Heat pump

A heat pump comprises:

• a compressor compressing refrigerant received from an accumulator;
• a water-refrigerant heat exchanger exchanging heat between the refrigerant discharged from the compressor and water;
• a main expansion valve expanding the refrigerant passed through the water-refrigerant heat exchanger;
• an outdoor heat exchanger exchanging heat between the refrigerant passed through the main expansion valve and outdoor air and connected to the accumulator;
• a main pipe connecting the water-refrigerant heat exchanger and the outdoor heat exchanger, the main pipe at which the main expansion valve is installed;
• an internal heat exchanger installed at the main pipe between the water-refrigerant heat exchanger and the main expansion valve;
• an injection pipe which has one end connected to a first point of the main pipe between the internal heat exchanger and the main expansion valve, which has the other end connected to the compressor, and at which the internal heat exchanger is installed;
• an injection valve installed at the injection pipe between the first point and the internal heat exchanger;
• a bypass pipe which has one end connected to a second point of the main pipe between the water-refrigerant heat exchanger and the internal heat exchanger, which has the other end connected to a third point of the main pipe between the internal heat exchanger and the first point;
• and
• a bypass valve installed at the bypass pipe.