This paper presents the work on evaluating the thermal performance and life cycle energy cost of using heat pumps for hotel swimming pools in subtropical climates. A roof top pool in a city hotel was investigated. The energy requirement for maintaining the pool water temperature was analysed in terms of the energy loss through four major processes, namely evaporation, radiation, convection and water refill. The governing equations for these four heat loss processes were developed based on a review of the literature. It was found that substantial energy savings could be achieved. Compared with conventional electric boilers and condensing/non-condensing gas fired boilers, the total energy savings during a 6 1/2 month (mid-October to April) heating season ranged from 39.9 to 46.3 MW h. A life cycle of 10 years was used to calculate the net present value of the energy cost. Over a 10 year life cycle, the energy cost could be reduced by HK$ 275,700 if a heat pump (with a mean seasonal coefficient of performance of 3.5) were used instead of a conventional electric boiler or condensing boiler.