I have a combi Vailliant about 12 years old. The thermo. For the hot water is set at the highest point in order to have a fairly hot shower and get hot water from taps in bathroom,otherwise the water would be lukewarm. Even so if I try to have a bath the water runs hot for about a minute then becomes luke warm. If the toilet is flushed whilst one is having a shower the water becomes scalding hot for about a minute then revert to bearable hot. The water pressure is normal. HELP HELP?? DOES THE SYSTEM NEED FLUSHING OR DO I NEED A NEW PUMP. Previously I had a diverter valve fitted because I could only get hot water if the central heating was on. But I think this prob was occurring well before.

We have had a similar problem to this which was caused by a faulty thermostatic valve in the shower, which was allowing cold water to enter the hot water system. The shower would get hot, but everywhere else in the house, the hot water would cool down to warm after a few minutes. If you can get easy access to the pipes, you can test this by running the shower a few minutes so that the it runs hot and the hot water pipe leading to it feels hot. Then turn it off and run the hot tap on the bath. If there is a faulty valve, the showers hot water pipe will feel cold very quickly as cold water is sucked through the valve.