Combustion is a rapid chemical reaction when oxygen combines with the fuel to produce heat. Lighting a match is combustion.

You require three things to produce combustion; heat, fuel, and oxygen. Since the air we breathe is only about 20% oxygen and 80% nitrogen, we end up having to supply much more air to get the oxygen we need.

There are three types of combustion:

• Perfect combustion
• Complete combustion
• Incomplete combustion

Perfect combustion is known as stoichiometric combustion and provides an exact fuel to air ratio and extracts all energy from the fuel at 100% efficiency. Perfect combustion does not exist in the real world as some of the hydrogen (H) and carbon (C) atoms will not join the oxygen (O2) and instead create incomplete combustion.

Complete combustion is what we aim for. 1 cu ft gas to 15 cu ft air for natural gas and 1 cu ft gas to 18 cu ft air is necessary for propane. Carbon and hydrogen combine with oxygen (O2) during complete combustion to produce carbon dioxide (CO2) and water (H2O).

Incomplete combustion is a result of an unbalanced combustion process. Not enough combustion air, incorrect gas pressure, flame impingement, wrong gas orifices, blocked venting, etc. Products of incomplete combustion are carbon monoxide, soot, and aldehydes.

Carbon monoxide (CO) is the most harmful product of incomplete combustion. It’s colorless, odorless, tasteless, and toxic. In large quantities, it can even be combustible.

Measuring CO in the exhaust is done on an air-free basis and is referred to as COAF. Since the added excess air for complete combustion dilutes CO readings, the COAF reading compensates for this dilution using the O2 sensor in the analyzer.

The maximum carbon monoxide an appliance is allowed to produce and exhaust out the venting is 400 ppm (coaf). As a general rule of thumb, an appliance well-tuned should not exceed 50 ppm CO. Check local codes for your allowable maximum.

During initial light off, a high level of CO is produced but should level off within a few minutes.

Dirty burners can cause insufficient air to gas ratio. If dirty burners are an issue, they should be removed and cleaned. A soft wire brush may be used to clear any rust or debris. Ensure the burners are in proper alignment to allow for the flame to spread evenly along with the burners flame spreader.

Gas pressures should be checked and set to manufacturer suggested settings. Follow the pressures listed on the appliance rating plate. You should not assume the gas valve comes with the correct regulator settings because multiple manufacturers could use the same gas valve since gas valves are not always explicitly manufactured for each appliance.

Aldehydes are irritating to the eyes, throat, and nose. They smell terrible. Aldehydes taste somewhat metallic and likely accompanied by carbon monoxide.

Soot is unburned carbon.

What this means on a real service call

On a furnace call, the combustion triangle is your quick diagnostic frame. If any leg is weak, combustion suffers. Too little fuel, too little air, or not enough heat for ignition, and the burner will not light, will not stay lit, or will burn poorly. Too much fuel for the air available, and you get incomplete combustion, soot, and carbon monoxide. Too much excess air, and you waste heat up the vent and can cool the flame.

Complete combustion is the goal: the right fuel-to-air balance, a stable flame, and clean products of combustion, mainly heat, water vapor, and carbon dioxide for natural gas. Perfect, or stoichiometric, combustion is the exact theoretical balance, but in real equipment a small amount of excess air is needed to make sure the fuel and oxygen actually meet and burn fully. That is why the air numbers look large: air is only about 20 percent oxygen, so a lot of air is needed to supply enough oxygen, and most of the rest is nitrogen that simply passes through.

Numbers and measurements a technician actually sees

The original post gives a useful starting point: about 1 cubic foot of natural gas to 15 cubic feet of air for complete combustion, and a carbon monoxide limit of 400 on an air-free basis, known as COAF. Do not judge combustion by flame color alone. Where you are trained and equipped to do so, use a combustion analyzer to check oxygen, carbon monoxide, and stack temperature, and compare the results with the appliance maker’s instructions. A CO reading that stays high after the first few minutes of operation, rather than leveling off after light-off, is a warning sign, not something to ignore.

Gas pressure is the other everyday number. Check supply and manifold pressures with a manometer and set them to the pressures on the rating plate and in the maker’s instructions, not to a number you remember from another unit. A burner that is overfired or underfired will not burn cleanly, even if everything else looks right.

A common mistake

A common mistake is to adjust gas pressure to fix a flame problem without checking air supply, burner cleanliness, and venting first. Dirty burners can starve the flame of air, a blocked vent can change draft, and a starved combustion air supply can do the same. Turning the gas up or down to mask those problems can make incomplete combustion worse and can create a safety hazard. Find the cause, clean or repair it, and then set pressures correctly.

A practical checking step

Work through combustion in order: first, confirm the fuel type and that the appliance is set up for it. Second, check combustion air and venting for blockage, and look for soot around the burner or vent, which points to incomplete combustion. Third, check and set gas pressures to the rating plate. Fourth, if you are trained and equipped, run a combustion test after the unit has warmed up and compare it with the maker’s guidance. Safety caution: if you smell gas, see heavy soot, or a carbon monoxide alarm sounds, do not keep running the appliance to finish a test. Shut it down if it is safe to do so, ventilate, leave the area if needed, and get qualified help. Carbon monoxide is colorless and odorless, so never rely on smell to judge safety.

Related posts

For the fuels themselves, see Characteristics of Gas. For what goes wrong, see Incomplete Combustion, and for air supply, see Combustion Air Supply.