Note: Some furnaces have a safety switch that prevents the unit from working while the cover is removed. Turn on electrical power to the heating system. Disconnect the fan at the fan motor or at the FAN LIMIT control terminal.Ĥ. Usually there is a light switch with a red cover plate that controls the power to the furnace.ģ. Turn off electrical power to the heating system. Verify that the control wiring has been installed in accordance with the manufacturer's installation instructions.Ģ. Honeywell Limit Switch Controls and Terminal Locationsġ.
#FAN CONTROL SWITCHES DOWNLOAD#
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. Carefully read your instructions for your particular device. The testing procedures are different for various fan limit switches. Testing a Combination Control- Fan Limit Switch 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. 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.Īs 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. As the warm air in the plenum passes across the spring it expands and turns a gear connected to the fan limit control dial. The fan limit switch probe contains a bimetallic spring which is inserted into the warm air plenum of the heating furnace. On the above picture of the Honeywell Switch the heater is powered by the red and yellow wires. 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. This may be used to test the controls or is certain systems where continuous operation is required for filtration or temperature control.
#FAN CONTROL SWITCHES MANUAL#
The button, which is usually white, is the manual override which allows the furnace blower fan to run continuously regardless of furnace temperature. The dial turns the blower fan on, off, and provides an upper limit temperature setting for safety.
#FAN CONTROL SWITCHES SERIES#
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. A fan limit control switch is found on both oil and gas fired heating furnaces of all types. The fan limit switch is designed to prevent this damage by shutting off the burner if plenum temperatures reach the high limit. 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. 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. This is why some devices are listed and combination fan and limit controls. 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. The fan limit switch is also a safety control which protects the furnace heat exchanger from damage due to overheating. Top (Black) and Bottom (Silver) of a Therm-O-Disc limit switch These are small bi-metal thermostats that are installed over a hole above the heat exchanger and plenum. Location of Fan Limit Switch inside Furnaceįor some units where a probe type fan limit switch may not be feasible a Therm-O-Disc fan control may be used. The probe can vary in length from 5, 8, or 11 inches for traditional Honeywell, White-Rodgers, Robertshaw, or generic models. 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. 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. 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. 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. 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.
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