The paper presents the design of a two-stage subcritical compressor heat pump operating in an air-to-water system and running on environmentally friendly refrigerants. The pump is dedicated to buildings where there is a high-temperature central heating system and wall-mounted radiators are used as heat receivers. The first stage of the unit was supplied with R290 refrigerant and the second stage with R1234 ze(E) refrigerant. The nominal heating power of the unit is 10kW for a lower source temperature in the range -20°C÷10°C. The unit's refrigeration system was designed and simulation tests were carried out to determine the unit's operating parameters for the lower source temperature in the range -20°C÷10°C, the condensation temperature of the stage I refrigerant for values: 20°C, 25°C, 30°C and the condensation temperature of the stage II refrigerant in the range 30°C÷60°C. The value of the coefficient of performance (COP) was determined in relation to the heating power generated in stages I and II to the electricity consumed, which value, depending on the operating parameters, ranges from 2.55 to 5.6. The value of the COP related to the heating power received from the stage II condenser to the total power consumed by the stage I and II compressor, depending on the operating parameters, ranges from 1.5 to 2.9. It was found that by using a stepless adjustment of the volumetric efficiency of the stage I and II compressor, it is possible to match the heating power of stage I to the cooling power of stage II and thus increase the COP value by 50%.