AbstractA computer model is developed to simulate the performance of air‐to‐water heat pumps heating a house via a radiator system. The performance characteristics of the heat pumps are derived from laboratory measurements. Hourly weather observations are used to calculate the heat demand of the house and the performance of the heat pump. The effects of changes in heat pump characteristics, changes of radiator size, and changes in heat demand of the house due to insulation, are compared in terms of their effects on annual energy consumpt