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How do you fix a yellow flame in a furnace?

A yellow flame in a furnace indicates incomplete combustion and can be caused by several issues such as lack of air supply, dirty burners, improper gas pressure, or a cracked heat exchanger. Fixing a yellow flame is important for safety, efficiency, and preventing carbon monoxide poisoning. This article will provide a step-by-step guide on diagnosing the cause and fixing a yellow furnace flame.

Check the Air Supply

The first thing to check is if there is enough air being supplied to the furnace for proper combustion. Furnaces require the right air-fuel mixture for complete burning of gas. Blocked air vents, a dirty air filter, or a malfunctioning draft inducer fan can restrict air flow leading to a yellow flame.

Here are some steps to check the air supply:

– Inspect the furnace air filter and replace if dirty. A dirty filter blocks air flow.
– Remove any blockages or debris around the exterior air intake and exhaust vents.
– Check if the draft inducer fan is running by listening closely when the furnace is on. Replace the fan if faulty.
– Inspect the blower fan operation and motor. If not blowing properly, it can impact air distribution.
– Look for signs of insufficient combustion air like soot around the burners.

Correcting any issues with air supply can help fix a yellow furnace flame.

Examine the Burners

Dirty or damaged burners can also prevent complete fuel combustion and cause yellow flaming. Follow these steps to inspect the burners:

– Turn off power to furnace and remove burner compartment cover.
– Check if burners are cracked, corroded or covered with debris. Use a wire brush to gently clean.
– Remove any dirt, soot or rust buildup on burners. Vacuum thoroughly.
– Ensure burner ports are free of obstructions. Use a needle to clear any blocked ports.
– Examine the burner flames when furnace is on. Flames should be blue and steady, not yellow or flickering.
– Replace burners if extensively dirty or defective. Match new burners to furnace specifications.

Properly cleaning and maintaining the burners can help fix issues with yellow flame color.

Check Gas Pressure

Improper gas pressure can also lead to incomplete combustion and yellow flames in the furnace. Here is how to check the gas pressure:

Step 1 – Turn off gas supply to furnace at the manual shutoff valve.

Step 2 – Remove the pipe plug from the gas control valve/regulator on the furnace. This is usually a screw-on plug.

Step 3 – Connect a manometer using the correct fittings. A manometer measures gas pressure.

Step 4 – Turn on the gas supply and check manometer reading with furnace on and off.

Step 5 – Compare manometer readings with manufacturer specifications on the rating plate. Gas pressure should be 4-13 inches of water column for natural gas, or 9-13 inches for propane.

Step 6 – If pressure is incorrect, adjust regulator on the valve clockwise (to increase) or counter-clockwise (to decrease).

Step 7 – Replace gas valve if unable to adjust pressure properly. Use valve specified by furnace manufacturer.

Proper gas pressure is essential for efficient combustion. Correcting pressure issues can resolve yellow flame problems.

Inspect Heat Exchanger

A cracked or damaged heat exchanger can also lead to incomplete combustion and yellow flames. The heat exchanger channels hot gases from the burner to heat air. A crack allows combustion gases to mix with supply air causing improper burner operation.

Here are steps for inspecting the heat exchanger:

– Turn off electrical and gas supply to the furnace.
– Remove outer panels to access heat exchanger portion.
– Examine heat exchanger surface for cracks, holes or perforations. Look for rust streaks.
– Check for signs of overheating like warped or discolored metal.
– Remove burner assembly for better visibility if needed.
– Perform a visual inspection with a flashlight and mirror.
– Conduct a leak test by pressurizing heat exchanger with air and applying soapy solution to check for bubbles.
– Replace heat exchanger if cracked or perforated. Only use manufacturer approved parts.

A defective heat exchanger is a safety issue and can lead to carbon monoxide leakage into the home. Properly identifying and fixing heat exchanger issues is crucial before operating the furnace.

Adjust Blower Speed

The blower speed affects air circulation which can impact flame color. An incorrect blower speed can lead to inadequate airflow for proper combustion. Here is how to check and adjust blower speed:

– Locate blower speed selection jumpers or dip switches on control board.
– Check speed setting against manufacturer recommended speed based on furnace model and BTU input.
– Adjust speed using jumpers or dip switches according to manufacturer instructions. Installing more jumpers increases speed.
– Recheck burner flame color after adjusting speed. Blue steady flames indicate good adjustment.
– Replace blower motor if unable to adjust speed properly. Use OEM replacement parts.

Adjusting the blower speed to manufacturer specifications can optimize airflow and fix some yellow flame causes.

Summary of Fixes

Here is a brief summary of the steps covered to fix a yellow flame in a furnace:

Issue Fix
Insufficient air supply – Check air filter, vents for blockage
– Ensure draft inducer fan is operational
– Check blower fan operation
Dirty burners – Clean burner ports and surfaces
– Remove soot and debris
Incorrect gas pressure – Check pressure with manometer
– Adjust gas regulator as needed
Defective heat exchanger – Inspect for cracks and holes
– Pressure test for leaks
– Replace if needed
Incorrect blower speed – Check speed setting
– Adjust speed per manufacturer

Diagnosing issues methodically and addressing the underlying cause is key to resolving yellow furnace flame problems for safe and efficient operation.

When to Call a Professional

For major repairs like heat exchanger issues, gas valve defects, or blower motor replacement, it is best to call a qualified HVAC technician. Professionals have the tools, expertise and training to correctly diagnose problems and fix complex furnace issues. They can identify any other underlying problems not visible through basic inspection. Safety should be the top priority when dealing with gas appliances, so don’t hesitate to call a technician for major repairs.

Prevention Tips

Here are some tips to help prevent yellow flame problems in your furnace:

– Replace air filter regularly as per manufacturer schedule, around every 1-3 months.
– Have an annual professional tune-up and cleaning done before winter.
– Check burners and heat exchanger for any cracks, corrosion or damage regularly.
– Keep all furnace vents and intake pipes clear of debris and obstructions.
– Monitor any changes in the color or behavior of the flame. Diagnose issues promptly.
– Ensure proper installation if any furnace modifications or repairs were done.

Proper maintenance and care can go a long way in preventing many yellow flame issues.

Safety Precautions

Observe these safety guidelines when working on your furnace:

– Turn off electrical and gas supply before any repairs.
– Allow system to cool down before servicing to prevent burns.
– Have a fire extinguisher on hand in case of accidental igniting.
– Test for gas leaks with a leak detector solution after working on gas lines.
– Never leave a malfunctioning furnace running with yellow flames due to risk of carbon monoxide poisoning.
– Ensure proper venting before restarting repaired furnace.
– Check for any unusual sounds or smells after repairs. Call a technician if anything seems abnormal.
– Refer to the manufacturer guide for any specific safety instructions.

Working safely is crucial when dealing with gas fired heating systems to prevent harm. Never hesitate to call an expert for assistance.


Diagnosing and fixing a yellow flame in a furnace requires a systematic approach to inspect the various combustion components and address the underlying issue. The most common problems involve insufficient air supply, dirty burners, improper gas pressure, heat exchanger defects, and incorrect blower speed. Following safety precautions is critical when servicing a furnace. For major repairs, always turn to qualified HVAC professionals. With periodic maintenance and prompt issue resolution, you can expect your furnace to operate safely all winter with a proper blue flame.