Heat pump system (100) comprising at least one 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 two different primary heat sources or sinks selected from outdoor air, a water body, the ground or exhaust air, at least one of two different secondary heat sources or sinks selected from indoors air, pool water and tap water, a respective temperature sensor (412,432) at each of said primary heat sources or sinks, a valve means (421,431,451) for selectively directing the primary-side heat medium to at least one of said primary heat exchanging means, and a control means (500). The invention is characterised in that, in a secondary-side heating operating mode, the temperature of said primary heat sources or sinksis measured, and in that the primary-side heat medium is directed only to the primary heat exchanging means associated with the heat sources or sinks with the highest temperature. The invention also relates to a method.