hi ,i have a vaillant turbomax ,about 8 years old and got a hot water problem. when I turn hot water tap on green light flashes to show demand , yellow ignition light comes on and boiler fires for 2 seconds then shuts off. then it fires again for 2 seconds and shuts off , does this constantly. heating works fine when on but if I open hot water tap does exactly same on then off. At the tap runs hot then cold , hot then cold. pcb board has been changed for new one but made no difference, any ideas please