What does it mean if a fuel-air mixture in a gas furnace contains natural gas over 14%?

When a fuel-air mixture in a gas furnace contains more than 14% natural gas, it indicates that the mixture is rich for combustion. A rich mixture means there is a higher proportion of fuel relative to the amount of air available for combustion. This results in excess fuel in the mixture, which can lead to incomplete combustion.

In this scenario, while a rich mixture may produce more heat initially, it can also create problems such as increased carbon monoxide production and potentially more soot formation within the furnace. Proper combustion requires a balanced ratio of fuel to air to ensure efficiency and safety. In contrast, a lean mixture, which contains less fuel and more air, might lead to more efficient combustion, but if it is too lean, the combustion process could become unstable.

The other options don't accurately describe the situation; suitable for combustion implies a proper balance which is not the case here, and too volatile does not specifically refer to the composition of the fuel-air mixture in terms of combustion efficiency. Understanding the correct mixture is crucial for the safe and effective operation of gas appliances.