Geothermal or some other relatively stable source. If you live near a lake you can drop a water sling on the bottom and use that as a source, it is also possible to bury a sling in a field next to your house at a frost-free depth (2.5 m or deeper in the north of Sweden) and use that. In built-up areas with many houses using deep wells for heat extraction there have been cases of those wells freezing up, this can also happen if the system is under-dimensioned.