Fan limits are a safety circuits that are designed to shut down the burners, in the event the furnace were to over heat during a heating call. They are located in the center of the heat exchanger, where the most heat is generated. If the fan where to fail while the burners where on, the heat exchanger would over heat. That would cause the fan limit to open the burner circuit. The fan limit would then not allow the burner to turn back on untill the heat exchanger has cooled to the set temperature.

If the air flow to the furnace was reduced, the fan limit would open the burner circuit as well, because the furnace would over heat.

The fan limit is a very important safety because it regulates the temperature range that a furnace will be operating in. Controlling the temperature range can extend the life of your furnace and prevent a fire.

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