The high limit switch in a gas furnace is a crucial safety device that prevents the furnace from overheating, which can lead to damage or safety hazards. When this switch keeps tripping, it can be a frustrating and concerning issue for homeowners. In this comprehensive guide, we’ll explore the common causes of a high limit switch tripping and provide step-by-step troubleshooting procedures to help you resolve the problem.
What is the Reason for the High Limit Switch Tripping?
The high limit switch in a gas furnace is designed to shut off the furnace when the temperature inside the unit exceeds a certain threshold, typically around 200°F to 250°F. This safety feature is in place to prevent the furnace from overheating, which can lead to damage or even a fire hazard. However, if the high limit switch keeps tripping, it’s a sign that there is an underlying issue that needs to be addressed.
Common Causes of a High Limit Switch Tripping in a Gas Furnace
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Dirty Air Filters: Clogged air filters can significantly restrict airflow, causing the heat exchanger to overheat and triggering the high limit switch. Regularly cleaning or replacing the air filters is essential.
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Blocked Vents: Obstructed vents or registers can prevent the proper circulation of air, leading to overheating and the high limit switch tripping. Ensure that all vents and registers are clear and unobstructed.
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Inefficient Blower Motor: A blower motor that is not providing sufficient airflow can cause the heat exchanger to overheat, leading to the high limit switch tripping. Checking and potentially replacing the blower motor may be necessary.
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Dirty or Faulty Flame Sensor: A dirty or malfunctioning flame sensor can give false readings, leading to excessive gas buildup and overheating. Cleaning or replacing the flame sensor may be required.
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Clogged “A” Coil: In systems with an “A” coil, debris can accumulate and clog the coil, affecting airflow and causing overheating. Cleaning or replacing the “A” coil may be necessary.
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Faulty High Limit Switch: In some cases, the high limit switch itself may be the problem, and replacing the switch may be the solution.
How to Troubleshoot a Gas Furnace High Limit Switch That Keeps Tripping
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Check Air Filters: Inspect the air filters for dirt and debris, and clean or replace them if necessary.
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Inspect Vents: Ensure that all vents and registers are clear and unobstructed.
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Evaluate Blower Motor: Check the blower motor for efficiency, and consider replacing it if it is old or inefficient.
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Clean or Replace Flame Sensor: Locate the flame sensor, clean it using steel wool or sandpaper, and replace it if cleaning does not resolve the issue.
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Check “A” Coil: Inspect the “A” coil for debris, and clean or replace it if necessary.
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Test High Limit Switch: If all other causes are ruled out, test the high limit switch itself and replace it if it is faulty.
Relevant Calculations and Thresholds
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Temperature Thresholds: The high limit switch typically trips when the temperature exceeds a range of 200°F (93°C) to 250°F (121°C), depending on the furnace model.
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Airflow Requirements: Proper airflow is essential, and a general rule is to ensure that the furnace receives at least 400 CFM (cubic feet per minute) of airflow per ton of furnace capacity.
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Filter Sizes: Ensure that the air filters are the correct size for the furnace. Using filters that are too small can restrict airflow, while filters that are too large may not fit properly.
Recommendations for Maintenance Practices
To prevent future occurrences of high limit switch tripping, it’s essential to:
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Regularly Clean Air Filters: Clean or replace air filters every 1-3 months, depending on usage and environmental conditions.
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Check for Airflow Obstructions: Regularly inspect vents and registers to ensure they are clear and unobstructed.
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Schedule Annual Maintenance: Have an HVAC technician perform annual maintenance to inspect and clean the blower motor, flame sensor, and other components.
By following these troubleshooting steps and maintenance practices, you can help ensure that your gas furnace’s high limit switch functions properly and prevent future issues.
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