no you are wrong. you just dont understand how shit works, and you dont understand your equipment options.
a tanked water heater has BETTER control than almost all instant hot water heaters(inline if you will).
a tanked water heater will reach and maintain a temp provided you do not draw it down to the point where the stratified layers of hot and cold water do not begin to mix agressivly.
typically the cold water enters the tank through a dip tube that travels to the bottom of the tank... not all the way to the bottom(sediment disturbance), but far down.
when you draw down the tank, you are picking up the hot layer of water on top of the tank. as you are drawing from the tank, cold water is entering this dip tube and displacing lost hot water... the cold water however largely remains low down and mixes slowly.
there are two heating elements. one low down where this cold water stratifies, and another high up.
the low one is usually a beef coil, 20 amps or so. and the upper coil is usually a bit smaller. the lower coil is the one that runs when you draw down hot water.
the upper coil is the one that runs for like 5 minutes every hour or so... just to maintain the temps in the tank.
if you size your tank such that you are not drawing down more than like... 30% of the tank volume at any one time, you will never have huge temp fluctuation during a draw down. the cold water will then remain largely stratified on the bottom where it will slowly equilibrate with the top layer ... the lower heating element will fire up and the whole tank will equilibrate in like 30 minutes or so... ready for the next draw down.
you could also install a thermostatic mixing valve as a sort of thermal pressure regulator.
basically it will mechanically respond to water temps adding more or less cold water to the stream of hot water to arrive at a constant temp.
but what ever.
