An improved cold climate air-source heat pump comprising at least a primary compressor, a booster compressor, a first heat exchanger associated with an indoor environment and a second heat exchanger associated with an outdoor environment and with one heat exchanger acting as an evaporator and the other heat exchanger acting as a condenser, an expansion device, flow conduit means for circulating a refrigerant fluid in a closed loop, a first sensor means for sensing the temperature of the internal ambient air, a second sensor means for sensing the temperature of the external ambient air, a third sensor means for sensing the temperature of the refrigerant fluid flowing between the booster compressor and the primary compressor, a fourth sensor means for sensing the operative state of the primary compressor, a primary compressor operation control means responsive to inputs from the first sensor means, and a booster compressor operation control means responsive to inputs from the second, third, and fourth sensor means. The booster compressor operation control means is configured to initiate and shut down operation of the booster compressor based on inputs from the various sensor means, to ensure safe operation of the system and improved durability of the air-source heart pump.
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