I have a vaillant 828 combi boiler the central heating is fine, but when we turn the hot water on it does not sustain the temperature, it keeps goin hot and cold

Suki, January 2009

im pretty sure it will be the plate heat exchanger that could be partially blocked, its not flowing through which is making boiler temp to high and tripping the thermostat, remove plate heat exchanger and soak it in cilit bang over night, then flush it through, had many jobs where this been the problem. and works great for minimal cost, would get a quailfied engineer to do as its not a good idea to be messing around in a boiler when u not know what u r doing.

hudds gas man, April 2009

I had this on an Alpha 24, it was the conector to the DHW temperature sensor, it was loose, so the temperature would go up and down. On this model, lights on the front indicated the problem.

Graham Phipps, January 2009

l have 24ci glo worm combi and the hot water keeps going cold and the light flash red the gas comes on and off the heater work fine

george, January 2009

Not sure on the makings of this boiler but this problem of hot then cold usually points to the thermisters..

Garry, January 2009