It is possible to use a ground source heat pump to heat cast-iron radiators, but efficiency will be compromised as a heat pump works best at heating water to a maximum temperature of 50°C. For a radiator system you would need temperatures to be 70-80°C. You could resolve this by having larger radiators, so that the output temperature of 5o°C is compensated for by the number of emitters; or you could use a GSHP with a boiler to top up the temperature to the required level. The disadvantage would be the additional cost of the boiler. Wood chip or wood pellet boilers are the most environmentally friendly options.
For the best possible heat efficiency you will need to ensure your home has good insulation and is properly draught-proofed, so you won’t need such a high level of heating. The most efficient way to use a GSHP is with underfloor heating as the temperature of the water does not have to be so high, yet the ambient temperature of the rooms will still be comfortable. For average-sized dwellings, a GSHP is likely to cost between £8,000- 12,000 to install (excluding underfloor heating). The cost savings will be in the reduced heating bills. GSHPs work using electricity. A 25kw gas boiler consumes on average £750 of gas per year. A 6kw GSHP producing 25kw of heat output and operating efficiently (at 400% with underfloor heating) consumes about 5,000-7,000 watt hours per year at about £400-500. Get quotes from three suppliers and ask them to cost a GSHP-only option and one with a boiler, so you can see what suits your circumstances. Try Econic (01603 700999), EarthEnergy (01326 310650, http://www.earthenergy.co.uk/), or Viessmann (01952 675000, http://www.viessmann.com/). For more general information on GSHPs try the Energy Savings Trust (0800 512012, http://www.est.org.uk/) and the Centre for Alternative Technology (01654 705950, www.cat.org.uk).