What happens to combustion efficiency if there is too much excess air?

When there is too much excess air in a combustion process, the combustion efficiency decreases. This is primarily due to the fact that excess air can lead to incomplete combustion, where not all the fuel is burned efficiently. The additional air requires heating, which takes energy away from the primary combustion process, resulting in a lower overall efficiency.

Moreover, the presence of excess air can lead to a dilution of the fuel-air mixture. The optimal ratio of fuel to air is crucial for achieving complete combustion and producing maximum heat output. Excess air causes the operating temperature to drop, which impacts the combustion process and can lead to increased emissions of unburnt hydrocarbons and other pollutants.

Thus, having more air than necessary does not enhance combustion efficiency but rather impairs it by wasting energy and leading to suboptimal operation of the heating system.