Generating and moving domestic hot water seems to be a popular area for ideas. I couldn't see a category, but they seem to cluster at "home: water".
[Did I do this right?]-- maffu, Dec 20 2008 You're right in that there are some hot water ideas in Home: Water, but that category as a whole isn't so full that it needs to be split up - I'd normally wait for it to fill up a little more.-- jutta, Dec 20 2008 random, halfbakery