When furnace burners do not stay on, a home can quickly become uncomfortable and energy bills may spike. This article explains common reasons burners shut off prematurely, diagnostic steps, safety considerations, and practical solutions homeowners and technicians can use to restore reliable heating. It focuses on causes, troubleshooting, and repairs that align with best practices and safety standards.
| Symptom | Possible Cause | Typical Fix |
|---|---|---|
| Burner lights then goes out | Faulty flame sensor or draft issue | Clean/replace flame sensor, check venting |
| Inefficient ignition | Dirty burners or improper gas pressure | Clean burners, verify gas pressure |
| Intermittent operation | Limit switch or thermostat problem | Test/replace limit switch or thermostat |
| No continuous flame | Blocked condensate drain (condensing furnaces) | Clear drain, test safety float |
How Gas Furnaces Ignite And Stay Lit
Understanding the ignition sequence helps pinpoint why burners won’t stay on. Most modern furnaces use an electronic ignition (hot surface ignitor or intermittent pilot) controlled by the control board and monitored by a flame sensor or rollout limit switch.
When the thermostat calls for heat, the inducer motor starts, the ignitor heats or a spark ignites, gas valve opens, and burners ignite. The flame sensor confirms combustion and signals the control board to keep gas flowing. If any component fails, the board shuts the gas valve as a safety precaution.
Common Causes Of Burners Not Staying On
1. Faulty Or Dirty Flame Sensor
The flame sensor detects a stable flame. If it is dirty, corroded, or misaligned, it may fail to sense flame and shut the gas off after a few seconds. This is a frequent cause of burners lighting briefly then extinguishing.
2. Dirty Or Blocked Burners
Burners covered in dust, rust, or spider webs can produce a weak or unstable flame. Poor combustion may trip safety devices, leading to flame loss or poor heat output.
3. Improper Gas Pressure Or Gas Valve Problems
If gas pressure is too low, burners may light weakly and go out. A failing gas valve or regulator can also cause intermittent supply, requiring professional testing and adjustment.
4. Venting And Draft Issues
Blocked flue or improper draft can cause combustion gases to accumulate, triggering pressure switches or rollout sensors that shut the burners down for safety.
5. Inducer Motor Or Pressure Switch Failures
The inducer motor ensures correct venting during startup. If it fails or the pressure switch doesn’t close, the control board will not allow the gas valve to open or will shut it down.
6. Limit Switches And Rollout Switches
High-limit switches trip if heat exchanger temperatures exceed safe levels. A stuck or faulty limit switch will interrupt burner operation. Rollout switches react to flame escaping the combustion chamber and also shut the burners down.
7. Thermostat Or Control Board Malfunctions
An inaccurate thermostat or failing control board can send intermittent or incomplete signals, causing the ignition sequence to abort prematurely.
8. Condensate Blockage In Condensing Furnaces
Condensing furnaces drain condensate through a trap and drain line. A clogged line or blocked trap activates a switch that prevents burners from staying on.
Safety First: Precautions Before Troubleshooting
Working on gas furnaces involves risks. Always turn off power at the service switch and shut off the gas before performing internal inspections unless the step requires live testing by a qualified technician.
If there is any smell of gas, evacuate the area and call the gas utility or emergency services. For complex repairs such as gas valve replacement or flame rectification testing, hire an HVAC professional.
Step-By-Step Troubleshooting Guide
Initial Checks
Confirm thermostat settings, check air filters, and ensure the gas supply is on. Replace a dirty filter and reset the thermostat to rule out simple causes.
Observe The Ignition Cycle
Turn the thermostat up and watch the sequence: inducer start, ignition, burner light, then whether the burners stay lit. Note any error codes on the control board LED for guidance.
Inspect And Clean The Flame Sensor
Locate the flame sensor (a small metal rod near burners). Remove and clean gently with a fine abrasive pad to remove carbon build-up. Reinstall and retest. Cleaning often resolves short-run flame issues.
Check Burners And Ignitor
Inspect burners for debris and corrosion. Clean with a soft brush and vacuum. For hot surface ignitors, check for cracks; if cracked, replace the ignitor. Spark igniters should produce a strong blue spark.
Test The Pressure Switch And Inducer Motor
Listen for the inducer motor and ensure it runs smoothly. A multimeter can test continuity through the pressure switch; if it fails to close, check hoses and venting for blockage or replace the switch as needed.
Verify Gas Pressure And Gas Valve Function
Gas pressure checks require a licensed technician. Professionals use manometers and follow local codes to ensure safe and correct gas valve operation and regulator settings.
Examine Venting And Flue
Inspect the vent pipe for obstructions (bird nests, debris) and ensure joints are sealed. For direct-vent or power-vent systems, verify the exhaust fan operates properly and that intake air is unobstructed.
Check Condensate Drain And Float Switch
For condensing furnaces, clear the condensate drain and check the trap for algae or ice. Test any float switch that disables operation when the trap is full.
Inspect Limit And Rollout Switches
Test limit switches for continuity when the furnace is cool. Rollout switches often have a manual reset button; if they trip repeatedly, identify the root cause—such as blocked burners or cracked heat exchanger.
Review Control Board Error Codes
Many boards flash LED codes. Cross-reference codes with the furnace manual to identify the failing component. This narrows down repairs and avoids unnecessary parts replacement.
Common Repairs And When To Call A Pro
DIY Safe Repairs
- Cleaning flame sensor and burners
- Replacing air filters and clearing vents
- Resetting tripped rollout switches
- Clearing condensate drains
These tasks can often be completed by a competent homeowner using basic tools and following the manufacturer instructions.
Repairs Requiring A Professional
- Gas pressure checks and gas valve replacement
- Control board diagnostics and replacement
- Heat exchanger inspection for cracks (safety-critical)
- Complex inducer motor or sealed combustion repairs
When safety devices repeatedly trip, or when a smell of gas appears, contact a qualified HVAC technician immediately.
Preventive Maintenance To Keep Burners Stable
Regular maintenance reduces the chance burners won’t stay on. Schedule annual tune-ups before heating season to inspect and clean components, check gas pressure, and verify safe operation.
Replace air filters every 1–3 months, keep the furnace area free of clutter, and ensure vents and flues remain clear of debris and animal nests.
Costs And Typical Timeframes For Repairs
Costs vary depending on the issue: cleaning a flame sensor or burners might cost $75–$200 if performed by a technician. Replacing an ignitor or flame sensor typically ranges $100–$300 installed.
Major repairs like control board or gas valve replacement can range from $300–$1,200. Heat exchanger replacement or furnace replacement is often $1,500–$7,000 depending on unit size and model.
When Burners Keep Going Out Repeatedly: Safety Risks
Repeated burner shutdowns can indicate unsafe conditions such as cracked heat exchanger or gas leaks. A cracked heat exchanger can allow combustion gases, including carbon monoxide (CO), to enter living spaces.
Install and maintain CO detectors on each level of the home and never ignore persistent burner shutdowns. If CO alarms activate, evacuate and contact emergency services.
Useful Diagnostic Tools And How They Help
- Multimeter: Tests continuity of switches, sensors, and control circuits.
- Combustion analyzer: Measures combustion efficiency, CO, and O2 for tuning burners.
- Manometer: Verifies gas pressure (professional tool).
- Vacuum and soft brushes: For cleaning burners and combustion chambers.
Using the right tools ensures accurate diagnosis and safe repairs.
Tips For Talking With An HVAC Technician
Provide clear symptom descriptions: when burners light, how long they stay on, any odors, and error codes. Note the last maintenance date and any recent work done on the system.
Ask technicians to explain findings, show the failing part, and provide a written estimate. Confirm that replacement parts meet manufacturer specifications and ask about warranties.
Key Takeaways For Homeowners
Common Fixes: Clean flame sensor and burners first; this resolves many short-run issues.
Safety: Persistent shutdowns, gas smells, or CO alarms require immediate professional attention.
Maintenance: Annual inspections and regular filter changes reduce failures and extend furnace life.
Professional Help: Hire licensed HVAC technicians for gas pressure, control board, or heat exchanger work.
Additional Resources
Refer to the furnace manufacturer’s manual for model-specific error codes and procedures. For safety guidance, consult the U.S. Consumer Product Safety Commission and local utility providers for gas-related emergencies.
Local HVAC associations can provide lists of licensed professionals and consumer guides to hiring contractors.