The integrated low and high temperature heating water system consists of heat pump heat source and boiler heat source. The heat pump heat source abstract heat from low temperature heat source and produce hot water up to 65°C. During mild weather, the 65°C hot water is sufficient for building heating. During cold weather conditions, the boiler heat source will have to be used to produce hot water at 90°C or higher to satisfy the building comfort. To improve the system economy, the integrated system has to maximize the use of the low temperature heat source since it is free. This paper presents a theoretical models and analysis to optimize the system design parameters.