Quantity and temperature of flue gas influence the heat loss that exit the stack. Quantity of fuel that be burned and amount of air which be supplied to the burner influence the quantity of flue gas. While temperature of flue gas is influenced by cleanliness of steam boiler, minimum slag on the surface of pressure part (boiler bank, air heater, economizer, etc). Cleanliness of steam boiler is responsible of soot blower and air heater. To make complete combustion process, good mixing between fuel and excess air should be considered and also operation of soot blower and air heater must be used routinely to minimize the heat loss.
The heat loss in stack consider how to keep flue gas exit temperature is low (but it is limited to prevent corrosion which occur at acid dew point because corrosion material will condense in the surface of cooler metal) and consider how well the temperature of flue gas is absorbed by the pressure part of steam boiler.
The heat loss in stack can be calculated based on dry gas loss, moisture loss, and humidity loss. Dry gas loss calculation is affected by weight of gas that leaves the system in unburned fuel. Moisture loss calculation is affected by weight percentage of hydrogen and moisture in fuel. Humidity loss calculation is affected by weight of moisture in air in unburned fuel.