Heat pumps are an essential technology for decarbonisation of domestic heating in the UK. This paper reports on the performance in use of a group of ground-source heat pumps, and in common with other UK studies finds that the seasonal performance is not as good as that reported in trials from continental Europe and that the system controls are unsatisfactory. Control improvements are investigated via a model of the dwelling and heat pump as a combined system, from which the thermal time constant of the building is identified as a critical factor that needs to be considered in retrofit projects incorporating heat pumps. The validity of the conventional practice (and advice from installers to users) of allowing heat pumps to run continuously is tested and bounded. Techniques for improving control are outlined and reasons for the poorer performance in the UK examined with the conclusion that heat pumps need to be better matched in capacity and control to the size and thermal characteristics of UK dwellings. Implementation of these findings by heat pump manufacturers and installers could promote a more rapid transition to renewable heat both in the UK and internationally wherever similar housing stocks and climates exist. (C) 2011 Elsevier B.V. All rights reserved.