Heat pumps meet the needs of the residential sector through the following three purposes: domestic water heating, space heating and cooling. This paper presents experimental research into the energy performance of an air-to-water heat pump in real summer changeover regimes, given that these are periods that significantly affect the reduction of efficiency. The transition regimes between the two regimes of operation with the most pronounced changes were especially considered. These were the transition regimes characteristic of operation in the summer period, namely: the transition from the space cooling regime to the domestic water reheating regime, and vice versa. In the observed period, the focus has been on 60 readings, with a resolution of 1 minute. In that period two changes in the operating mode were made, from the stationary space cooling regime (when the heat pump prepared water for the underfloor cooling at 17ºC) to the domestic water reheating mode (when the heat pump prepared hot water in the tank at 43ºC during the day), as well as switching to the previous, cooling mode. The research results show that in the observed period the heat pump worked in stationary regimes of space cooling and domestic water reheating, respectively, with average performance values of 3.52 and 4.75, and average exergetic efficiency of 6.93% and 33.38%.