opal2 10.5 needs 1.5 bar and 11 litres of water flowing to lift pressure switch if you only have .7bar (3-5 litres flow)so pressure switch will not engage heatig elements,as you turn the temp up you are resricting the flow this then increases the pressure in the supply as less water is flowing through the unit so the unit thinks there is more pressure than there is, now on 10kw the temp rise on 3-5 litres a min is going to be over 50deg they say it is possible to get 3rd deg burns from water at 60deg + so be very careful