Combustion is a chemical reaction that changes a fuel source, such as propane, into a useful form of energy, such as heat. Combustion requires all three ingredients: fuel, oxygen, and an ignition source. However, all three ingredients must be present in the proper proportions for combustion to occur. If a mixture of gas and air contains more gas than is needed for ideal combustion, then it is referred to as a rich burn. If a mixture of gas and air contains less gas than is needed for ideal combustion, then it is referred to as a lean burn.
The ideal combustion ratio ("complete combustion") for propane is 1 part propane (4%) to 24 parts of air (96%). Even at this ideal ratio, combustion products, known as flue gases, still occur. However, under this scenario, those products are harmless carbon dioxide and water.
Incomplete combustion (due to either rich or lean burns) may produce harmful combustion by-products such as carbon monoxide and aldehydes.
Most appliance burners are designed to burn a gas mixture that is as close to the ideal (4% propane) as possible, and may be regularly inspected by a qualified technician to ensure the maintenance of the ideal burn ratio.