I am building a waste heat recovery system that will use the hot waste exhaust gases (about 350 to 500 degrees) to preheat the incoming cold water when the water heater is in use. This will reduce the run time forthe water heater as the preheated water will not need as much energy to get to the set temperature. The is a product from a company call GFX gray water heat recovery that make a product that could be used for just such a use. I am building mine from stainless steel for the duct with copper tubing coils around it.
arcandspark (The Dallas Alternative Energy Meetup Group)
