failure
Dirty flame sensor, failed flame sensor, damaged burner, bad high limit switch
Thermostat problems, bad compressor/condenser fan capacitor, lack of refrigerant, failed compressor
Main causes: errant thermostat settings, lack of electrical power, clogged drain line, wiring failure, control failure.
Main causes: Errant thermostat settings, lack of power, bad interlock switches, lack of exhaust ventilation, wiring failures.

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