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High Temperature Limit Switch

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The furnace high temperature limit switch is a safety device allowing operation within a specified temperature range.

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Definition:

The high temperature limit switch is a safety device used on a furnace that monitors the internal furnace temperature and shuts down the gas valve to the burner if temperature limits are exceeded. Once the temperature drops to the lower limit of the switch, the gas valve is turned back on allowing the burner to operate again.

The limit switch consists of a long temperature sensor probe which is located in the furnace which is attached to a switch mounting plate fastened to the outside of the furnace. That mounting plate  usually contains two switch terminals for the probe. The switch location varies by manufacturer and design of the furnace but is often located between the burners / combustion chamber and the vent / draft inducer fan motor.

A common operating range for these switches is 160°F for the upper limit (turning the burner off) and a lower temperature limit of a 30°F drop, or a lower limit setting of 130°F (turning the burner back on). Another common range is 155°F / 125°F.

These valves are often damaged by poor air filter maintenance. A very dirty air filter will cause low air flow and heat buildup near the combustion chamber which will cause the switch to cycle on and off frequently, eventually causing the switch to fail completely from frequent over heating or get stuck in the off or "open circuit" position.
 

Also Known As:

  • High temperature limit control switch
  • Furnace plenum thermostat
  • High temperature cut out switch

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