Key Post: geothermal heating system/ground source heat pump

What about the air source, same system but witout the cost of the underground pipework?

the problem with air source is that the temperature in the air varies quite an amount more than that of the soil and will give problems on colder days!!

But are they not vastly cheaper to buy and install?

Yes. A GSHP can be a combination heating/cooling and hot water heating system, GSHPs can save you up to 50% on your water-heating bill by preheating tank water.Most of a GSHP installation is underground.GSHPs are very quiet, providing a pleasant environment inside & outside of the home. GSHPs have no noisy fan units to disturb outdoor activities, on or near the house .At European Ground Source Heat Pumps Association website you can get all the detail in respect to Gshp installation and designs.