If your fan limit control will not turn off the blower, check the dial settings and calibration immediately.

The fan limit control is a temperature sensitive switch mounted on the furnace that tells the blower when to start and stop based on plenum temperature. First verify the dials: the blower on temperature should be set around one hundred twenty degrees Fahrenheit and the blower off temperature should be between eighty five and ninety degrees Fahrenheit. If the blower runs continuously while the plenum is cool, raise the blower on set point slightly to avoid the control thinking the plenum is still warm. If the blower never comes on during warm cycles, lower the blower on setting and confirm the operating thermostat and heat call are functioning. When adjusting, make small changes of five degree increments and allow a full heat cycle to complete before further adjustment so you can observe true behavior. If the control is old, shows corrosion, or responds erratically despite correct settings, replacement with a new fan limit control is the most reliable fix.

Common mistakes include misreading the dials or setting values too close together which can cause short cycling; keep at least thirty degrees separation between the blower off and blower on settings to prevent rapid switching. Another frequent issue is assuming the thermostat or control board is faulty when the fan limit is out of calibration; verify plenum temperature with a handheld thermometer at the sensor before swapping other parts. A third problem is neglecting the age and condition of the control; if the device has visible rust, burned contacts, or inconsistent response replace it rather than spending extensive time troubleshooting marginal components.

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