A heat pump includes a compressor for compressing refrigerant. The compressed refrigerant is divided into a first portion and a second portion. Simultaneously, the first portion of the refrigerant is used in a first vapor-compression circuit to heat or cool a space and the second portion of the refrigerant is used in a second vapor-compression circuit to heat a fluid. In a single external source heat exchanger, both the first portion of the refrigerant in the first vapor-compression circuit and the second portion of the refrigerant in the second vapor compression circuit exchanges heat with an external source fluid.