Presents a mathematical model of an instantaneous, condensing, gas-fired hot water boiler, which is verified by experiments. States the model employs empirical heat and mass transfer coefficients in order to calculate convective heat transfer to the baffled, finned-tube heat exchanger. Radiative heat transfer that occurs with the fibre-matrix radiant burner and the coil is determined by the exponential wide-band model. Finds that for a wide range of operating conditions the water temperature rise and flue loss can be predicted accurately.

KEYWORDS: Calculating, boilers, condensing boilers, swimming pools, gas fired boilers, models, performance, instantaneous water heaters, space heating, water heating