I have a vaillant combi boiler - it is 4 years old. I've noticed in the last week that it isn't heating up to the temperature which it used to. It heats it up to the maximum temp and then cools down to lukewarm water.
I checked the pressure which was slightly low so increased it - it doesn't seem to have made any difference. Have turned up the boiler temperature which is reflected in the maximum temp.
It only seems to happen on the hot tap on the bath - could it be the tap that's causing this? Or is it the boiler?
Have you checked it is firing properly when the taps are running? Is the flame blowing out? If your water pressure is too low this can cause an automatic cut-out IIRC. You should really get a corgi guy in to look at it I think.
We've had some problems with ours recently - well acually ever since we moved in. Water on full flow = luke warm water no matter how long you leave it to run. Water on a bit more than a trickle = hot water after ages. We had the boiler serviced as there were pressure issues, and afterwards we needed something replace, along the lines of "external pressure valve" (can't remember the exact name but can find out for you). We now have red hot water, even on full flow!
It still does take about 30 seconds to stop being freezing cold though - DH says this is just what combi boilers are like and I've been meaning to ask on here if everyone else finds this?