Modelling the performance of condensing boilers

Recent research on new and hybrid renewable heating systems has increased the need for accurate and reliable boiler models for use in system simulations. The majority of existing boiler models are based on curve fits of manufacturers' data, giving values of efficiency as a function of part/load ratio. These models have limitations when used in rigorous system simulation studies. A few models based on accepted heat transfer principles have been developed in the last ten years. However, there is little or no published information regarding the calculation of heat transfer when the boiler is operating in condensing mode; this is a limitation in view of the extent to which condensing boilers are now specified. The paper presents an extension of an existing boiler model to enable it to calculate performance under condensing operation and shows that this can be carried out to a satisfactory level of accuracy using the sensible heat ratio method.