News wrote:

...If you allow a tank of water to cool and you re-heat it with
a condensing water heater the re-heat is very efficient in fuel use.

As usual, News, we need numbers to make this latest vague claim of yours
meaningful, ie 1) interesting vs trivial, and 2) capable of being proved
true or false.

As stated, it's like saying "It's harder to climb stairs from the 1st to
2nd than 2nd to 3rd floors of a house." It is, with greater gravitational
force, but who cares, since the difference is utterly miniscule?

If the difference were non-miniscule, it might still be a waste of time
to discuss, unless we can put the effect to good use in some practical way.

So, exactly what do you mean by "cool"... 80 F?
And exactly what do you mean "very efficient"... 80 or 90%?
And what's your efficiency claim when heating 110 F water to 120 F?
And what's your efficiency claim when heating 80 F water to 120 F?

And how would you put this to practical use? Omit the tank insulation,
so the water quickly cools to 70 F when the heater is off? :-)

Please stop wasting our time...

Nick