What is the main factor that determines the combustion efficiency of a gas appliance?

The air-to-fuel ratio is the primary factor that determines the combustion efficiency of a gas appliance because it directly influences the completeness of the combustion process. An optimal air-to-fuel ratio ensures that there is enough air mixed with the fuel for complete combustion, resulting in maximum energy output and minimal emissions.

When the air-to-fuel ratio is balanced, the chemical reaction between the fuel and oxygen proceeds efficiently, producing water vapor and carbon dioxide as byproducts. If the ratio is not correct—either too much fuel or too little air—the combustion process becomes inefficient, leading to unburned fuel and increased emissions, such as carbon monoxide, which is harmful and can be a safety hazard.

Therefore, while other factors such as gas type, appliance temperature, and gas line size may influence an appliance's operation, it is the air-to-fuel ratio that fundamentally governs how effectively the fuel is being used and, consequently, the overall combustion efficiency of the appliance.