Rinnai tankless water heaters are made by the a Japanese company that started up in the early years of the twentieth century and rose to be the biggest company in the world specializing in making these tankless water heaters and heating equipment.
Rinnai have been a major player around the world for a long time - they started a specialist range in the US back when President Ford was the President (Rinnai launched in the States in 1975).
The best known rinnai tankless hot water heaters are the Continuum, the showpiece of their range. These units encapsulate the essence of tankless gas water heaters - they are small and powerful.
Comparable in size to a desktop PC, the continuum unit can heat up to eight and a half gallons a minute, every minute. To give you an idea of what translates to, a small faucet uses three quarters of a gallon a minute, a big faucet two and half gallons.
The modern eco shower heads (low flow units) take about two gallons a minute, about the same as a dish or clothes washer. The older style shower units can blast out as much as 3.5 gallons a minute, but if you have these it might be a wise investment to move to low flow units.