Photograph of  this gas flame which gives a clue that there may be an operating problem and an unsafe gas furnace in this buildingBlue vs Yellow Flames & Efficient Oil or Gas Fuel Combustion

  • BLUE vs YELLOW COMBUSTION FLAMES - CONTENTS: Theory of Blueray & What It Means to be Combustion Efficient. Comparing the true efficiency of oil burning vs. gas burning heating equipment. Efficient oil burner & gas burner combustion theory. Theoretical Fuel to Air Ratios for Complete Combustion applied to the design of the BlueRay heating equipment line
  • POST a QUESTION or READ FAQs about Blue vs Yellow flames in heating equipment
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Gas flame colors indicate proper or improper equipment operation.

Here, courtesy of aerospace engineer Herman Vogel, we discuss the relation of blue flame and efficient oil or gas combustion in engines or in heating equipment.

Mr. Vogel explains how the blue flame theory found its way into the later unsafe BlueRay Heating equipment line. Our page top photo shows a blue-colored flame photographed on a heating boiler burner using natural gas fuel.

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Theory of Blueray and What it Really Means to be Combustion Efficient

Oil burner gun operation (C) D Friedman - AudelHerman Vogel, Aerospace Engineer

I used to design combustors for Pratt&Whitney Aircraft (P&WA).

Later on it was top-secret government work for the InfraRed Countermeasure world where we created combustor-driven systems that would confuse heat seeking missiles from acquiring hot jet-engine targets.

This was back in the late 60's early 70's, a few years before the concept of BlueRay came out!



Why Blue Flames Indicate Superior Combustion Efficiency

Our combustors used JP [jet propulsion] fuel and always ran a blue flame (yellow flames existed only in-between re-setting for new thrust states requiring readjustments in fuel and air flows).

Our sketch (at left) shows how an oil burner gun atomizes and sprays heating oil into the combustion chamber - Audel Oil Burner Guide [free online textbook.]

Measure a blue flame temperature and you get around 3,000F. Measure a yellow flame temperature and you get about 1,400 to 2,000F at best (depending on how well the combustor atomizes liquid fuel and mixes it with oxygen). Just from temperature alone, one recognizes the superior aspects of blue flames.

Blue flames run closer to stoichiometric conditions of combustion, that is they burn as if using a pure gaseous fuel as opposed to liquid.


Gases basically burn 100% efficient or stoichiometrically. Jet engine manufacturers like P&WA, GE, and RR all take great pains in trying to design their combustors to first convert liquid fuel into fine atomized liquid droplets, then convert these droplets into pure vapor before they are allowed to mix with pressurized air (only oxygen part of air) for near complete combustion.

This yields about 95+% efficient combustion today. What happens here is that the fuel surface area, available for mixing with oxygen and burning more effectively, increases by factors of 1000 over that available with only atomized droplets of fuel.

Blue Flame Efficiency and the History of BlueRay Heating Boilers in the 1970's

The BlueRay folks got wind of these concepts back in the early 70's (perhaps even hiring some of the jet engine engineers?) to create their widely touted and highly efficient combustion process technology.

It's a shame that their product couldn't survive the rigors required by both the residential and commercial heating furnace customers. Unfortunately, it took a lot more tender loving care to keep the BlueRay flame "blue" than your typical yellow flamed combustors. And as is pointed out
at BLUERAY Recall, poorly tuned BlueRay's had a tendency to burn sooty and rich, emitting Carbon, CO and NOx in their exhaust.

Two Measurements of Combustion Efficiency

If I burn a pound of fuel, and get 3,000F flame temperature out of it as opposed to 2,000F, I'm getting more heat (efficiency) out of the same amount of fuel. In fact, true thermodynamic combustion efficiency of any combustor (jet engine, car, furnaces, etc.) can be measured in two ways, with both relying on the stoichiometric chemical reaction formula for complete combustion;

  1. We theoretically calculate the released adiabatic ideal heating value (IHV), per unit fuel, assuming complete burning and compare it to its measured heating value (MHV) obtained from a furnace-gun and calorimetric system, the absolute combustion efficiency is then simply MHV/IHV
  2. We theoretically calculate the released adiabatic ideal flame temperature (IFT) value per unit fuel with complete burning and compare it to its measured flame temperature (MFT) value using a high temperature, platinum-rhodium thermocouple (Pt-Rh TC) held directly into the flame, the absolute combustion efficiency is then simply (MFT - Tin)/ (IFT - Tin). Since a TC is simpler and easier to use than gathering data with a calorimetric setup, option#2 is recommended. For JP fuels: IHV = 18,950 BTU/Lbm, and IFT = 3,850F.

    Therefore yellow flames have an absolute maximum burning efficiency of roughly (2,000 - 70) / (3,850 - 70) = 51%, while blue flames have an absolute maximum burning efficiency of (3,000 - 70) / (3,850 - 70) = 78%.

What is the True Efficiency Indicated by Yellow Flames? - How Oil Competes with Gas as a Heating Fuel

Now, if a yellow furnace system (with heat gun, combustion-chamber and exhaust-pipe) can at best only deliver about 51% efficient running, according to absolute thermodynamic principles, how do furnace companies and maintenance technicians claim efficiencies of 85% or even 90% or better? They have collaborated, industry wide, to re-define oil-burning efficiency according to more favorable terms that can compete with the gas industry.

While there is nothing wrong with that, the basic problem lies in allowing them to conveniently forget that they should not compare their home-grown heating oil efficiency to "real" and absolute efficiency values as with burning methane or natural gas in furnaces or even JP-fuels in jet engines.

Comparing colours of gas flames: yellow = unhealthy, yellow tips = normal for LPG or propane, soft blue flame = normal for natural gas (C) Daniel Friedman adapted from Bosch (2014)

[Click to enlarge any image]

Our gas flame colour illustration shown at left, adapted from Bosch (2014),shows the properties of

More details are

A Comparison of Oil Fuel vs Gas Fuel Efficiency in Heating Equipment

Consumers don't know what they are really getting when the oil industry compare apples to pears. When a fuel-oil furnace that is burning No#2 home heating oil is said to burn 100% efficient (based on non-stoichiometric reactions, defined by the furnace industry as standard, and using CO and CO2 exhaust product ratios to "relatively" redefine efficiency), their real furnace flame temperature would actually measure only about 2,000F at best.

Obviously, this is nowhere near 100% operation based on the above calculated theoretical analysis. Now as their hypothetical flame decreases in temperature, the CO and CO2 fractions will change, and, depending upon the ratio of these changes, a new burner efficiency of under 100% gets quoted.

Again, these are not absolute thermodynamic values, rather they are relative burning efficiencies based on a reference flame temperature of 2,000F which was arbitrarily chosen to represent 100% combustion efficiency. One may as well state that if your furnace has been tuned and reads 85% efficiency, as arbitrarily based on its emissions of CO and CO2 , it is really burning at [51% x 0.85 or] 43% efficiency in terms of "absolute thermodynamic efficiency".

Compare this to any gas furnace burning 100% in "absolute thermodynamic efficiency" and you are effectively throwing out almost half of your heating oil value.

How Oil Fuel is Competitive with Gas Fuel for Heating buildings

While this is shocking and true, remember that oil heat has as yet certain redeeming qualities.

It is still cheap enough to allow this disparity to happen and yet be competitive in heating your home because of its 40% greater heating value and 7% greater flame temperature (IHV=18,950 BTU/Lbm, IFT=3,850F) compared to natural gas (IHV=13,660 BTU/Lbm, IFT=3562F).

While BlueRay technology tried to take advantage of this disparity in efficiency, sadly they lost out to the need for frequent maintenance visits and to keep the unforgiving technology properly tuned or produce killing CO gases.

Bad Design and BlueRay - Design Products For What People are Likely to Actually Do

Blueray II heating boiler - read the history of Blueray on this page.OPINION-DF: Perhaps the BLUERAY Recall and carbon monoxide hazards had as a root cause, the mistake of designing an oil burner that required a high level of expertise and great care in following tuning instructions precisely.

For example, when adjusting the air-fuel mix by changing the air shutter opening on the oil burner, the direction of change, from more lean to more rich versus adjusting from the more rich to the more lean position could be enough to leave the oil burner adjusted to an unsafe position that would produce dangerous carbon monoxide. Here is what the company's service bulletin said:

When setting the unit for proper air mixture always start with the air band fully open, close it until proper CO2 reading is reached.

Close it further - if CO2 continues to climb you are on the "right side of the curve" and should then re-open the air band to proper CO2 reading. If it does not continue to climb you are on the "wrong side of the curve" in air-starved mode and are producing carbon monoxide (CO).

The subtlety of having to care about the direction from which one makes an adjustment to a common oil burner device, when either way the adjustment appeared to end at the same setting was perhaps too much to ask of traditional oil heat service technicians who were accustomed to more than sixty years of oil burners hat were wonderfully tolerant of rough handling.

Across a very wide range of discussions of construction problems and failures, we often return to this point. Good product design should provide for what people are likely to do (in installation, service, maintenance, use) rather than what the designer thinks they should do.

This article series answers most questions about central heating and water heating systems to aid in troubleshooting, inspection, diagnosis, and repairs.


Continue reading at COMPLETE COMBUSTION, STOICHIOMETRIC - explaining the complete combustion of fossil fuels and the details as well as the significance (to non-engineers) of Stoichiometric Combustion, or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Or see BLUERAY Recall


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