Why Your Gas Furnace Fan Keeps Running Continuously
A gas furnace fan that runs continuously, even when the heating cycle is complete, can be caused by several factors. Here are the common causes and a comprehensive troubleshooting guide to help you diagnose and fix the issue.
What Causes the Gas Furnace Fan to Keep Running?
- Incorrect Thermostat Settings
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The most common cause is that the thermostat is set to “ON” instead of “AUTO.” When set to “ON,” the fan will run continuously regardless of the heating cycle.
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Thermostat Issues
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A faulty thermostat or one that needs new batteries can cause the fan to run continuously. Check for proper calibration, battery condition, and wiring connections.
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Fan Limit Switch Issues
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The fan limit switch may be set to manual override mode, causing the fan to run constantly. This switch is typically located near the blower motor or in the control box.
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Faulty Wiring or Components
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Faulty wiring, a shorted thermostat wire, or a bad control board can also cause the fan to run continuously. These issues often require professional diagnosis and repair.
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Other Mechanical Issues
- A tripped flame rollout switch or a bad relay on the control board can also be the culprit. These issues are best diagnosed by an HVAC technician.
How to Troubleshoot a Gas Furnace Fan that Keeps Running?
- Check Thermostat Settings
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Ensure the thermostat is set to “AUTO” instead of “ON.” If it is set to “ON,” switch it to “AUTO” and adjust the temperature to a lower setting than the current room temperature. Wait for 3-4 minutes to see if the fan turns off.
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Inspect and Replace Thermostat Batteries
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If the thermostat display is blank or dim, or if the thermostat is not working consistently, replace the batteries. This is a simple fix that can resolve the issue.
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Check Fan Limit Switch
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Locate the fan limit switch, usually under the furnace hood. If the switch is in manual override mode (the white button is pushed in), pull it out to reset it to “AUTO” mode.
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Inspect Wiring and Components
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Use a multimeter to check for continuity and voltage issues. Ensure there are no shorted wires or faulty connections. If you suspect a wiring issue, it’s best to call a professional.
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Check for Tripped Safety Switches
- If the flame rollout switch has tripped, it may cause the fan to run continuously. This requires professional diagnosis and reset.
What Tools are Needed to Troubleshoot a Gas Furnace Fan Issue?
- Multimeter: To check for voltage and continuity issues in the wiring.
- Screwdriver: To access the fan limit switch and other components.
- New Batteries: If the thermostat batteries need to be replaced.
What are the Estimated Costs for Fixing a Gas Furnace Fan Issue?
- Thermostat Batteries: $5-$10
- Fan Limit Switch: $20-$50
- Control Board or Relay: $100-$300
- Professional Repair: $100-$300 (depending on the technician’s rates and the complexity of the issue)
By following this troubleshooting guide, you can identify the root cause of your gas furnace fan running continuously and take the necessary steps to resolve the issue. If you’re unable to diagnose or fix the problem, it’s best to call in a professional HVAC technician for assistance.
