How to Troubleshoot a Low-Voltage Thermostat

Close Up Of Female Hand On Central Heating Thermostat
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Virtually all central heating and air-conditioning systems are controlled by a low-voltage thermostat. The thermostat tells the system when to turn on and shuts it off when the house temperature has reached the desired level. These thermostats are called low voltage because they run off a transformer that reduces line voltage from 120 volts to a level somewhere between 12 and 24 volts, depending on the brand of furnace you have.

When an HVAC (heating, ventilation, air conditioning) system doesn't work as it should, it can often be traced to a problem with the thermostat. This can actually be fortunate because problems with a thermostat are generally easier to fix than problems with the furnace or AC unit.

Here are three common problems with thermostats, as well as how to troubleshoot them.

No Heat or Cooling

If the HVAC system fails to turn on at all, there are a number of thermostat problems that should be considered before assuming the furnace or AC unit is to blame. In many cases, this symptom is the result of no electrical current reaching the thermostat. 

Possible Causes

  • Circuit breaker tripped or fuse is blown
  • Dirty thermostat components
  • Loose wires or open connection in the thermostat
  • A bad battery on a digital electronic thermostat

Possible Repairs

  • Reset the circuit breaker, or replace the burned-out fuse.
  • Remove the thermostat cover, and gently clean the components, especially the bimetallic coil and switch contact surfaces. Set the thermostat to its lowest setting, and clean the bimetallic coil with a soft brush or compressed air. Set the thermostat to its highest setting, and clean the coil again. Then, reset the thermostat to its desired setting.
  • Tighten any loose screw terminals. If a wire is loose from its terminal, place the wire back on the terminal clamp and tighten the screw.
  • Check for loose connections or disconnected wires on a low-voltage transformer, which is usually mounted on the furnace.
  • On electronic thermostat models, check to see whether the display is working and indicating power. If it's not working, replace the batteries in the thermostat.

System Fails to Set the Desired Temperature

Sometimes the furnace or AC system will run but fail to set the temperature you've selected.

Possible Causes

Possible Repairs

  • Remove the cover of the thermostat, and loosen the screws that hold the unit to the wall. Level the thermostat, retighten the screws, and replace the cover.
  • Adjust the heat anticipator. Try moving the lever one calibration mark away from the "longer" setting.

Furnace Frequently Turns On and Off

If a furnace or AC system turns off and on again frequently—a symptom known as short-cycling—it is usually because the electrical connections within the thermostat are not making clean, consistent contact. However, be aware that when outside temperatures are radically higher or lower than the desired indoor temperature, it is normal for the HVAC system to cycle fairly frequently. For example, on a bitterly cold winter's day your furnace might start and stop quite often. 

Possible Causes

  • Dirty thermostat components
  • Heat anticipator not set correctly

Possible Repairs

  • Remove the thermostat cover, and gently clean the components, especially the bimetallic coil and the switch contact surfaces.​
  • Adjust the heat anticipator. Try moving the lever one calibration mark closer to the "longer" setting.