If your furnace simply stopped working, and you've checked everything that could possibly go wrong with it, ask a heating specialist to check the furnace's high temperature limit switch. A high temperature limit switch is a unique but simple device that keeps your furnace safe when you use it. If something goes wrong with the blower fan, it can affect the limit switch. Learn more about a high temperature limit switch below.
How Does a Limit Switch Protect Your Furnace?
A high temperature limit switch, or fan limit switch, is generally mounted inside the plenum and near the heat exchanger. The switch also communicates electronically with the fan found inside the blower compartment. When the temperature inside the plenum reaches the proper setting, the limit switch turns off the fan.
Certain things can inadvertently affect the limit switch, including an old fan or clogged air filter. The fan might not spin properly or circulate air properly through the furnace. Dust may also cover the entire surface of the air filter inside the blower compartment and keep return air from reaching the fan. The temperature inside the heat exchanger will rise until the limit switch shuts it down. If the limit switch turns off repeatedly, it'll eventually place the furnace into "lock down" mode.
A locked furnace won't turn back on until you fix the problems in the fan and air filter.
How Do You Fix the Switch?
Some homeowners attempt to repair their locked furnaces themselves. They may do this by resetting the limit switch. But if you don't address the problems in the fan and air filter, resetting the limit switch will be futile. It's important to hire a professional HVAC contractor to check and reset your furnace for you.
A contractor will most likely change the air filter and clean the blower motor fan first. Next, a contractor will reset the limit switch and wait for the furnace to heat up. If the limit switch turns off the fan after the furnace's reaches the right temperature, the contractor solved the problem. But if the limit switch continues to lock down the furnace, a contractor will replace the switch. The switch may simply be bad or faulty.
If your gas furnace shuts down unexpectedly or without any apparent reason, contact a heating specialist right away, such as at Dependable Heating & Cooling. The problem may be as simple as a bad or faulty high temperature limit switch.