I have a Vaillant Combi boiler VCW-T3W which fails to heat hot water correctly. When the tap is turned on the water initially heats to the correct temperature however it then quickly cools to at best luke warm. If the cold water tap is run simultaneously the temperature increases but not to max. In the boiler the burners are running continiously from the moment the hot water tap is turned on. The central heating is working correctly. I suspect this is a valve problem but do not have the boiler manual and have not been able to find it at the Vaillant website. Does anyone have any information / thoughts?