Long recognized in Europe and Asia as a major energy- and space-saving solution, tankless water heaters have begun to make significant inroads in the North American market.
In 1970, when I started in the heating industry, there was a battle going on between the folks who believed in catching and controlling the air in a hydronic system by holding it inside a plain-steel compression tank, and those who thought it was best to catch the air and just get rid of it using automatic air vents and diaphragm-type compression tanks.
There will always be fringe markets for products that use renewable energy to provide marginal heating comfort. These markets survive because early adopters willingly sacrifice superior comfort as the perceived price for using renewable energy.