Troubleshooting a Home Furnace and Air Conditioning Thermostat

  • 01 of 04

    Repair a Home Furnace and Air Conditioning Thermostat

    Hand turning dial on thermostat
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    The furnace thermostat is the control device that determines when and how much heat will be produced by the furnace. This tutorial describes how to troubleshoot the low voltage thermostat often made by Honeywell, White-Rodgers, Lux, Robertshaw and others. This type of thermostat runs off of a transformer reducing the incoming line voltage from 120 volts down to between 12 and 24 volts, depending on your furnace.

    When a furnace does not work it may be caused by problems with the furnace or the ther...MOREmostat.

    See How to Troubleshoot a Gas Furnace for information on possible furnace problems, causes and repairs.

    But now let's take a look at troubleshooting problems that could be caused by a faulty thermostat and how to make the repairs.

    Continue to 2 of 4 below.
  • 02 of 04

    PROBLEM: Furnace Produces No Heat

    France, young couple dressed warmly
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    Furnace Produces No Heat

    Possible Causes



    Possible Repairs

    • Reset circuit breaker or replace fuse.
    • Remove thermostat cover and gently clean components especially the bimetallic coil and the switch contact surfaces if the thermostat does not have a mercury switch.
    • Set the thermostat to its lowest setting and clean the bimetallic coil with a soft brush or...MORE compressed air. Set the thermostat to its highest setting and clean the coil again. Reset the thermostat to its desired setting.
    • Tighten any loose screw terminals. If a wire is loose from its terminal, place wire back on terminal clamp and tighten screw.
    • Check for loose connection or disconnected wire on low-voltage transformer usually mounted on furnace
    • On electronic models, check to see if the display is working indicating power. If it is not working, replace the batteries in the electronic thermostat.
    Continue to 3 of 4 below.
  • 03 of 04

    PROBLEM: Heat Either Does Not Reach or Exceeds the Set Temperature

    Electrician Installing New Programmable Thermostat
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    Heat Either Does Not Reach or Exceeds the Set Temperature

    Possible Causes

    • Thermostat mounted crooked.
    • Heat anticipator not set correctly.


    Possible Repairs

    • Remove cover of thermostat and loosen screws holding unit to wall. Level the thermostat. Re-tighten screws and replace cover.
    • To adjust the heat anticipator, please see Heat Anticipator Adjustment.
    Continue to 4 of 4 below.
  • 04 of 04

    PROBLEM: Furnace Turns On and Off Frequently

    Young woman repairing the thermostat of a radiator
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    Furnace Turns On and Off Frequently ("short cycling")

    Possible Causes

    • Dirty thermostat components
    • Heat anticipator not set correctly.


    Possible Repairs