Storage tank for hot water systems

A heat pump system comprising:

• a compression device to compress a refrigerant to a high pressure;
• a heat rejecting heat exchanger for cooling said refrigerant;
• a tank that stores a cold fluid medium in a cold reservoir that cools said refrigerant in said heat rejecting heat exchanger and stores a hot fluid medium that is heated by said refrigerant in a hot reservoir, said tank including a moveable interface plate that separates said cold fluid medium from said hot fluid medium in said tank, wherein said cold fluid medium flows from said cold reservoir of said tank to said heat rejecting heat exchanger, exchanges heat with said refrigerant in said heat rejecting heat exchanger to form said hot fluid medium, and said hot fluid medium flows from said heat rejecting heat exchanger to said hot reservoir of said tank during a water heating mode;
• an expansion device for reducing said refrigerant to a low pressure;
• a heat accepting heat exchanger for heating said refrigerant;
• and
• a first sensor and a second sensor that detect a position of said moveable interface plate, and said water heating mode begins when said first sensor detects that said moveable interface plate moves above said first sensor and said water heating mode ends when said second sensor detects that said moveable interface plate moves below said second sensor.