Why Is My Furnace Fan Blowing Constantly

The furnace fan blowing constantly can waste energy and signal simple settings issues or more serious electrical or control faults. This article explains common causes, step-by-step troubleshooting, safety tips, and when to call a professional to restore normal system operation.

Common Cause	What To Check	Likely Solution
Thermostat Fan Set To On	Thermostat Fan Mode	Change To “Auto”
Stuck Fan Relay Or Contactor	Audible Clicks, Fan Runs Constantly	Replace Relay Or Control Board
Blower Motor Control (ECM) Settings	Motor Constant-Run Feature	Adjust Settings Or Replace Motor
Wiring Short Or C-Wire Issue	Recent Thermostat Changes	Inspect Wiring, Repair Short
Fan Limit Switch Or High-Limit Fault	Overheating Symptoms	Replace Switch, Check Heat Exchanger

Start with low-risk checks: thermostat fan setting, examining thermostat wiring visually with power off, and verifying that no fan schedules are active. Use a screwdriver to remove the furnace access panel only if comfortable and power is off.

If comfortable with basic electrical testing and with power off, verify that the G terminal on the control board is not shorted to 24VAC common (C). For live testing, use caution and prefer a professional when uncertain.

Step-By-Step Troubleshooting Checklist

1. Confirm Thermostat Fan Is Set To “Auto” Not “On”.
2. Inspect Thermostat Wiring: Ensure G Wire Is Correct, C Wire If Needed.
3. Reboot Or Reset The Thermostat To Clear Software Errors.
4. Turn Off Power And Inspect Furnace Wiring For Damage.
5. Check Fan Relay: Listen For Relay Clicks When Thermostat Calls For Fan.
6. Observe Motor Behavior: ECM May Run At Low Speed For Circulation.
7. Look For Error Codes Or LED Flashes On The Control Board.
8. If Overheating Or Strange Smells Occur, Turn Furnace Off And Call A Technician.

When To Call An HVAC Professional

If thermostat changes and basic wiring checks do not stop constant fan operation, a technician should inspect the furnace. Call an HVAC pro when: the control board or relay is suspected faulty, there are electrical shorts, the blower motor runs but other components fail, or there are signs of overheating or safety switch activation.

Licensed technicians have the tools to test 24VAC control circuits, motor windings, relay outputs, and to replace control boards or motors safely.

Energy Cost And Indoor Air Quality Trade-Offs

A continuously running fan increases electricity consumption and might produce mild HVAC wear but can improve air filtration and circulation. Using a central fan continuously can help distribute filtered air and maintain consistent temperatures in multi-level homes.

To balance energy and air quality, set the thermostat fan to “Auto” and use timed or smart air circulation features if filtration or humidity control is the goal.

Common Fixes And Expected Costs

Issue	Fix	Typical Cost Range
Thermostat Setting	Change Fan To “Auto”	$0
Thermostat Wiring Or Replacement	Rewire Or Replace Thermostat	$0–$300
Stuck Fan Relay	Replace Relay Or Control Board	$100–$600
ECM Motor Fault	Reprogram Or Replace Motor	$300–$1,000+
Limit Switch Replacement	Replace Switch, Inspect Heat Exchanger	$100–$400

Preventive Maintenance To Avoid Constant Fan Problems

Routine furnace maintenance reduces the risk of control failures. Recommended actions include annual professional tune-ups, replacing filters regularly, cleaning blower assemblies, and inspecting wiring for damage or corrosion.

Smart-home users should verify thermostat firmware updates and compatibility advisories. Keeping a maintenance log helps track changes that might correlate with new fan behavior.

Safety Considerations

Never attempt advanced electrical repairs without disconnecting power to the furnace. High-voltage and 24VAC control circuits can cause injury or damage. If the furnace shows signs of overheating, burning smells, or unusual smoke, turn off the system and ventilate the area.

For heat exchanger concerns, such as cracked components or carbon monoxide risks, cease operation and contact a certified HVAC technician immediately.

Key Takeaways And Practical Tips

• Check The Thermostat First: Most constant-fan issues are thermostat-related and fixable by changing the fan setting to “Auto.”
• Inspect Wiring After Thermostat Changes: Miswiring or missing C-wires can cause continuous operation.
• Listen For Relay Signs: A stuck relay commonly causes nonstop fan operation and usually needs replacement.
• Understand Motor Type: ECM motors can be programmed for continuous low-speed circulation; this may be intentional.
• Call A Professional For Electrical Or Safety Concerns: Control board, motor, or overheating faults require licensed service.

Additional Resources

For further help, consult the furnace manufacturer’s manual, thermostat installation guide, or resources from reputable HVAC organizations. Certified technicians can perform diagnostic tests such as control voltage checks, relay coil resistance testing, and motor current analysis to pinpoint the fault.

If the furnace fan continues to run constantly after basic checks, scheduling a professional inspection is the most reliable way to ensure safety and restore efficient operation.