carbon this is from other half ----all heat pump, ground source, air source etc only produce low grade heating, that is generating heat up to 50 deg c.
This makes them ideally suited for underfloor heating systems, that only use water temp to that level. The alterative to underfloor heating systems using low water temperatures is to size radiators using a deltatee calculation of 30(a typical radiator system would use a deltatee of 60). This would typically double the size of radiators to your room making it not only aesthetically but cost prohibitive.
In all cases heat pump technology should not be considered unless you can ensure the very highest levels of insulation to your property, roof, walls, windows and underfloor, without this level of insulation running costs would be exhorbitant. Your friend must have 2 bore holes as this is classed as water to water heat recovery, which is the most effective(typically 3 to 400% efficient), the water is drawn from an underground water source circulated through the heat exchanger, where the heat is extracted using refridgeration technology and returned down another bore hole, usually downhill of the first borehole. These systems generally attract heavy scrutiny from numerous agency's e.g river authority's etc and is only assuming you have an under ground water source.
It may be that the bore hole is a vertical ground loop, which is a closed loop system,where the water in the system is piped down pipes into a pre drilled shaft and returns back to the heat exchanger, absorbing the latent geological
heat from the earth. The alternative being a range of ground loops in a paddock buried in excess of 1 mt below the surface, as this alternative extracts less heat the requirement is to install more pipe, typically the size of a 5 aside football pitch for a medium size house. All ground source systems require damp soil conditions to work best.
Unless you are replacing the system for an oiled fired or solid fuel system the running costs will way exceed that of a mains gas system.
If you are on mains gas I would not contemplate heat pump tecnology.
At present energy audits still do not recognise these technolgies fully, their arguments being that fossil fuels have to be burnt to generate the electricity to drive these systems, and are therefore not as green as they elude in comparison to solar technology, which are almost free energy, so the sap ratings are not that great.
Hope this helps.