March 1, 2014

Over the past three or four weeks we have been advised of several tragedies resulting from carbon monoxide poisoning.   Due to the ridiculously cold winter we have experienced this year heating systems have been running just about non-stop.  I have several friends in the northern part of our country that have heating bills topping the $700 per month level.  Gas-fired heating systems MUST operate properly or death can occur as a result of an over-abundance of carbon monoxide in the products of combustion.  This, unfortunately, has happened all too frequently this winter.  Let’s take a very quick look at how this can occur, and why proper maintenance of heating systems is absolutely critical.


Carbon monoxide (CO) is a colorless and odorless gas that is decidedly poisonous.   CO is NOT found in natural gas but is generally the product of incomplete combustion.   The presence of carbon monoxide in the products of combustion has an important relationship to combustion efficiency.  CO (2) is a measure of complete combustion whereas CO is a measure of incomplete combustion.   The American Gas Association (now the Canadian Gas Association) says a maximum of 800 parts per million (PPM) for CO (carbon monoxide) must not be exceeded for safe operation.  In the United States, the two most-used gases are natural (primarily methane) and liquefied petroleum (LP).   Every gas-fired burner must exhibit CO levels below 800 PPM for acceptability.  The effects of carbon monoxide may be seen from the following chart:



You can now see why the 800 PPM is considered to be the very maximum level of CO in products of combustion.  CO can be a very DEADLY constituent resulting from incomplete combustion.

In looking at atmospheric gas burners, we note that two (2) sources of air are available; i.e. 1.) Primary and 2.) Secondary.


Air which is mixed with gas before that gas/air mixture is ejected and ignited at the ports is called primary air.  A definite minimum amount of primary air is required for complete combustion no matter what gas type is being used.  That minimum percentage of primary will vary depending upon the type of gas, its specific gravity and its heating value.


All other air supplied to the burner is called secondary air.  In an ideal case, primary air and secondary air should be 100% of the air needed for complete combustion.  All air supplied is called total air.

Ideally, 10 cubic feet of air is needed for 1 cubic foot of natural gas if complete combustion is to be accomplished.  Suppose that 15 cubic feet of air is supplied to a burner for each cubic foot of gas.  The total air is 150% (15/10 X 100%).  Excess air is 50% (150%-100%).  Now suppose that 5% of the air supplied from that 15% is primary air.  The remaining 10 cubic feet would be secondary air and excess air.

Approximately 20 cubic feet of air is needed for 1 cubic foot of propane and 30 cubic feet of air is needed for butane.  Please note that this is a volumetric measurement with the respective flow rates being in FT³/ Hr.  It is proper to consider Ft.³/Hr of fuel gas combined with Ft.³/Hr of primary air for complete combustion.  This mass flow rate would be the total mass flow rate for the mixture of gas and air entering the mixing tube.

Ideal burner characteristics may be noted as follows:


  1. Blue flame with possibly some yellow tips when using propane or butane as the fuel. ( NOTE:  These burners are firing natural gas with a heating value of approximately 1075 Btu/Ft³ )
  2. Distinct individual flame pattern. You can count the number of ports by counting the number of individual flames emanating from those ports.
  3. No blowing or lifting of flames; i.e., separation of the flame from the burner port.
  4. No lazy flames.  (This is an indication of too little primary or secondary air.)
  5. No flash-back of burner flames.
  6. No offensive noise during ignition, operation or extinction.
  7. No offensive odors emanating from the combustion process.
  8. Flame heights are uniform around the burner periphery.  (NOTE:  In looking at the simmer burners below (smaller burners), you will notice that the flame heights are not equal.  This is by design and involves the configuration of the burner grates mounted above the burners themselves. )



The burners above show flame patterns producing complete combustion with minimal CO in the products of combustion.



Now, I would like to show you a picture of a burner system that is NOT firing properly.  Sometimes it is easier to discuss proper operation by looking at a burner behaving badly.  This design, shown below, breaks ALLof the rules given above.  This is a gas grill basically used for “tail-gate” cooking.  The propane tank and grill are integral parts of the trailer.  The trailer is attached to the car with a bumper hitch, and then towed to the game behind the vehicle.  I was asked to “pass judgment” on the design and indicate to the designer what corrective actions I would recommend.  It did take some time.



Here is what we know:

  • Yellow flames; an indication of inadequate primary air and incomplete combustion.  This condition will produce sooting (or carboning) and will generate an intolerable amount of carbon monoxide.  If this product were used in an enclosed space, there would be definite issues with the accumulation of carbon monoxide.  We can expect some yellow tipping because the fuel gas is propane.  Typically, propane and butane produce yellow burner tips but this is much too much and represents a condition that must be rectified.
  • Flame height is very irregular which tells me there are real issues with primary air injection, burner alignment and issues with the burners being level relative to the mounting system.  The orifice size metering gas to the burners is very suspect and, as I discovered, much too large relative to the design capability of the burners for propane.
  • Very probable that there is an issue with delivery pressure.  A “lazy” flame indicates the pressure needs to be checked and corrected if inadequate.  A system firing on propane should have 11.00 inches water column (W.C.) downstream of the regulator and available at the burner orifice(s).  It is difficult to see from the picture, but the gas delivery system to the orifices was almost serpentine in configuration; tubing everywhere!  The tubing was 0.25 inch in diameter with each of the four burners orificed to fire 50,000 Btu/Hr.  This was truly a “master blaster” but with a gas delivery system very suspect relative to pressure losses.  Please remember—pressure losses in a gas delivery system are the enemy and are to be avoided at all costs.
  • In looking at the “superstructure” of the burner system, there is a real issue with misalignment of the burners during movement of the trailer.  The burners can become displaced, thereby creating a hazardous condition.  This definitely needs to be corrected to provide additional stability of the entire system.
  • This last point is somewhat academic, but the designer did not consult any design standard prior to initiating the project.  It was all “off-the cuff”.  Cut and try.  If it works fine, if not fix it.  No real attempt at obtaining the ANSI standard for gas-fired grills or gas-fired products and following that standard.


Believe it or not, there were corrections made to the design and much better performance did result.  That product is now is in the “field test” phase and will be introduced in late summer—ready for kickoff.


Now let us examine the basic operation of an atmospheric gas burner.  We do so by looking at a typical atmospheric the gas burner design shown as follows:.




Gas is supplied to the burner orifice by virtue of pressure from the distribution system or the gas bottle; i.e.; propane or butane.  For natural gas, that pressure is generally delivered between 9.00 and 12.00 inches water column.   The gas is then regulated to a delivery pressure between 3.5 and 4.00 inches W.C for natural gas.  If the gas is propane or butane, the delivery pressure will be between 10.5 and 11.00 inches W.C.  A regulator is located on the gas bottle so adjustments may be made to achieve the proper delivery pressure.  It is not uncommon to have an additional regulator mounted on the gas-fired device.  All regulators have “pressure ports” from which the gas pressure may be measured so if there is any doubt about delivery pressure, please check.   A simple “U” tube manometer is completely adequate for this measurement.  Simply measure the difference between the heights of the columns.  This difference is the pressure relative to atmospheric.

As mentioned earlier, the air for combustion is called primary air and is provided at atmospheric pressures from outside the appliance.  Gas is metered by virtue of the burner orifice.    Primary air is drawn through the air shutter as the gas is “streamed” from the orifice opening.  The injection of the gas creates a negative pressure relative to the ambient pressure so atmospheric air is entrained through the air shutter and the mixing face.   The orifice diameter controls the gas flow rate, generally measured in Ft³/ min or Ft³/Hr.   The combination of gas and air occurs in the mixing zone or burner tube.  It is very important that the internal design and length of the mixing tube allow for proper combination so a homogeneous gas /air mixture can result and be delivered to the burner head prior to ejection through the burner ports.  The venture “throat” serves as a converging / diverging nozzle to increase the velocity of the gas/air mixture prior to entering the mixing tube.  The mixture is then swept into the burner head and distributed to the individual burner ports for ignition.  The ports act as a restriction to flow; thus, port loading has a great effect on primary air injection.  (I would again direct your attention to the glossary in the appendix for the definition of port loading.)  It is also important to note that internal burner roughness has an effect on the injection of primary air through friction losses.  It is definitely possible to increase primary air by enlarging the burner ports, thereby reducing individual port loading.  When this occurs, care must be taken to make sure that no flashback results.  Flashback occurs when the velocity of the ejected gas/air mixture is less than the velocity of the flame front through the individual ports.  When this occurs, burning may actually “flash back” to the burner orifice.  This condition is prohibited by ANSI (The American National Standards Institute).   Ignition takes place at one port with carryover to the remaining ports.  Again, ignition time should be at or less than four (4) seconds to preclude an accumulation of uncombusted gas. This time is also prescribed by ANSI.   Continued ignition takes place around or along the burner ports until gas is no longer supplied to the burner.   This control is generally accomplished by virtue of a thermostat that senses temperature or closure of a gas valve done manually by the user.

I definitely hope this very short explanation will help AND provide needed information relative to safety that must be considered when operating heating systems.


  1. Great article! That is the type of info that are supposed to be shared across the net.

    Shame on the seek engines for not positioning this put up higher!

    Come on over and talk over with my site . Thank you =)


    • cielotech Says:

      Hello Liners–Thank you so much for your very kind comments AND I really appreciate you logging on to take a look at my posts. Carbon Monoxide is a horrible gas and one that take a huge toll each year. I agree with your comments completely. Take care. B.


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