The flame sensor is a device located at the burner assembly and can be found in furnaces, boilers and other gas-burning appliances.
The purpose of the flame sensor is to confirm that whenever the gas valve is open, a fire or flame is present. If there were nothing to ignite, once gas is emitted, a dangerous build-up might occur. Flame sensors have a short time in which to detect a flame. If no flame is detected, the unit will automatically shut down. If this shutdown happens after a few times, a safety lockout usually happens that shuts the unit down for more than an hour. In the case of carbon build-up on a dirty flame sensor, a furnace will keep shutting down - a common complaint by homeowners. Carbon build-up is often caused by dusty particles in the air that burn to a sensor, especially if the furnace is located in areas that typically collect a lot of dust, such as basements and attics.
Flame sensors can be cleaned by using emery cloth and steel wool.
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