A three-dimensional (3-D) code which can predict the complete performance of high efficiency boilers is described in this paper. The model is in two parts. The combustion chamber is modelled in the first part and the heat exchanger in the second. The thermal efficiency is calculated from the predicted results. The computational model was based on the two-dimensional (2-D) TEAM code which has been extended into 3-D and extensively modified by the present authors. In the code, the gaseous combustion was modelled by Simple Chemical Reaction System concept; the Radiation process was handled by the ‘Discrete Transfer’ method. The modelling results of the combustion chamber were used as input for the calculation of the consequent finned tube heat exchanger. The heat exchanger calculation included conduction, convection and radiation. Predictions were performed for a low Reynolds number boiler and compared with existing data. Good agreement was obtained.