1) What’s a Limit Switch?
Limit switches are components in every piece of heating equipment, be it gas furnaces or air handlers with electric heat.
2) Are Limit Switches Important?
Yes, especially the high limit switch. It’s named appropriately and is a safety measure to turn off the gas furnace or AHU with electric heat strips if the temperature inside the unit goes above the degree limit of the switch. Example: 160* degree limit switch is in place; the blower motor stops working and stops moving air through the unit. However, the flames are still on or the electric heat strips are still on and the temperature continues to rise. When the inside of the unit reaches the high limit of 160*, the switch turns off the unit.
3) Common Limit Switches
High limit switch – discussed above
Fan Control limit switch – The fan control switch monitors the temperature inside the heating equipment. When the desired temperature is reached, the fan switch turns on the blower motor to push warm, cozy air through the ducts and out the registers.
Causes that Trip a Limit Switch:
A dirty/clogged filter – restricts and blocks proper air flow into the furnace or AHU and will cause the temperature to rise. If the temperature raises high enough, the High limit switch will trip and will shut off the unit. Check the filter regularly and change it as needed.
The blower motor – check if the blower motor is working…is air moving out of the registers? Manually change the thermostat fan setting from “auto” to the “on” position and check air flow from the registers. If the blower motor is not working and no air is being blown through the furnace or AHU, it will reach the “degree limit” of the switch and the high limit switch will trip, shutting off the unit.
There are many parts and controls working together to keep that comfortable warm temperature in our homes. If the filter is clean and the blower motor is working, a limit switch can wear out and fail. There is always a reason that causes limit switches to trip and it may be time to call for a furnace repair service professional to thoroughly check the heating equipment for safety and proper operation.
