Techno-economic assessment of cascade air-to-water heat pump retrofitted into residential buildings using experimentally validated simulations

•TRNSYS modelling and simulations validated against laboratory and in situ results.•Predicted annual COPs almost below 2.5.•Higher running costs of the heat pumps compared to gas and 90% efficiency oil boilers.•Potential of carbon emissions reductions from 14% to 57%.•Comparison of three system configurations between heat pump and thermal energy storage.