Returning to the soap opera of excessively hot water in the system (I discovered that it is not only on the kitchen tap, but on all taps on the boat).
I checked the entire system.
The problem is not the thermostat, because I have the same condition or worse when I am anchored (heating by the engine).
The heater's cold water inlet hose was very hot, deducing that the hot water is returning.
I installed a check valve at the heater inlet, and the temperature problem was solved, however the pressurization pump is activated every 10 seconds when a tap is turned on (especially set to cold).
I will remove the check valve to prevent damage to the pump.
SV Wayag, SM 323
Currently Alicante, Spain