About Furnace Thermocouples

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The furnace is the heart of any heating system. It generates the heat that is distributed to all parts of a home to keep it warm.


Whether fed by oil, natural gas or propane gas, all furnaces run at extremely high temperatures and must be controlled. Without proper safety measures a furnace can be very unsafe. The thermocouple is one of the safety mechanisms.

Definition

  • A thermocouple is a safety shutoff device used in gas-fed appliances. It controls the main gas valve providing a safety control to prevent gas buildup if the pilot flame goes out. A thermocouple is used in gas-fed furnaces, ovens, and water heaters.

    A pilot is a device which produces a continuously burning flame. The pilot is used to ignite a gas burner to start the furnace.

Function

  • A thermocouple works as a temperature sensor. It senses whether the pilot flame is hot enough to maintain burning the gas for the furnace burner.

    The thermocouple has two wires made of dissimilar metals welded together at one end. The tip of the welded part or probe is positioned in the hottest part of the pilot flame. The other end is connected to the pilot gas valve.

    As the thermocouple heats up, a small amount of electricity is generated and routed to 24-volt transformer. This operates the supply valve that feeds gas to the pilot.

    The level of heat generated by the furnace is regulated by a thermostat. It controls the gas valve which adjusts the amount of gas supplied to the burner.

    As long as the pilot remains lit, the thermocouple remains hot. If the pilot flame goes out the thermocouple cools down and the voltage drops. That in turn triggers the transformer to stop keeping the supply valve open. With the supply valve closed gas is prevented from leaking out and building up, thereby preventing an unsafe condition.

Types

  • Thermocouples differ in terms of the metals and wire thickness. Different combinations of paired metals and wire diameter are called calibrations. The calibration determines the temperature range of different thermocouples.

    The choice of thermocouple depends upon the temperature range needed for different equipment. Most furnaces run from 1000 degrees to 1800 degrees Fahrenheit and use a Type K thermocouple.
    .

Ignition

  • The method of starting up or igniting the gas furnace burner can be done with a standing pilot or by electronic ignition.The standing pilot system uses a flame that continuously burns. When the thermostat calls for heat the pilot ignites the main burner.

    Newer furnaces use a hot surface igniter system (HSI) instead of standing pilots. When the thermostat calls for heat the igniter glows red hot and lights the main burner. The HIS system uses silicon carbide or silicon nitride, which ignites when electricity is applied to it.

    The standing pilot system requires that pilot flame is always burning. When the flame goes out it must be re-lighted. This can be done with a match or a lighter.

    Hot surface igniters require electricity to work. Any disruption of power will prevent igniting the main burner.

Replacement

  • A common symptom of a faulty thermocouple is that the pilot flame will not stay lit. A thermocouple wears out because it runs at high temperatures generated by the pilot and the burners. The higher the operating temperature the more quickly the thermocouple will deteriorate. It is recommended that Type K thermocouples be replaced every one to two years. Higher temperature thermocouples may need replacement as often as every three months.

    Hot surface igniters continuously undergo extreme temperature change, becoming red hot then cooling. Eventually they crack and will need replacement. Silicon carbide igniters last for three to five years whereas silicon nitride igniters last for six to ten years.

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  • Photo Credit Lennox G61 MP Gas Furnace (TP Technical Publications, Litho USA)
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