The furnace flame turning off when the blower fan starts is a common and potentially dangerous HVAC issue. This article explains likely causes, diagnostic steps, safety considerations, and repair options so homeowners can understand what to expect and when to call a professional. Follow safety precautions before attempting any inspection or repair.
| Symptom | Possible Cause | Typical Fix |
|---|---|---|
| Flame extinguishes As Blower Starts | Draft or limit switch trips; pressure switch issue | Inspect venting; replace switch; clear blockage |
| Short delay, then flame out | Weak flame/low gas pressure; faulty gas valve | Measure pressure; adjust or replace valve |
| Fan runs but no ignition | Control board or relay failure | Test and replace control board or relays |
How A Gas Furnace Is Supposed To Work
A furnace ignition sequence typically includes the thermostat calling for heat, induction of the blower or draft inducer, ignition of the burner(s) and continuous combustion while safety switches confirm proper draft and temperature. The blower or main fan usually starts after burners are firing and heat exchangers reach a set temperature, depending on system type. Any interruption in the safety or control circuit can extinguish the flame.
Common Causes Why Flame Goes Out When Fan Starts
1. Faulty Pressure Switch Or Draft Inducer Issues
The pressure switch confirms the inducer fan creates proper draft before the control board allows ignition. If the switch is marginal, it may pass the initial check but open when the main blower starts changing airflow and pressure. Symptoms include intermittent ignition and error codes related to pressure or inducer failures.
2. Limit Switch Or Heat Exchanger Overtemperature
The high-limit switch shuts burners off if the heat exchanger temperature exceeds safe limits. When the fan starts it can change airflow and heat transfer, causing a limit switch to trip or a weak limit sensor to falsely detect overheating, extinguishing the flame. Dirty filters or restricted ducts increase this risk.
3. Gas Supply Problems Or Weak Gas Valve
A slight drop in gas pressure when the fan motor draws additional power or when multiple gas appliances operate can extinguish the flame. A weak or failing gas valve may not maintain sufficient manifold pressure when blower motors and other loads run. Listen for unusual gas valve sounds or detect flame instability.
4. Control Board Or Relay Malfunction
The furnace control board coordinates ignition and blower operation. A failing relay or board component may energize the blower relay in a way that interrupts the gas valve circuit or ignition control, causing the flame to drop out when the fan starts. Look for burnt traces, error LEDs, or intermittent behavior.
5. Flame Sensor Dirty Or Misaligned
The flame sensor detects the presence of flame to keep the gas valve open. If the sensor barely senses flame because it’s dirty or misaligned, the additional airflow or vibration when the blower starts can momentarily reduce the sensor signal, causing the board to close the gas valve. Cleaning the sensor often resolves this issue.
6. Electrical Voltage Drop Or Weak Blower Motor
If the blower motor draws excessive current at startup, voltage to the ignition or control circuits may dip, causing the flame to extinguish. Conversely, a motor on the brink of failure can create electrical noise or voltage instability that interferes with the control board. Check voltage during startup and inspect motor condition.
Safety First: Precautions Before Troubleshooting
Turn off power and gas at the furnace before inspecting internal components. Use a multimeter only if competent with electrical systems. If there is any smell of gas or consistent flame failure, evacuate and call the gas company or a licensed technician. Carbon monoxide risk increases with incomplete combustion or blocked venting.
Step-By-Step Troubleshooting Guide
Tools And Materials Needed
- Multimeter with AC/DC and continuity
- Screwdrivers and nut drivers
- Shop vacuum and brushes
- Replacement flame sensor and air filter (if needed)
- Manufacturer wiring diagram or manual
Visual And Basic Checks
Inspect vents and flue for obstructions such as bird nests or debris. Replace dirty air filters and confirm duct dampers are open. Look for error codes or LED blink patterns on the furnace control board and record them for reference.
Testing The Flame Sensor
Turn off power, remove and visually inspect the flame sensor. If it is coated with soot, clean gently with fine emery cloth or fine steel wool, then reinstall. Restore power and test operation. A cleaned sensor often cures intermittent shutdowns linked to blower activation.
Checking Pressure Switch And Venting
With the furnace running, observe the inducer motor and listen for steady operation. Use a manometer or a simple tube manometer if available to measure draft switch differential. If the pressure switch opens when the main blower starts, verify vent restrictions, gasket integrity at the inducer housing and the switch tubing for blockages or cracks.
Inspecting Limit Switches And Airflow
Replace very dirty filters and confirm return air pathways are not blocked. Temporarily bypassing a limit switch for testing is not recommended due to safety risks, but technicians can test switch continuity and temperature actuator operation. Persistent trips often indicate airflow restriction or failing switches.
Electrical Tests
Measure line voltage at start and during blower activation for significant dips. Check control board relays for proper operation and inspect wiring connections for corrosion or loose terminals. If voltage sags or relays chatter, further electrical diagnosis is needed.
Repair Options And Typical Costs
Estimated costs vary by region, furnace model, and whether a licensed technician is required. These figures are approximate ranges for U.S. homeowners.
| Repair Item | Typical Cost Range |
|---|---|
| Clean/Replace Flame Sensor | $50 – $150 |
| Replace Pressure Switch | $150 – $350 |
| Replace Limit Switch | $100 – $300 |
| Control Board Replacement | $300 – $800 |
| Gas Valve Replacement | $250 – $600 |
| Blower Motor Repair/Replace | $300 – $900 |
These costs include parts and labor; emergency or weekend service can raise prices. Always obtain an itemized estimate from a licensed HVAC contractor.
When To Call A Professional
Contact a licensed HVAC technician if any of the following apply: persistent flame outages, error codes indicating combustion safety failures, suspected gas leaks, or if troubleshooting requires gas valve or control board replacement. Licensed contractors have tools for pressure, combustion, and electrical diagnostics, and can test for carbon monoxide and gas pressure safely.
Preventive Maintenance To Avoid Future Problems
- Change air filters every 1–3 months depending on usage and filter MERV rating.
- Schedule annual professional furnace inspections and combustion analysis before the heating season.
- Keep vents and chimney/flue clear of obstructions, and install bird guards if needed.
- Clean flame sensors, burners and inducer housing periodically.
How To Document Issues For A Service Call
Record the thermostat behavior, exact sequence of events, and any LED error codes or patterns. Note whether the flame goes out immediately when the blower begins or after a short delay. Take photos of the wiring diagram and label disconnected wires when troubleshooting to help the technician diagnose faster.
Frequently Asked Questions
Is It Dangerous If The Flame Goes Out When The Fan Starts?
Yes. Repeated flame interruption can lead to incomplete combustion and carbon monoxide risk. It also stresses the gas valve and control components. If the issue occurs frequently, stop using the furnace and call a professional.
Can A Homeowner Replace The Pressure Switch Or Gas Valve?
Pressure switches can sometimes be replaced by experienced DIYers, but gas valve replacement involves working with gas and should be performed by a licensed technician to ensure safety and code compliance.
Will Replacing The Air Filter Fix This Problem?
A dirty filter can cause airflow-related limit trips that extinguish the flame, so replacing the filter may resolve the issue if airflow restriction is the primary cause. It is not a universal fix.
How Long Will A Repair Take?
Minor repairs such as sensor cleaning or filter replacement can take under an hour. Replacing a control board, blower motor, or gas valve may take several hours including testing and combustion tuning.
Key Takeaways For Homeowners
Intermittent flame loss when the fan starts usually points to pressure, airflow, control, or gas supply issues. Prioritize safety: shut off the furnace and call a licensed HVAC professional for persistent or unclear problems. Regular maintenance reduces the chance of these issues and extends furnace life.
Additional Resources And References
Homeowners seeking more technical guidance can consult furnace manufacturer manuals, local building codes, and resources from organizations such as the Air Conditioning Contractors of America (ACCA) and the U.S. Consumer Product Safety Commission for carbon monoxide safety information.