This is one of the questions I address in my book, Sustainable Energy - without the hot air (available from amazon, and for free (pdf) on the web). The chapter on "Smart Heating" highlights heat pumps. They are definitely going to be one of the pillars of the post-fossil-fuel future. With a typical suburban population density, there is indeed not enough ground area for everyone to get all their heating out of the ground, if they are not careful to put heat back at other times of year. If people suck too much heat, without putting it back, then the ground will gradually become frozen. There is a similar potential problem with ground source air-conditioning, where people use the heat pump the other way round a lot, dumping heat in the ground (or sucking cold, if you like). Eventually the ground warms up, and the A/C doesn't work so well. This has happened in central London. There are two fixes: (1) ensure that annual heat sucking matches annual heat dumping (by matching winter heating to summer A/C, and perhaps even adding solar hot water panels to the roof, to get extra heat to dump during the summer); or (2) use a heat pump but with a different heat source, for example the air. As already noted, air-source heat pumps are not great if you have -40 degree winters. But in many countries with mild winters (eg Britain) I think air-source heat pumps are the best choice for green heating. Sustainable Energy - without the hot air also has an appendix on heat pumps in which the relevant equations are worked out from first principles. David MacKay Cambridge

Thanks very much for the feedback on Dasher. I have updated the Dasher FAQ which you can get to from the "Any questions" link on Dasher to answer several of the questions people asked today (26.6.2002). Let me highlight two points:

• If you find Dasher "scary", or "it is writing things I don't want to write", please read the tips for novices and the three page explanation. It's like learning to drive a car. Don't speed; Don't drive forward until you know where you want to go; Go slowly for a minute or two and you'll soon get better.
• According to his assistant, Stephen Hawking can write at 6-9 words per minute with his present system, on a good day. With Dasher, we can write at 20 or 35 words per minute, depending on whether we use eyetracker or mouse.
• If anyone wants a job in Cambridge working on Dasher, we are looking to recruit a good programmer to clean up the rough edges.

Have fun! - David