Symptoms of a Faulty Thermostat

Symptoms of a Faulty Thermostat

Early detection can prevent heating and cooling system problems

By Robert Ferguson

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Properly diagnosing a faulty thermostat is key to preventing heating and cooling system failure. The thermostat is the medium between the air conditioning/heating system and you. There are three types of thermostats, an older mechanical style with a mercury bulb inside, a digital type powered by batteries and a digital type powered by the HVAC unit. A few simple tests can help diagnose a faulty thermostat. Though not foolproof, they are worth trying before calling in a professional to fix the problem. When working around electrical components, use extreme caution.

Battery-operated digital thermostats

Battery-operated digital thermostats have backlights. The light runs off the power of the battery. If the backlight is not working, replace the batteries. If the thermostat has a programmable function, it will work off the battery as well. Replace the batteries with new ones even if the backlight is working. When batteries become weak, it can affect the programmable function of the thermostat, causing it to lose its settings.

24-volt digital thermostats

This type of digital thermostat draws its power from the main system and has no batteries. If the area is prone to power outages, it will lose its program every time there is an interruption in the power. If the backlight is not illuminated or the thermostat is just not working at all, reset the circuit breaker that powers the HVAC unit. The thermostat wires connect to the air handler, which is usually located inside. There is a circuit breaker on the air handler itself that could need to be reset as well.

Mechanical thermostats

Mechanical thermostats have a mercury bulb inside that needs to be level. If the thermostat is not level, you will get false temperature readings in your home or business, so make sure the thermostat is level.


Doors and windows affect thermostats by creating a draft when they are opened and closed. Thermostats need to be located close to the air return and out of direct sunlight. Another problem with drafts is the hole behind the thermostat. They all have one because of the wires coming out of the wall. Plug the hole with some insulation or newspaper and cover it with a piece of tape. Filling the hole with caulk is also an option.

About the Author

Robert Ferguson is a licensed building contractor with more than 30 years of experience, focusing primarily on residential remodeling, repair, renovation and construction.

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