These water heaters include solar systems and water condensing heaters.
A solar panel attached to the roof absorbs the sun’s heat and warms an antifreeze-like fluid, which then runs to the water tank to heat the water and then back up to the roof to be heated again. These are best used in sunny areas that do not have many overcast days. The initial installation cost is quite expensive, so be sure to consider all aspects of this heater before purchase.
A condensing water heater captures exhaust gases that would normally escape through the flue and runs them around a coil in the base of the water tank, heating the coil, which then heats the water.