The house is designed not to need much heating, they reckon we could get away with no heating, we're installing a small gas boiler to top up hot water (solar panels) and linked to radiant panels (a bit like underfloor heating in the walls) as a backup. The mechanical heat recovery/ventilation system should also help minimise heat requirements. We've been told by lots of people that ground source heat pumps can be highly inefficient, in order for it to be effective the system has to be designed very well.

The key to our house is insulation, walls, floor, ceiling all super insulated, all windows triple glazed.

There are a few of threads about it at www.aecb.net (you should join, a very useful organisation).