Abstract: A heat pump liquid heater for heating a liquid comprising: a heat pump; a liquid tank in heat communication with the heat pump, wherein the liquid tank comprises the liquid; and at least one metal condenser tube immersed into the liquid, wherein the metal condenser tube forms at least one coil. The metal condenser tube has a flattened double-tube configuration and a cross-section defined by concentric ovals such that at least a portion of the concentric ovals is in contact with one another thereby minimizing space between the flattened double-tubes. The heat pump comprises a tube-in-tube heat exchanger and a compressor, wherein the tube-in-tube heat exchanger recovers heat from refrigerant returning from the liquid tank and transfers the recovered heat to refrigerant going to the compressor thereby superheating the refrigerant.