I had the idea of installing a drain heat recovery system on my high efficiency boiler exhaust. It captures wasted heat in the exhaust, by running half of the domestic water fed to the house through copper coils wrapped around a copper exhaust pipe. This preheats all of the water in your house, which reduces your water heater usage. I've only read about one other person experimenting with this. The only thing I didn't mention is the inside of the copper pipe should be coated with some sort of paint or epoxy so the corrosive exhaust condensate doesn't corrode the copper.