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Furnace Safety Devices explained

There are several safety devices that are active once the gas heating furnace has the main burners lit and there are several different types or brands for each device type. Typically most gas furnaces have what is called the spill switch or limit switch. This is a device that can detect heat or flame outside the burner tubes within the gas burner cabinet.


Normally mounted on the gas burner manifold itself this device will shut off the main gas valve if it senses any heat or flame and will usually lock out the furnace. On newer furnaces this will produce an error code that can be read on the heating system control board based on the number of flashes or sequence of flashes from the status LED.

Several spill switches also come with a manual reset button. This is usually a little red button on the top of the safety device. If the spill switch has for some reason tripped this button must be pushed to reset this safety device before the heating system control board is reset.


Another common safety device is the high limit switch or Plenum Thermostat which usually can be found above the main gas burners mounted to the back wall of the gas burner cabinet. This device senses the temperature inside the air stream of the furnace at or near the heat exchanger. Each one is usually specific to the furnace and requires a specific temperature range. Upon sensing too high of a temperature this device will shut off the main gas burner of the heating system.

These safety devices are normally a self reset type but some may also have a little red reset button. The most common cause of high temperature from a furnace is a clogged air filter because with less air flow more heat is transferred to the air and it will get hotter.
After several trips or shut downs these devices usually will fail and require replacing to function correctly.  

Note: The heating system control board can be reset by shutting off the power to the furnace for a few seconds and turning it back on.      


CAUTION - Always be very careful with electricity as it can be lethal, if you are not comfortable performing any of these steps please do not perform them, of course you are performing these steps at your own risk.

Important Note: The heating system control board is sometimes very sensitive to static electricity, avoid touching this board to prevent failure.

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