Testing and adjusting your fan limit switch in your gas or oil-fired furnace can prevent costly repairs to your blower and heat exchanger.
The purpose of a fan limit switch is to automatically control the furnace blower allowing it to run for a predetermined time to allow the heat exchanger to cool down after the thermostat temperature has been achieved. The limit switch also prevents the furnace blower from distributing cold air through the home until the oil or gas burner has warmed the heat exchanger and supply air plenum. In other words the fan limit switch is a time delay device that starts the fan about 30 seconds after the call for heat and allows it to run about 30 to 90 seconds after the set point has been reached.
On older units the switch is an analog combination fan and limit control while newer units may use electronic controls but may also have a sensor connected to the control board.
The fan limit control switch is typically located inside the furnace cabinet where a probe is inserted into the supply air plenum after the heat exchanger. The probe can vary in length from 5, 8, or 11 inches for traditional Honeywell, White-Rodgers, Robertshaw, or generic models.
Location of Fan Limit Switch inside Furnace
For some units where a probe type fan limit switch may not be feasible a Therm-O-Disc fan control may be used. These are small bi-metal thermostats that are installed over a hole above the heat exchanger and plenum.
Honeywell Fan Limit Switch Model L4064
Top (Black) and Bottom (Silver) of a Therm-O-Disc limit switch
The fan limit switch is also a safety control which protects the furnace heat exchanger from damage due to overheating. After the unit shuts down the burner the fan will continue to run if the temperature inside the supply plenum is above the setting on the limit switch. This is why some devices are listed and combination fan and limit controls. Under normal conditions in the heating mode, return air moves over the heat exchanger to warm the supply air and thereby remove heat from the heat exchanger.
If a furnace blower fan fails to start but the gas or oil fired burner is running, the heat exchanger would overheat and may warp and crack.
The fan limit switch is designed to prevent this damage by shutting off the burner if plenum temperatures reach the high limit. A fan limit control switch is found on both oil and gas fired heating furnaces of all types.
In the Honeywell L4064 Series furnace controls you can clearly see the metal dial providing temperature control settings, and a button which can be set to force the blower fan to run continuously or to permit it to run automatically when the furnace is warm.
The dial turns the blower fan on, off, and provides an upper limit temperature setting for safety.
The button, which is usually white, is the manual override which allows the furnace blower fan to run continuously regardless of furnace temperature. This may be used to test the controls or is certain systems where continuous operation is required for filtration or temperature control.
Some models also have an electric heater inside the limit switch to force the fan on even if the plenum hasn’t reached the cut-in temperature. On the above picture of the Honeywell Switch the heater is powered by the red and yellow wires.
Sensing the Air Temperature
The fan limit switch probe contains a bimetallic spring which is inserted into the warm air plenum of the heating furnace. As the warm air in the plenum passes across the spring it expands and turns a gear connected to the fan limit control dial. As the dial rotates tabs on the back of the dial open or close electrical contacts inside the switch to turn the fan on or off and at the upper limit to turn off the furnace oil or gas burner as well.
Bimetallic Spring in Fan Limit Probe
As you set the cut-in and cut-out temperature set points on the face of the dial you are repositioning the tabs on the back of the dial. For example say the spring turns a quarter turn when the air temperature reaches 140 degrees, if you move the tab past this point the spring would have to expand more to reach the new location meaning the temperature would have to be higher to get to this point.
Testing a Combination Control- Fan Limit Switch
The testing procedures are different for various fan limit switches. Carefully read your instructions for your particular device. If you do not have the instructions, you can download them from the manufacturer’s website or call them and have them sent to your home.
Honeywell Limit Switch Controls and Terminal Locations
1. Verify that the control wiring has been installed in accordance with the manufacturer's installation instructions.
2. Turn off electrical power to the heating system. Usually there is a light switch with a red cover plate that controls the power to the furnace.
3. Disconnect the fan at the fan motor or at the FAN LIMIT control terminal.
4. Turn on electrical power to the heating system. Note: Some furnaces have a safety switch that prevents the unit from working while the cover is removed. You may need to bypass this safety to test the limit switch by using a piece of tape or a clamp to depress the micro-switch on the frame of the unit.
5. Set the thermostat to call for heat by setting the thermostat above the room temperature. Verify that the thermostat is in the heating mode and the fan is on AUTO.
6. Burner Operation: The oil or gas burner should turn ON and then turn OFF in a very short period of time as the temperature in the supply air plenum reaches the FAN LIMIT set point. The dial should be moving to confirm its operation. If the burner does not turn off as expected, shut off the power to the furnace. Failure to shutdown the system can damage the heat exchanger by overheating. If the Fan Limit switch did NOT work normally, leave the system turned OFF and call for service from a professional HVAC mechanic.
If the Fan Limit switch worked by shutting off the burner:
1. Turn off electrical power to the heating system.
2. Reconnect the fan motor power or control where you had disconnected it.
3. Turn on electrical power to the heating system.
4. Set the thermostat to call for heating, but don’t set it too high or you will have to wait longer to verify the limit switch’s operation. The furnace blower should come on when the plenum temperature has reached the FAN ON setting and watch the dial move on the limit switch.
5. The furnace blower should turn OFF when the plenum has cooled down to the FAN OFF setting.
6. Reinstall the furnace cover and return all thermostat settings to their previous positions.