Is There a Problem Behind Furnace Flame Color: Blue, Green, Orange

Blue furnace flame is ideal – the color you want to see when you take off the panel/cover and look into the burners.

What does an orange furnace flame mean? Why is my furnace flame yellow? These colors are not ideal, and by that, we mean inefficient and even potentially dangerous.

Even if this concept is new to you, checking the color of the burner flames might be something that interests you.

The Color of Burner Flames in a Properly Functioning Furnace is Bright Blue

The burner flames of a properly functioning natural or propane gas furnace should be predominately blue. The flame from each burner should have a “whooshing” sound and have a lighter blue triangle in its center. The flames should be elongated as their heat is directed into the holes or channels of the heat exchanger. There may also be a very small tip of yellow on the flames, but the main color should be blue.

What Causes Gas to Burn with a Blue Flame

A blue flame indicates that the gas is clean – there are few impurities and little moisture in it – and, importantly, that it is burning thoroughly. It must have a sufficient supply of oxygen for complete, blue, combustion.

Homeowner Tip: It is a good idea to check furnace flame color a few times each heating season to ensure your furnace is burning true blue!

If you enjoy the technical side of HVAC equipment, the next couple paragraphs might interest you. For fuel, furnaces use either natural gas or propane gas, also called, “LP” for liquid propane and “LPG” for liquified petroleum gas. Same stuff. Both are categorized as “pure hydrocarbons” and are widely used as fuels for heating in countless homes worldwide.

Being pure hydrocarbons, they burn with a clean, hot blue flame. This is because when they are ignited, the result is complete combustion of the carbon molecules.

Of course, in order to burn, the gas must also be mixed with oxygen. With the proper mixture of either of these gases and oxygen, natural gas will produce that pure blue flame at a temperature of about 3,560°F, while propane burns slightly hotter at about 3,600°F.

Did you know? Any gas appliance that you have, such as a gas stove, gas grill, or gas furnace, will burn with a bright blue flame when combustion is efficient, complete and not producing harmful amounts of gases like carbon monoxide. An exception to this is a gas fireplace, which is designed to have more orange and yellow in the flames to make it look more realistic – like a wood-burning fireplace.

What Causes a Red, Orange or Yellow Flame

Now, to get more specific. The color of a flame is a result of two things: the makeup of fuel that is being burned (what it is and what’s in it) and the temperature of the flame. We have already said that pure hydrocarbons such as natural or propane gas will burn with a blue flame. Many other materials that will burn are made of a variety of other chemicals and compounds that do not burn blue. The most common other flame colors are red, orange, and yellow, with the main color also being an indication of their temperature.

So, why is furnace flame orange, yellow or red? Even green? Dirt in the furnace, impurities or water in the gas are major causes.

Think about wood in a fireplace or campfire. It usually burns bright orange because of the presence of sodium in them. Pine and other coniferous trees contain a lot of turpentine and resins which cause them to burn fast and hot – giving off a flame more yellow than orange. Oaks and other very hard woods are harder to get burning, but burn more slowly, giving off an orange flame – not as hot as Pine, but longer-lasting.

When it comes to natural gas and propane, any color but blue is an issue that must be addressed.

The Problem with Yellow or Orange Flames in a Furnace

Since the color of a properly functioning natural or propane gas furnace flame is blue, the presence of yellow, orange, or even green flames tells us that something is wrong. Here are common reasons for furnace flame to be orange or yellow.

Dirt in the Burner

This is the most common cause.

There could be built up dirt or a small accumulation of soot in the orifice of the burner – or it could be something else. And, as we have seen, any flame other than blue is not as hot. This means that your furnace will not be operating at peak efficiency. And it won’t be cranking out as many BTUs of heat as it should be, leaving your home or rooms farthest from the furnace inadequately heated.

Check this out – You can clean your own furnace burners! For ambitious and handy homeowners, we have completed a How to Clean Furnace Burners guide with step-by-step instructions and other useful tips.

Bad Oxygen-to-gas Mixture

The mixture of gas and oxygen may not be correct. One common reason is that the gas orifice is the wrong size, and too much gas or too little gas is being released, which throws off the proper air-to-gas ratio. Some adjustments can be made, but adjusting the fuel mix is best left to a furnace repair technician.

Improper Combustion

A bad oxygen/gas mix often results in incomplete combustion – and that is a major cause of excess carbon monoxide being produced and potentially leaking into the living areas of your home. Carbon monoxide is a silent, odorless killer, as the warning goes, so a furnace flame the wrong color should not be ignored.

Moisture in the Mix

This isn’t as common as a bad oxygen blend, but it can happen. It might be a bad supply of gas from your propane company. Did you notice the orange or yellow furnace flame shortly after having your tank filled? Call to ask them if there have been other complaints.

Or it could be moisture getting into the burner from a humidifier attached to your furnace (if you have one) or one elsewhere in your home. Sounds odd, but it is possible. A reliable HVAC technician says, “if the water [from a humidifier] contains salts or minerals, any salts or minerals in the water will end up floating around the house as tiny specks. Those, evidently, are what is contaminating your gas flames as they are sucked in along with the air required to burn the gas.” The homeowner in this instance turned off the humidifier, and the color of the flame went back to blue from orange.

Pro Tip: If you have a gas range, what color is its flame? If the flame on the range or any other appliance is also orange or yellow, then it could be moisture in the gas supply or contaminated moisture in the air caused by the humidifier. Start by turning off the humidifier. If that doesn’t improve flame color, call an HVAC technician.

Here is a clue. Any popping, flickering, or other abnormal flame pattern is an indication that the furnace is not operating properly. If you observe any of these signs, they should not be ignored. They can be an indication of a more serious problem that could result in much wasted energy, damage to your furnace, or harm to the inhabitants of your home from excess carbon monoxide.

How to Fix a Yellow or Orange Flame on a Gas Furnace

If your furnace runs heavily in winter, and hasn’t been cleaned in the last few years, it is due for maintenance. You can clean the burners yourself – we linked above to the Pick HVAC page on the subject. You can go deeper and clean the heat exchanger with this guide.

Remember the humidifier issue too, and give that a try if your home has one.

Beyond that, we recommend consulting an HVAC professional for guidance, furnace maintenance and repair.

We can’t recommend that you attempt to make adjustments to the oxygen mix on your gas furnace. The best thing you can do is to contact a reputable HVAC repair service in your area. If the problem is minor, they will be able to correct it fairly quickly. If it is more serious, they will inform you of the action that needs to be taken.

Written by

Rene has worked 10 years in the HVAC field and now is the Senior Comfort Specialist for PICKHVAC. He holds an HVAC associate degree and EPA & R-410A Certifications.

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