Heat pump system comprising a heat medium circuit (210, 220, 230, 240, 250, 310, 320, 410, 420, 430, 440, 450, 460) in turn comprising a compressor (211), an expansion valve (232, 242), at least one primary heat exchanging means (422, 433, 452) between a primary-side heat medium and a respective primary heat source or sink selected from outdoor air, a water body or the ground, at least one secondary heat exchanging means (314, 315, 316) between a secondary-side heat medium and a respective secondary heat source or sink selected from indoors air, pool water and tap water, and a control means (500). The invention is characterised in that the speed of the compressor can be controlled, in that an opening of the expansion valve is adjustable, in that the speed of the compressor is controlled, and in that an output temperature of heat medium flowing out from the expansion valve is controlled by controlling the opening of the expansion valve given the controlled speed of the compressor. The invention also relates to a method.