The ABC’s of Home Heating – High Temperature Limit Switch
The high temperature limit switch (also known as a furnace plenum thermostat) is a safety device on a furnace that is designed to cut the gas supply when the burner temperature exceeds the safe operating range. When the temperature drops back within the safe range, it will allow the gas valve to open again.
The switch commonly operates at an upper limit of 160°F (causing the burner to turn off) and a lower limit of 130°F. The limit switch has two main components: a long temperature sensor located inside the furnace, and a switch mounting plate located on the outside of the furnace.
High Temperature Limit Switch Damage
One of the most common causes of high temperature limit switch failure is improper furnace maintenance, especially dirty air filters. A clogged air filter will reduce air flow, causing heat to buildup in the combustion chamber. This will cause the limit switch to cycle on and off continuously, leading to premature failure.
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