Can a High Limit Switch Cause a Furnace Blower to Run Continuously?

A high limit switch plays a critical role in maintaining the safety of your furnace by monitoring the temperature of the air within the heat exchanger. When the temperature exceeds a predetermined threshold, the high limit switch opens to prevent potential overheating. However, under certain circumstances, this switch can malfunction and cause the furnace blower to run continuously.

The Role of the High Limit Switch

The primary function of the high limit switch is to protect the furnace from overheating by interrupting power to the blower and burners when the heat exchanger reaches a predetermined temperature. This switch is typically designed to close when the temperature drops below a safe level. In cases where the switch malfunctions, it may remain in the open position, leading to persistent operation of the blower.

The Impact on the Furnace's Operation

When a high limit switch opens, the furnace's control system may interpret this as a signal that the temperature is still too high. As a result, the control board might not de-energize the blower until the high limit switch recloses or the temperature drops to a predefined lower set point. This can cause the blower to run continuously, even when the heat exchanger is no longer in danger of overheating.

Diagnosing and Resolving the Issue

If you are experiencing continuous operation of the furnace blower, it is advisable to check the high limit switch and associated components for proper operation. Here are some steps you can take:

Inspect the High Limit Switch: Check if the switch is physically stuck in the open position. Sometimes, dirt or debris can cause the switch to malfunction.

Install a New High Limit Switch: If the switch is faulty, replacing it with a new one can resolve the issue. Ensure you choose a switch that meets the manufacturer's specifications.

Test the Control Board: If the high limit switch appears to be functioning correctly, the problem might lie with the control board. Replace the control board if necessary.

Check the Blower Motor: Even if the switch and control board are working properly, a faulty blower motor can cause continuous operation. Test the blower motor and replace it if needed.

Preventive Maintenance

To avoid such issues in the future, consider implementing a regular maintenance schedule for your furnace. This includes:

Inspecting and cleaning the high limit switch and related components.

Checking the integrity of all wiring and electrical connections.

Testing the blower motor and control board for any signs of malfunction.

Regular maintenance can help prevent unexpected issues with your furnace and ensure it operates efficiently and safely.