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The fan limit control turns the fan on and off in furnace applications. It works off of temperature and also provides a high temperature safety limit.This is a switch, actually two switches, which turns the furnace blower on and off based on the temperature of the heat exchanger. A helicle bi-metal probe extends from the back of the fan limit control. This probe is located inside the heat exchanger and the bi-metal of the probe reacts to temperature.
When the furnace heat exchanger heats up the bi-metal twists causing the round dial (shown in the picture) to turn. Based on where the manual selection switch is set, when it reaches the manual setting contacts inside the switch close and the furnace blower motor turns on. This provides a time delay for the fan to come on which allows the furnace heat exchanger to heat up. This eliminates (or limits) cold air from being blown out of the ducts when the furnace blower first starts. After the burners go out when the call for heat is over, the bi-metal in the fan limit control cools and the bi-metal begins to make the dial turn the other way. When the temperature in the furnace heat exchanger cools enough and the bi-metal reaches the low setpoint on the dial the contacts inside the fan limit control open and the blower shuts off. This allows the heat exchanger to cool off by disapating the excess heat in the firnace heat exchanger and makes better use of the heat produced by the furnace. Instead of allowing the heat to remain in the furnace this feature in the Honeywell fan limit control uses as much heat as possible in the heat exchanger which adds efficiency to the furnace.
The Honeywell fan control is also a temeperature control limit switch. Another limit control switch which is integral to the Honeywell fan control is a high temperature control limit switch. The circuit which directly runs through this switch controls the burners to the furnace. The temperature control limit switch inside the Honeywell fan control breaks the circuit to the burners if the temperature exceeds the high limit setpoint. This is a safety feature all furnaces are equipped with in case the temperature gets too hot inside the exchanger. If this happens the furnace blower contacts remain closed so the furnace blower disapates the excess heat in the furnace exchanger.
