Thanks guys! Some really useful points there - sorry we got your head hurting Justin! I almost felt the same reading your calculations!

Not that it offers any great technical detail but it looks like Dick Strawbridge had one installed and were selling them via their eco website, although I can't seem to see it on his website now...



I'm not particularly interested in buying one of these units, I just thought I'd better check I wasn't missing out on a good trick though! Thanks for everyones input.