Ok, first disclaimer, I have no idea what I'm talking about. Now that I've said that, could you do some sort of aparatus that bubbles the hot air through the water? I don't know how efficient this would be...I'd imagine smaller air bubbles would transfer heat better, but that's just a guess. Now shoot down my crackpot idea (but you have tell me why it won't work) :) Josh -- A common mistake that people make when trying to design something completely foolproof is to underestimate the ingenuity of complete fools. -Douglas Adams On Fri, 11 Feb 2005 15:39:51 +0000 (GMT), Howard Winter wrote: > The Centre for Alternative Technology, near Machynllyth > in Wales, did an experiment using an underground water > tank for heat storage - I don't know if the write-up is > still on their site (http://www.cat.org.uk) but I d > remember it wasn't as successful as they'd hoped, and > they discontinued it after a year or two. I seem to > remember the biggest problem was insulation - they had > *a lot*, but it wasn't enough. They used a drip-roof to > collect the heat, I think - how would you transfer the > heat from hot air to the water? That seems to me rather > trickier than the other way round. -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist