Thermal efficiency and heat-transfer characteristics of a novel integrated condensing gas water heater

Natural gas water heaters are widely used as the primary equipment for domestic hot-water supply and space heating in residential buildings. However, condensing gas water heaters commonly suffer from complex structural configurations and insufficient recovery of condensation latent heat. This study proposes and applies an integrated inverted shell-and-tube condensing heat exchanger in a natural gas water heater. Corresponding physical and mathematical models were developed based on the experiments to evaluate the heat-transfer behavior and overall performance of the condenser. Experimental results show that, under the rated thermal load of 100 kW, the thermal efficiency based on the lower heating value reaches a maximum of 112.9%. Under the low-temperature radiant floor heating condition with supply and return water temperatures of 50 °C and 30 °C, respectively, the thermal efficiency is approximately 107%. Numerical simulations were conducted to investigate the heat-transfer characteristics of the heat exchanger. The results indicate that when the water vapor volume fraction in the flue gas increases from 10% to 30%, the dew-point temperature rises accordingly, leading to markedly enhanced condensation and latent-heat recovery, thus increasing the heat transfer. The geometric dimensions of the heat exchanger were also investigated. It was found that in the high-temperature sensible heat region without condensation, the enhanced heat transfer coefficient (η) increases with the patency coefficient (ξ, defined as the ratio of the transverse tube spacing to the transverse tube center distance), while in the low-temperature condensing region, an optimal ξ of approximately 34% was identified for maximizing η. This study provides insights and engineering references for compact, structurally integrated, and easy-to-manufacture condensing-type small commercial gas water heaters.