The thermostat in your home may be small but it is a very important device. It determines the timing and amount of heat and air conditioning used to moderate indoor temperature. A low voltage thermostat runs off a transformer and reduces the line voltage from 120 to the range of 12 to 24 volts based on the type of furnace installed. When the thermostat goes on the fritz, troubleshooting will identify the necessary repairs, which are not usually difficult to make.
You may need:
Issue 1: The furnace is not producing heat or air conditioning. This could be caused by a blown or tripped circuit breaker, which can be repaired by replacing the fuse or resetting the circuit breaker. If this is not the problem, look for an open connection or loose wires leading to the thermostat. Use a screwdriver to tighten loose screw terminals. If a wire has loosened itself from a terminal, put it back on the terminal clamp and tighten the screw. Find the transformer (it is typically mounted on the furnace), check for a disconnected wire or loose connection, and reconnect or retighten.
Remove the cover of the thermostat to determine if the components are dirty. If so, clean them gently, focusing on the switch contact surfaces and bimetallic coil, if present. After setting the thermostat to the lowest setting, use some compressed air or a soft brush to clean the coil. Adjust the thermostat to the highest setting and reclean the coil. Reset the thermostat to the desired temperature.
For a digital electronic thermostat, the battery may be bad. Verify that the display is working and if it is not, replace the batteries.
Issue 2: The heater or air conditioner will not reach or is exceeding the set temperature. A crookedly mounted thermostat or improperly set heat anticipator may be to blame. If the thermostat is crooked, remove the cover and loosen the screws that attach the unit to the wall. Use the level to level the thermostat, then retighten the screws and replace the cover.
To adjust the heat anticipator, locate the small disc in the center of the thermostat that features calibration marks and the word “longer.” The lever arm fastened to it is the anticipator adjustment. Move this lever one calibration mark away from the setting marked “longer.” Allow the furnace to run for two to three hours so the temperature can stabilize.
Issue 3: The furnace frequently turns off and on, which is referred to as short cycling. The thermostat components may be dirty or the heat anticipator may be incorrectly set. If the components are dirty, follow the component cleaning instructions in Step 1. If the heat anticipator requires adjustment, move the anticipator lever one calibration mark closer to the setting marked “longer.” Let the furnace run for two to three hours, allowing the temperature to stabilize.