Abstract: System (200) for heating a structure (10), wherein the system comprises a first heat pump (210), arranged to transfer heat to a first hot-side external liquid from a cold-side external liquid circulating in the ground (20) or a water body (21); and a second heat pump (220), arranged to transfer heat to a second hot-side external liquid; wherein the system is arranged to heat the structure (10) via heat exchange with at least the first external liquid. The air-to-liquid heat pump uses carbon dioxide as an internal-loop heat medium, and the system comprises a third heat exchanger (230) being arranged to, cool the internal-loop heat medium by heat exchange with the cold-side external liquid, the cooling of the internal-loop heat medium being to a temperature of less than a critical point (CP) of the internal-loop heat medium. The invention also relates to a method.