Tankless are rated on gpm and temp rise. If you have 6 gpm incomming
at the main and your tankless is rated for 6 gpm @ 100f then you can
open every faucet in the house and get water 100f hotter then incomming.
With units out putting near 200,000 btu as standard for Rinnai and
Takagi you have alot of power to heat. Tankless have modulating gas
valves, my small unit modulates from apx 25000-117000 Btu. Mistakes are
made by underestimating incoming flow and lowest winter water incoming
temp and undersizing the Ng line, wich must be tested with everything
competing on.
Sears best Power miser Ng water heater have an energy factor in the
60s. Tankless are from 82-90+. This is your true energy rating of true
efficiency. My 800$ Tankless will pay for itself in 3 years from my well
insulated electric tank. That is an investment return of 33% per year, I
can think of no other better investment ive made.