High Limit Control On A Gas Furnace

There is a difference between a limit switch and a rollout switch.

High limit control on a gas furnace.

A limit switch is a device that s located under your furnace s supply plenum. The high limit switch in your gas furnace is a small part with two very important jobs. Limit switches turn off the burner and turn on the circulating fan if there is a high temperature condition in the furnace. Which way to set the white auto manual fan control switch.

It s also known as a fan limit switch or simply a limit switch. The high temperature limit switch is an important device used in forced air furnaces powered by gas or heating oil. Manual reset excess temperature control commonly known in the industry as a hi limit control this control should be set approximately 50 f 10 c higher than the oven setpoint or maximum rated temperature. The limit switch includes a sensor in the heat exchanger to determine when the air is warm enough to circulate.

Once the plenum reaches a specific temperature the limit switch sends a signal to the air handler to begin running and delivering warm air to your home. When the temperature reduces to normal the burner will come back on. Check with your owner s manual for suggested temperatures. The switch makes sure the furnace is operating safely and at its desired settings.

Once the air reaches a preset temperature the limit switch turns on the blower. The third marker should be set around 200 degrees as a safety shut off for the burner. This is the role it plays in every cycle of your furnace s normal operation. It reads the temperature of the plenum and waits until it gets appropriately warm.

Set the markers to the appropriate settings for your furnace. For a general guide set the two lowest ones from the furthest left to 100 and 130 degrees to control when the fan comes on and goes off. The hi limit switch will trip and shut down the heaters once the excess temperature setting is reached. A high limit control switch will turn off the gas to a furnace if the temperature becomes too high.

This video will help you to diagnose and replace a faulty limit control on your gas furnace. The limit switch shuts off the burner when the temperature exceeds a preset temperature. If the furnace oil or gas burner is very high capacity or if the furnace fan limit controls have been set to cause this effect the burner may on some systems cycle on and off periodically while the warm air blower continues to run. Should the unit produce too much heat.

The flames would raise the temperature inside the unit and have no place to go getting hotter and hotter. But that s not the only thing that the high limit switch does.