What Is a Furnace High Limit Switch?

Is the inside of your furnace a mystery? Learning about key components, like the furnace high limit switch, can give you a better idea of how the heating system works and help you understand what could be wrong if it's not working correctly.

This May Also Interest You: The 4 Types of Furnaces

What Is a High Limit Switch on a Furnace?

A furnace high limit switch is a small but vital component that controls the furnace's temperature. The switch is located near the heat exchanger, where it monitors the inside temperature. It helps the blower function properly and keeps the furnace from overheating.

How Does It Work, and What Is Its Purpose?

When you set the thermostat, the furnace's burners start heating the air. The high limit switch monitors the air inside the furnace until it reaches the desired temperature. It then turns on the blower fan to send the heated air through the home. The switch also tells the blower when to stop running.

The high limit switch ensures the furnace doesn't get too hot by turning off the burners to allow the air around the heat exchanger to cool down. Once the temperature reaches a safe limit, the switch lets the burners turn on again.

Limit switches on furnaces keep the equipment from overheating, which could cause a fire or damage the furnace's internal components. Too much heat can cause the heat exchanger to crack and lead to a carbon monoxide leak.

More Related Articles:

• Need a New Furnace? Here’s What You Should Know About Costs

• How Much Does It Cost to Switch to an Electric Furnace?

• Do You Need a Zoned HVAC System?

• How Much Does It Cost to Replace an Oil Furnace?

• Is an HVAC Tune-up Worth the Price? (Yes, Yes It Is)

How Do You Test an HVAC High Limit Switch?

Signs of a problem with your high limit switch include:

• Short cycling. Listen to how your furnace runs. If it turns off and on frequently, it's short cycling, meaning it stops running before it reaches the correct temperature.

• Continuous fan operation. The high limit switch tells the blower fan when to run. If the switch fails, it might cause the fan to run continuously instead of shutting off when your home reaches the temperature on the thermostat.

• No fan. The switch may fail to trigger the blower fan at appropriate times.

• Overheating. A faulty high limit switch may fail to cut off the burners when the internal furnace temperature exceeds safe limits, causing the system to overheat.

Testing and Troubleshooting a Furnace High Limit Switch

Testing the switch can help determine if something is wrong. Always turn off the power to the furnace first. Check the high limit switch furnace wiring to ensure it's secure and in good condition without any loose connections or fraying.

A multimeter, which measures electrical properties, can determine if the switch is working correctly. Disconnect the switch and touch the probes of a multimeter onto the terminals. You'll want the multimeter on the continuity setting. If the switch is functioning properly, it should show continuity when the switch is at room temperature.

Next, use a hair dryer to heat the switch a little, being careful not to overheat it. Applying the heat should break the circuit, which should show on the multimeter as open. If this happens, the furnace high limit switch is functioning as it should.

Got a Limit Switch That Keeps Tripping?

A properly functioning high limit switch will trip if the furnace overheats. This could be caused by something as simple as a clogged air filter. Other potential issues include dirty blower fans, dirty coils or blocked supply vents. If your furnace overheats frequently, call a technician for heating, ventilation and air conditioning (HVAC) systems to make sure a serious problem like a cracked heat exchanger or a circuit board issue isn't the cause.