How does a Furnace Limit Switch acts as a safety device?

How does a Furnace Limit Switch acts as a safety device?

One of the best things about modern furnaces is that they have built-in protections against many of the things that can go wrong with your system. One of these safety devices, which also plays a role in the regular heating process, is the furnace limit switch.

image A limit switch is usually an automatic reset thermostatic switch that opens at the temperature rise to a set temperature and closes when the temperature falls below a set temperature. There is usually a built-in differential so that the burner does not cycle fast and cause a failed ignition.

It is designed to shut off the furnace before the heat exchanger gets too hot, both to avoid damage from overheating and to maintain an effective temperature of the heat exchanger.

image A limit switch is a device that is located below the supply plenum of your furnace. Read the temperature of the plenum chamber and wait until it heats up properly. Once the plenum reaches a specific temperature, the limit switch sends a signal to the air controller to start running and deliver hot air to your home.

A limit switch also acts as a safety device for the furnace. If the temperature inside the plenum heats up too much, the gas supply to the burners is shut off to prevent the furnace and heat exchanger from overheating.

It can be connected directly in series with the gas valve or with a limit circuit of the control module. Other limit switches, sometimes called a high limit, are designed as over-amplification safety devices and do not restart automatically. They are fuse links or can be reset manually and can be connected in series with a deployment or other safety switches.

If the furnace has experienced a high-temperature condition, due to inadequate combustion of the air, these switches will be turned off from the furnace. Reset the switches by pressing the button on the switch. If the switch trips a second time, turn off the heater and request service.