The blower motor in older furnaces cycles on and off at specific temperatures because the fan limit control is set to modulate the blower at setpoints for efficiency.

The fan limit control is a temperature sensing device mounted on the furnace plenum that measures heat and tells the blower when to start and stop so the heat is moved into the ductwork rather than vented away. Factory settings commonly call for the blower to turn on around one hundred twenty degrees Fahrenheit and to turn off around eighty five to ninety degrees Fahrenheit to prevent short cycling and to maximize heat transfer. To check this without special tools, let the furnace run through a full heat call and feel the plenum near the control; the blower should start when that metal feels noticeably warm and stop after the heat call ends and the plenum cools to the off range. If the blower starts too early or too late, first verify the control is mounted securely and that its sensing bulb or probe has good contact with the plenum metal, because poor contact gives inaccurate readings. To recalibrate, consult the furnace manual for the correct adjustments and slowly turn the setpoint screw while observing several heat cycles so you avoid overshooting and creating short cycling. If the control is corroded, has intermittent contacts, or the temperature sensing element is visibly damaged, replace the control with a direct replacement of the same type and temperature ranges for reliable operation.

The three most common issues are incorrect setpoints from factory drift, poor thermal contact between the sensor and plenum, and failing electrical contacts inside the control. If setpoints are wrong the blower will run too long or too short, so adjust the on and off screws carefully to the recommended one hundred twenty degrees for on and eighty five to ninety degrees for off and retest; if the manual gives different ranges follow the manual. If the sensor is loose or sitting on insulation, remount it flat to bare metal and use thermal compound if needed so the sensor tracks true plenum temperature. If the control clicks or acts sporadically the internal relay may be worn, so replace the fan limit control rather than trying to jury rig the contacts.

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