An air-to-water heat pump system comprising a compressor , a condenser , an expansion valve , a lower heat source unit and an upper heat source unit connected in a thermodynamic cycle, wherein the lower heat source unit is configured to be supplied with external air, and the lower heat source has at least two alternately working evaporators forming the lower heat source of the unit with a fan mounted axially in relation to the lower heat source unit in its upper part, and provided with a defrosting unit, the air-to-water heat pump system further comprising a set of valves comprising two pairs of motor-actuated three-way valves and connected to the at least two evaporators , a shutter provided for each evaporator and of the lower heat source unit, wherein the evaporators are connected in series in a closed circuit with the condenser ; wherein an outlet port of the first evaporator connects via the through hole of the second three-way valve with a suction side of the compressor , connected on a discharge side to the condenser , the outlet port of which connects via the bypass of the third three-way valve with the inlet of the second evaporator , wherein the outlet of the second evaporator is connected via the bypass of the fourth three-way valve with the expansion valve and via the through hole of the first three-way valve the expansion valve connects with the inlet of the first evaporator ; wherein the outlet port of the second evaporator connects via the bypass of the second three-way valve with the suction side of the compressor , connected on the discharge side with the condenser , the outlet port of which connects via the through hole of the third three-way valve with the inlet of the first evaporator , wherein the outlet of the first evaporator is connected via the through hole of the fourth three-way valve with the expansion valve and via the bypass of the first three-way valve the expansion valve connects with the inlet of the second evaporator ; wherein the shutter is formed on the lower heat source unit to shield the frosted evaporator going into defrosting mode. wherein an outlet port of the first evaporator connects via the through hole of the second three-way valve with a suction side of the compressor , connected on a discharge side to the condenser , the outlet port of which connects via the bypass of the third three-way valve with the inlet of the second evaporator , wherein the outlet of the second evaporator is connected via the bypass of the fourth three-way valve with the expansion valve and via the through hole of the first three-way valve the expansion valve connects with the inlet of the first evaporator ; wherein the outlet port of the second evaporator connects via the bypass of the second three-way valve with the suction side of the compressor , connected on the discharge side with the condenser , the outlet port of which connects via the through hole of the third three-way valve with the inlet of the first evaporator , wherein the outlet of the first evaporator is connected via the through hole of the fourth three-way valve with the expansion valve and via the bypass of the first three-way valve the expansion valve connects with the inlet of the second evaporator ; wherein the shutter is formed on the lower heat source unit to shield the frosted evaporator going into defrosting mode. <EOS>