Hello.
pablo732 wrote: Kocił is no longer under warranty.
It is a pity, because usually such strange unusual symptoms, it is a disc fault in the case of NORDGAS, and it goes without warranty.
I assume that the boiler is clean, properly adjusted, the cables in the boiler are not damaged, burned, etc.
From experience I know that usually the problem with DHW is associated with a flow sensor or temperature sensors. Flow sensor excluded, moreover, the symptoms described in your second post do not match.
Stoning or obstructing the DHW circuit can also be ruled out - not these symptoms. Unless the exchanger is overgrown on the central heating circuit and overheats at higher burner power, as it works on hot water, but then there should also be problems with working on central heating. In the event of limescale, there should also be noises indicating this: in the kettle, knocking, etc.
To these symptoms can be pulled up this temperature sensor on the exchanger (coil) of the boiler. If at a higher temperature it gets a temporary break, or short circuit, or its characteristics are broken, then the burner can be turned off without reporting the fault by the boiler. I have encountered such a sensor fault causing similar symptoms.
If you have this sensor that was exchanged for hot water, replace it for testing on the boiler exchanger (the fixing plate is a little tighter but can be fastened) and check for the same symptoms.
What else can cause the burner to switch off without reporting the fault by the boiler.
Temporary interruption in the safety thermostat circuits (chimney draft thermostat and STB thermostat on the boiler exchanger). Very short breaks extinguish the burner, and the boiler may not report an error, and after a few seconds the burner fires again, and so on.
Similar symptoms are given by short breaks in the gas valve coil circuits.
These elements are common for CH and DHW, and the problem occurs only on DHW, but this does not exclude these elements, because when the boiler starts working on DHW, it usually works with more power, and heats up to a higher temperature, and thermal faults of the sensors and protections can only be revealed at higher temperatures, so the problem can only occur on hot water
And what does the ignition-ionization electrode look like? Is it clean, not bent or scorched, etc.? Does this electrode not come out of the flame, as the burner goes into more power? Maybe the problem is on the side of the electrode, which loses ionization, as the burner goes into more power for hot water
On the left side at the bottom of the boiler you have a gas valve. Cables come to him. One lower black is thicker with a plug, and two thin on top of the gas valve connected to the sleeves. Disconnect one of these sleeves for testing, and check if the boiler still behaves the same way. After disconnecting this lead, the burner will operate only with minimal power.
And whether there was certainly a case of spontaneous switching off, switching the boiler to a different mode of operation, or changing the set temperature, because usually NORDGAS plate defects are accompanied by such symptoms, but this is not a rule.
Greetings.