My thermostat won't work

Written by eric jonas
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My thermostat won't work
Keep the cold weather outside where it belongs. (snow 2 image by Omely from

The thermostat regulates the heat produced by the furnace. However, what do we do when our trusted thermostat appears to have abandoned us, leaving us vulnerable to the extreme outdoor temperature seeping inside? When the problem is simply a programming error, a homeowner can follow the instruction manual to reset the thermostat. When the problem is due to the hardware failure, there are still some methods to determine the potential fault before running out for a replacement.

Skill level:

Things you need

  • Screwdriver

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  1. 1

    Check to see if the thermostat is losing time for no reason or the clock is turning off. When the furnace overheats, the high limit trips and the current to the transformer that powers the thermostat is shut off. The fan will continue to run to cool down the heat exchanger and as a result the homeowner may be left wondering whether or not the furnace is working. Once any of the furnace safeties are tripped, the power to the transformer is cut off. If you are certain that the power is connected properly, it is advisable to contact a professional to investigate. (Reference 1)

  2. 2

    Look at the thermostat to see if the display is functioning. If the thermostat is completely blank then the common wire is the issue. The common wire is the low-voltage wire that provides the thermostat with power. Remove the thermostat's faceplate to check to see if the common wire is attached properly to the thermostat and then follow that wire to verify it is connected to the furnace as well. If so, check the continuity of the wire to ensure that there is no break in the wire. Some thermostats allow the hook up with only two wires and no common wire. If there is no common wire running from the thermostat, check to see if there is a 10-watt, 250-ohm resistor between the common and the white wire. If a resistor is present, you will see a small box that runs between the two wires. (Reference 1 & 2)

  3. 3

    Check that the correct type of battery is in the thermostat if it is wireless. Regular alkaline batteries run out of power quickly and cause problems for thermostats. Replace the regular batteries with lithium batteries and check the backlight settings. Having the thermostat backlight on all the time drains the battery much quicker than having the backlight set to light up only when adjusting the temperature. (Reference 1)

  4. 4

    Inspect the wires to see if the heating and cooling wires are reversed. Look at the wiring instructions in the thermostat manual and make sure that the coloured wires line up with the correct thermostat and furnace terminals. (Reference 1)

  5. 5

    Insulate the wall behind the thermostat if the changes in room temperature do not trigger any change in the thermostat, which can happen due to air passing through the thermostat from within the wall. (Reference 1)

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