For gas heating, what would be considered too lean for combustion?

In gas heating, the combustion process requires a proper mixture of fuel and air to achieve efficient and complete combustion. A fuel-to-air ratio that is too lean means there is insufficient fuel relative to the amount of air available, leading to incomplete combustion. This can cause issues such as a decrease in heating efficiency, higher emissions of nitrogen oxides (NOx), and potentially flame stability problems.

A combustion system operating with too lean a mixture typically starts to experience these issues around the 4% mark. When the air-to-fuel ratio exceeds this threshold, the combustion can become inefficient, and the risk of combustion-related problems increases. It's at this point that the combustion process may not produce enough heat effectively or may create dangerous conditions like backfiring or flame lifting due to the instability of the flame.

The other percentages listed (2%, 6%, and 8%) either represent a richer mixture, which may still support proper combustion (in the case of 2%), or are too lean, leading to inefficiencies and the aforementioned risks. Thus, 4% serves as an important threshold for determining the lean limit for safe and effective gas combustion.