Aiming at a two-stage condensing heat pump hot water system, a model for the two condensers was developed based on partitioned lumped parameter method. Results from the model were compared with the experimental data, and the predicted results agree well with the experimental data. Based on this model, the application of this system was analyzed in indoor swimming pools. When the ratio of tube length of the first condenser to the second one is set as 23.67%, both swimming water and shower water can be precis...