Surprise. Three different kinds of copper K, L, M with pressure drops of the order of 45% higher in K compared to M. Looks like the type of copper will have to be discovered too.
The math is 100% known for this problem. Nothing to do with Kepler's situation.
But it is not known to any of us here. Yet. We're still missing the copper equation as opposed to hose, the circular turn equation, not sure about the elbow if it is copper rather than what? Once we have everything...
You are misapplying the manual: the 37 degrees-3 LPM pair assumes a standard input temperature that is not available here so it is a pointless thing to say or test for.
How about this for a reason: you do not grasp that the 37 degrees is up to the taste of the user, that the 23 degrees is up to the weather, that the maximum flow available at the balcony varies depending on what the city is consuming, and that the problem does not always occur and cannot be...
This link you gave is about hoses, is there another formula for copper pipes maybe ?
https://www.engineeringtoolbox.com/water-pressure-loss-hose-d_1525.html
Asymptotic, you do not really have an alternative that fits the observations in #40. Can't blame it on heater malfunction, it is almost new. Can't blame it on a clogged filter, the little mesh filter inside the heater is clear. Can't blame it on anything other than low TOTAL flow. Those 6 kW of...
Let us not forget the plumber's claim is about pressure, not flow rate. Pressure at the point where his piping ends and devices begin. We still care about flow so we might as well ignore him, even if when he is right.
The temperature of the hot water increases, the temperature of the mixed water decreases. This is equivalent to pumping more water through the heater while the cold tap is closed.