The following write-up contains nothing new but some useful information for millers to remember all the time. What are the factors that limit the heat transfer efficiency in a boiler? In the past, millers were not so interested in heat transfer efficiency in the boilers as the boilers played the dual role of a steam generator as well as an incinerator to dispose off the excess biomass. Now with the advent of grid connected co-generation, the millers have to think of highly efficient boilers, meaning that the heat transfer efficiency of the boiler must be as high as possible. Every bit of fuel must be converted to steam and electricity, and supplied to the national grid to generate additional profit for the company. Let us have a closer look at the factors involved in the heat transfer in a boiler. Heat is transferred from the burning fuel in a boiler by means of (a) radiation (b) conduction and (c) convection and the media through which this happens are (i) the inert gas layer or dry surface (ii) the boiler plate and (iii) the water film or wet surface.