What are Furnace - Air Handler Limit or Fan switches?
They have different names, but they operate all the same way. Limit switches have a snap disc inside that expands and activates an ON or OFF switch.
A limit switch is a safety device to turn OFF power when the temperature exceeds the rating.
A Fan switch starts the fan/blower motor when the FAN-ON temperature is present.
Here are a few alternative terms used for limit switches:
High limit safety switch, high-temperature limit sensor, snap disc switch, thermo-disc switch, thermal limit switch and so on.
Here are a few alternative terms used for fan switches:
Fan limit switch, low limit fan switch, low limit snap disc, fan control switch, low limit exhaust sensor, thermal sensor, proof of fire switch and much more.
For your information: wiring diagram for fan and high limit switches:
Free wiring diagrams and troubleshooting tips for fan and high limit thermostat snap discs are included with your order!
How an air handler fan switch works:
The contacts on a fan switch are usually open; they close on temperature rise.
Let's look at a gas furnace: When the thermostat calls for heat, the gas valve opens, and the burners ignite. At this time, the heat exchanger is still cold; you don’t want the fan pushes cold air through the house at first.
Once the system warms up, the fan switch closes and powers the fan ON.
When the heating cycle stops, there is still a lot of heat available in the heat exchanger. The fan switch keeps the fan running until the warm air is cooling down below the operating range of the switch.
How a furnace limit switch works:
A limit switch is a safety switch with normally closed contacts; the connections open at the designed maximum temperature and interrupt the power supply to the heater.
An automatic limit switch will reset itself and resume the supply power after the heat falls below the rated temperature. designation. e.g., an L190-40F will cut off at 190ºF and return power at 150ºF.
A manual reset limit switch operates like a regular limit switch. But after reaching the nominated limit temperature, the switch trips and will stay in OFF position until a technician reset it. They are the last safety element in a system; if they interrupt the power supply, there is something wrong (fan motor bad, air flow restricted, filter blocked, plugged evaporator and so on). That's the reason why they do not automatically reset; the homeowner needs to call a service technician to inspect and repair the system.
Information: Most furnaces have more than one high limit switch, sometimes an automatic reset and a manual reset.
What you need to know if you want to replace a limit or fan switch:
They come in two different sizes: The smaller one is a 1/2" size, the bigger one a 3/4" switch.
Limit switches have two different mounting options:
1. Airstream mount to sense the surrounding air temperature, the sensing cup looks inside a box. E.G., through a hole in the heat exchanger.
2. Surface mount to read the temperature at the surface of a metal box, E.G., the flat bottom of the switch touches the outside wall of the heat exchanger.
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