I am hoping someone can help, as I have looked through this very helpful forum and nothing matches my boilers symptoms! I have a Vaillant turbomax plus 828E and when the heating is not on and hot water is demanded the water comes out luke warm, the temperature on the display reads just under 40 deg C, and does not really change from this. However, the green light flashes and the orange light comes on, so it is seeing a demand for hot water, and you can hear the boiler starting up. I thought it must be a diverter valve fault, but when this occurs the pipe to the central heating stays cold, so i don't think it can be this. However, when the central heating has been on for a little while and then hot water is demanded then the water temperature is much better - although not as hot as it was a week or so ago when this fault first started. Could this be the secondary heat exchanger being blocked up? Any help appreciated?