Help with Junkers W250K water heater

Can't remember exactly which knob does what on that model, but one of them controls the amount of gas that will pass through the burners and subsequently the outlet water temperature, the other controls the flow of water through the water heater which if passing through too fast will not allow the water to get hot enough, fast enough, usually in the winter months one has to alter the settings on these units and slow the water flow through them a little to give the water "more time" in the heat exchanger to allow the heat to be applied to the water - thusly increasing the water temperature. Best trick is to mark the settings before you tweek with the knobs so you can reset them if you are unsucessful.
Hope this helps.