Hi, so what was the end of this story? I have the same problem with my Daikin Altherma 2 long time. I have change the boiler, bigger diameter pipes (25 mm plastic Uponor system) from Daikin to boiler (200 litres). Have I bypass valve, but I don't know if that valve is a must, because I have seen Dainkin 2 like ny without bypass valve.