The new generation medium range of Biomass fuels fired boilers are the Combi-Boiler. A combination boiler made up of a water wall pre-furnace and a conventional three pass fire-tube shall boiler with a straight radiation furnace to accommodate burning of liquid or gaseous fuels. The unique design of the pre-furnace consist of lateral walls made of fined tubes connected to the boiler drum through headers in an “A” shape arrangement, for the following advantage.