Keeping your furnace in healthy working is essential for ensuring safe and efficient operation. The flame color indicates whether or not your furnace is burning its fuel safely and efficiently or not.

For homes with furnaces that burn fuel, it’s always a good idea to check out the flame on the burner on a regular basis. The color and behavior of the flames can be a good indicator of the health of your heating system. If you notice that the flames are flickering or if they are any other color than blue, you should shut off your heating system immediately and call a professional.

Flickering flames and incorrect flame colors (anything other than blue) are indications that your furnace is leaking wasting energy and leaking carbon monoxide.

What Color Should My Furnace Flame Be?

The burner flames from your natural gas furnace should look blue or almost completely blue. A healthy natural gas furnace flame is characterized by a roaring blue flame with a light blue triangle in its center. There may be a tiny tip of yellow. A blue flame indicates safe and efficient combustion, meaning that the gas is being burned efficiently and not being wasted. Blue flames are hotter and indicate more complete combustion.

If your flame is yellow or orange, or if you see pops of yellow or orange, that is a sign that you have a combustion problem that should be addressed by a professional. Long streaks of any color, such as yellow, orange, or green means your furnace needs adjustment or cleaning from a professional HVAC technician.

Whether it is your gas stove, gas furnace, or any other gas appliance, you always want a blue flame.

If your furnace burner flames are any other color than blue, turn your furnace off and contact a licensed HVAC contractor for service right away!

Warning: NEVER attempt to fix a furnace flame’s color on your own. Correcting furnace combustion is a job best left to a professional. Learn more about the dangers of DIY furnace repair.

Why Does a Blue Flame Indicate More Complete Combustion?

If you remember the Bunsen burner days of your science education, you may recall how different flame colors result from varying amount of oxygen.

By increasing the oxygen supply, you get higher temperatures, less soot, and more complete combustion. When the flame does not receive enough oxygen, you can see the flame color change to a red, orange, or yellow color. You may even notice dark soot rising from the tip of the flame.

Due to the lack of oxygen, the flame is not as hot and although you cannot see, smell, or taste it, more carbon monoxide is released into the air. The coolest flame is a yellow/orange color.

Depending on the gas involved, different amounts of oxygen are required for complete combustion. For instance, propane requires an air-to-gas ratio around 24:1. For natural gas, the ratio is closer to 10:1.

When using a Bunsen burner, you can adjust the color of the flame by opening or closing the air holes. Unfortunately, adjusting furnace flame color is not so easy. Never attempt to adjust furnace flames on your own—always contact a professional.

What Is the Danger of Incorrect Flame Colors?

A blue flame from your furnace or gas stove indicates complete combustion of the gas. If you have a yellow, orange, or red burner flame, this usually means that your burner is not receiving enough air for complete combustion.

Besides wasted gas, higher energy bills, and more soot, the main danger of improper combustion is the increased amount of carbon monoxide (CO) produced by the combustion process.

Carbon monoxide, known as the “silent killer,” is an odorless, colorless, and tasteless gas that can lead to headaches, nausea, hallucinations, and possibly blackouts and death. Many people describe the symptoms of carbon monoxide poisoning as flu-like.

According to the Centers for Disease Control and Prevention (CDC), over 400 Americans die from unintentional CO poisoning every year and more than 20,000 Americans visit the emergency room due to carbon monoxide leaks.

The gas is produced whenever fuel is burned. When it binds to hemoglobin the blood,

Due to the seriousness of CO poisoning, we highly recommend installing CO detectors on every level of the home and outside of each sleeping area. 

Additional CO Safety Tips Include:

  • Check or replace CO batteries during Daylight Saving Time in the spring and fall.
  • Consider purchasing and installing low-level CO monitors or CO monitors with digital readouts. Schedule professional inspections every year for your heating system, water heater, and all other gas, oil, and coal-burning appliances.
  • Your CO detector should be battery-powered or have a battery backup.
  • Interconnected CO detectors are best.
  • Make sure your gas appliances are venting properly. Periodically check your outdoor vents to make sure they are not blocked by leaves, snow, nests, debris, or anything else.
  • Get your chimney checked or cleaned every year.
  • Never use your gas range or oven for heating.
  • Never use a generator or portable gas heater indoors or an enclosed space. Your generator should be at least 20 feet from any door, window, or vent.
  • Never run a car, truck, or automobile inside of a garage, even if the door is open.
  • If your CO detector goes odd while operating your indoor fuel-burning appliance, turn off the appliance and do not turn it on again until a professional has inspected it. Evacuate the household and contact your gas company and fire department.

Signs of Improper Combustion

In addition to yellow or red flames, pay attention to the flame pattern. Is it wavering or irregular in any way?

You may also notice an accumulation of soot around the furnace or gas-powered appliance. Other signs of unsafe and incomplete combustion include acrid smells and carbon monoxide detectors going off.

What Should I Do to Prevent Improper Furnace Combustion?

The best thing you can do as a homeowner is to be aware of the problem and periodically check the colors of your gas furnace. Incomplete combustion is a serious safety issue for your home and family.

  • Schedule professional furnace/heating maintenance at the beginning of every heating season, preferably before you first need the services of your furnace.
  • Inspect the color of your furnace flame color at the beginning of every heating season and every month thereafter.
  • If your flame is wavering or any other color than blue, contact a professional HVAC contractor right away. The furnace’s burners should be inspected and cleaned for proper operation.
  • Dirty burners can become blocked, leading to incomplete combustion and an accumulation of soot inside the furnace.

In conclusion, blue flames indicate that your gas appliance is operating safely and efficiently. If you notice your gas flame is not blue, turn the unit off and contact a professional for service. With a blue flame, you’ll save money and be a lot safer.

Learn more heating safety and energy-saving tips for your home.


For your next furnace service, contact OnTime Service:  (205) 872-1944 .

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