I have a problem with an open-vented system returning an 06 flow error code after a minute or so when on DHW only. CH is aok. When CH is on and then DHW is called for, system runs ok and hot water pumps to storage tank. Pump is new and DHW motorized valve is opening fully. Installation has a gate valve bypass from pump output to boiler return set at a couple of turns open. Surprisingly, fully opening this did not solve the problem!
