What?

After some searching and research, it looked like heat pumps would be a non starter, because the output temperature of ground source heat pumps and HFC-based air source heat pumps is too low to plug into my existing and conventional heating system. They generally deliver water at about 50c, and then use a "conventional" electric heater to boost the temperature to 65c. They can work down to temperatures of -10c, but during the winter of 2010, North Devon experienced temperatures of -13c. The last thing you want is your heat pump not to work on the coldest night of the year.

My heating engineer, Paul Furber of Furber Heating Ltd. directed me to Sanyo heat pumps based on the CO2 cycle. These deliver water at 65c with no need for a supplementary heater.

Perfect: you can store the water directly and use it for your bath or shower, and pump it round your existing heating system. Even better, they work all the way to -25c. This should be OK, because the lowest temperature ever recorded in southwest England was -16.7c at Cullompton in Devon in January 1940.