A fan limit control is a safety device on a furnace that monitors heat and tells the blower when to turn on and off to protect the system.

A typical mechanical fan limit uses a temperature sensing element and a bi metal disc that bends or a motorized sensor that tracks heat rise in the plenum and closes a circuit to start the blower when safe. To check one first power down the furnace and remove the access panel, then locate the limit assembly near the heat exchanger or plenum and observe whether the sensing element and any rotating sensing cup or shaft move freely when you warm the plenum gently with a hair dryer at low heat. If the fan limit is sticking because of dirt or corrosion you can often free the mechanism by careful cleaning with a soft brush and contact cleaner on the terminals, but do not force or bend the bi metal element because that changes calibration. For systems with a motorized sensing cup make sure the cup is not obstructed and that the motor drive turns smoothly; replace the small motor if it binds or does not rotate. When replacing a faulty limit choose a unit with the correct temperature settings and compatible mounting for your furnace model and always follow the manufacturer instructions for wiring and calibration. After installation restore power and test by running the furnace through a heat cycle and verifying the blower comes on at the specified temperature and shuts off as the plenum cools.

Common issues include a stuck bi metal element that fails to close and keep the blower running when required, which you fix by replacing the limit if cleaning does not restore free movement. Another common problem is electrical contact corrosion or loose terminals that prevent the blower circuit from completing, so tighten connections and clean contacts or replace the limit if pitting is present. A less obvious issue is incorrect limit selection or miscalibration after a service change causing premature blower shutdown, so confirm the replacement part matches the original temperature set points and test operation after any change.

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