The other heat losses from a boiler consist of the loss of heat by radiation and convection from the boiler casting into the surrounding boiler house.

Normally surface loss and other unaccounted losses is assumed based on the type and size of the boiler as given below

        For industrial fire tube / packaged boiler = 1.5 to 2.5% 

        For industrial watertube boiler = 2 to 3%

        For power station boiler = 0.4 to 1%

However it can be calculated if the surface area of boiler and its surface temperature are known as given below :