I was researching the very same thing a few months back. I live in the
Philadelphia area so in the winter the water can get pretty cold. I
found a DYI article about an electric version (I don't have gas in my
house), and it looked pretty good, but I remember there being an issue
with a limit on the max temp the water could be raised as based on the
temp coming from the ground. I was looking for something for 3 baths,
washer, and dishwaser. Cost was about $800 for the unit.

One link I found was
http://www.bobvila.com/VideoLibrary/...deo-20517.html