It depends on the geology underneath (I have no idea what is under the area they are currently covering) the amount of heat exchange can vary a lot, even within

It depends on the geology underneath (I have no idea what is under the area they are currently covering) the amount of heat exchange can vary a lot, even within the same type of soil/rock. As well the diameter of the bored hole implies a given diameter of the inner pipes, and they have obviously different thermal exchange capacity, though you can use smaller pipes using higher pressure in them. Here there are some "generic" references (it is the site of an Italian firm that builds geothermal inverters, their English could be bettered): http://en.rossatogroup.com/guide/pompe-di-calore/pompe-di-calore-geotermiche/126-come-realizzare-un-impianto-geotermico-con-pozzi-verticali.html http://en.rossatogroup.com/guide/pompe-di-calore/pompe-di-ca...