The article explains why a furnace may fail to shut off, covering electrical, mechanical, and control-system causes. It offers actionable checks and repair guidance for homeowners and highlights when to call a professional. Understanding common causes and safe troubleshooting steps helps restore efficient heating and prevent damage or safety risks.
| Symptom | Likely Causes | Quick Remedies |
|---|---|---|
| Furnace Runs Continuously | Thermostat issue, shorted fan limit switch, stuck relay | Check thermostat settings, replace thermostat or limit switch, call HVAC tech |
| Burner Won’t Turn Off | Shorted gas valve, faulty control board, flame sensor problems | Power cycle, clean flame sensor, professional repair |
| Blower Won’t Stop | Stuck fan relay, defective fan control, overheating | Inspect relay, ensure proper airflow, replace relay or control |
Some repairs—gas valve replacement, control board swap, complex electrical diagnostics—require licensed HVAC technicians. Call a professional if there is a gas smell, if the homeowner is uncomfortable working with gas or high-voltage parts, or if simple troubleshooting fails.
What To Expect From A Technician
A qualified technician will perform safety checks, test the thermostat and control board, inspect the flame sensor and gas valve, measure voltages across relays, and check airflow. The technician should provide a clear repair estimate and explain parts replaced and preventative maintenance advice.
Preventive Maintenance To Avoid Continuous-Run Problems
Regular inspections reduce the risk of a furnace that won’t shut off. Replace filters every 1–3 months, schedule annual professional tune-ups, keep the area around the furnace clear, and observe for unusual noises or odors that indicate emerging issues.
- Change Filters: Improved airflow prevents overheating and control problems.
- Annual Tune-Up: Professional cleaning and calibration reduce sensor and ignition faults.
- Thermostat Check: Replace old mechanical thermostats and verify smart thermostat firmware.
- Watch For Rodent Damage: Protect wiring and insulation from pests.
Cost Considerations For Common Repairs
Repair costs vary by region and the extent of the problem. Thermostat replacement can range from inexpensive DIY models to higher-cost smart units when professionally installed. Control board and relay repairs typically cost more due to parts and labor. Gas valve replacement is one of the higher-cost repairs and must be done by a licensed technician.
| Repair | Typical Cost Range |
|---|---|
| Thermostat Replacement | $50–$300+ (installation extra) |
| Flame Sensor Cleaning/Replacement | $50–$200 |
| Control Board Or Relay | $200–$800 |
| Gas Valve Replacement | $300–$1,200 |
Safety Reminders
Working on furnaces involves gas, high voltage, and moving parts. Never ignore a persistent gas smell; leave and call professionals. Turn off power at the breaker before inspecting components. When in doubt, choose a licensed HVAC contractor to ensure safe, code-compliant repairs.
Quick Troubleshooting Checklist
- Verify thermostat is set to “heat” and fan to “auto.”
- Lower thermostat setpoint to test if furnace responds.
- Replace thermostat batteries and reboot smart thermostats.
- Replace air filter and check vents for obstructions.
- Inspect furnace for gas smells, loose wiring, or obvious damage.
- Clean the flame sensor and check the ignition sequence for errors.
- If burners or blower remain on, shut off power and call a technician.
Further Resources
For reliable guidance, consult the furnace owner’s manual, the U.S. Consumer Product Safety Commission for gas appliance safety tips, and EPA resources on indoor air quality. Local utility companies often provide trusted contractor lists and diagnostic help lines.
Key Takeaway: A furnace that won’t shut off usually stems from thermostat settings, control board or relay failures, flame sensing issues, gas valve faults, or airflow restrictions. Prioritizing safety, following the troubleshooting checklist, and scheduling professional service when needed will restore safe, efficient operation.