This paper presents the performance analysis and retrofit assessment of a domestic high temperature air source heat pump coupled with thermal energy storage in terms of different system configurations. TRNSYS simulations were used to model the dynamic system, which has been validated against field trial results. The validated models were then simulated with different system configurations under the same boundary conditions to assess its performances. Simulated results showed that continuous coupling between the heat pump and storage to heat the house had the worst performance, while the operation of only the heat pump heating the house obtained the highest efficiency. Although the heat pump cannot compete with the gas boilers in terms of running costs, between 6.6% and 33% of carbon cut can be achieved if the heat pump was retrofitted.