What is the effect of using an air heater in a boiler?

Using an air heater in a boiler significantly increases the efficiency of the boiler. This is achieved by pre-heating the combustion air before it enters the furnace. When the combustion air is warmed up, it facilitates a more complete combustion of the fuel, resulting in higher energy output from the same amount of fuel. Moreover, pre-heating the air reduces the amount of fuel consumed, as less energy is required to heat the incoming air to the optimal combustion temperature. As a consequence, the overall efficiency of the boiler is enhanced, leading to more effective energy use and reduced emissions.

The other options do not accurately reflect the primary function or benefits of using an air heater in this context. The induced draft fan and forced draft fan relate to air movement but do not directly address the efficiency aspect, which is the focus here. The mention of a balanced draft system is unrelated to the specific impact an air heater has on boiler efficiency, further emphasizing why the correct choice points directly to efficiency improvement.