In a forced-air heating system, the blower motor fan in a gas or oil-fired furnace turns on in response to the setting of the fan limit switch. The limit switch energizes the fan circuit when warm air in the plenum of the furnace reaches a preset temperature. When the desired temperature in the home is reached, the furnace burner shuts off, but the blower fan continues to operate until the temperature of the plenum falls to the lower limit switch setting. You can change a limit switch with a few tools, a manual for your model of furnace and basic electrical know-how.
Things You'll Need
Turn off the circuit breaker at the main electrical panel that supplies power to the furnace. Tag or otherwise mark the breaker to alert others that the power is to remain off.
Remove the furnace controls access panel. Refer to the furnace manual for the location of the panel. Most access panels can be removed without tools.
Locate the furnace fan limit switch. The switch is mounted on the furnace air plenum and is easily recognized because of its wiring once the access panel has been removed. Two of the wires are low voltage and go to the limit switch, and two wires are line voltage and go to the fan switch.
Test the line voltage connections on the limit switch with a voltage tester to verify that no electrical power is present. Touch one probe of the tester to the L1 terminal and the other probe to the L2 terminal to perform the test.
Insert a flat-head screwdriver into the horizontal slot just above the L1 terminal wire. Press in on the screwdriver to release the terminal clamp while pulling out on the wire. Label the wire with masking tape and a marker pen according to its location for installation on the new switch.
Release the L2 terminal wire and the two low voltage wires using the same technique as in Step 5. Label the wires with masking tape and a marker pen.
Locate the screws that secure the fan limit switch to the furnace plenum. Turn the screws counter-clockwise with a screwdriver to remove.
Pull the limit switch straight out from the plenum to remove.
Compare the old limit switch with the replacement switch to ensure that the temperature-sensing probes are the same length. Verify that the set-points for the fan power cut-in and cut-out are the same on the new replacement as on the old.
Insert the new limit switch into the furnace plenum and secure with the mounting screws. Tighten the screws clockwise using the screwdriver.
Insert the L1 and L2 line power wires into their respective terminals up to the shoulder of their insulation. Each of the terminal clamps are self-locking. Ensure that each wire is placed in the appropriate terminal by relating the tape marking to the terminal location.
Insert the low-voltage wires into their proper terminals according to the taped marking.
Install the furnace access panel.
Restore electrical power to the furnace.
Run the furnace through a complete cycles to check the operation of the new fan limit switch.