Most conventional systems consist of boilers and hot water tanks. Instantaneous water heaters are becoming common in radiant floor systems that provide great efficiency.
Greener alternatives are also available by Michaudco. Solar water heating, geothermal heat pumps, and masonary heaters are some of the best around that will pay back over the life of the system as well as help you do your part in the fight against global warming and ongoing oil depletion.