The use of simulation as a design tool is increasing in a number of areas, such as the design of process and thermal plant, and heating, ventilation and air-conditioning (HVAC) systems. In such situations the requirement for short execution time precludes the use of the classical field-problem approach. This paper describes the development of a three-component, one-dimensional model that enables the thermal and emissions performance of commercial (100 kW to 3 MW) boilers to be calculated via a blend of first-principle and empirical relationships. Model development was closely linked with a testing programme for gas-fired and oil-fired commercial boilers.