It's stable, cheap, carries heat OK enough, so it gets pumped through the pipe in the ground to collect heat and bring it back up to the heat pump.
If there's a leak it's bad since it's quite toxic to organic life. If you collect in soil rather than drilling into rock tree roots might push around the collector pipe a bit over the years but I've never heard about that being a problem.
If there's a leak it's bad since it's quite toxic to organic life. If you collect in soil rather than drilling into rock tree roots might push around the collector pipe a bit over the years but I've never heard about that being a problem.