About Combustion

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.

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