vaillant turbomax 824

[colw1]

hi my vaillant seems to have lost its hot water temperature, when i say this you open the hot tap and the boiler starts up the water heats and ofter 10 seconds or so the boiler shuts down then re lits and the process keeps going on over and over until the bath is full, whilst this is going on the temperature reading goes up rapidly the reduces the back up again. the problem I have is that the water is not very hot with this very short period of time
oh yeah I have tried the hot water dial on top setting as well
please could anyone suggest what is wrong
thanks col,

 

[ChrisR]

a) Probably sludge, which can be cleaned, diy maybe just
b) maybe a faulty temp sensor, diy ok, depending
c) maladjusted gas valve, not diy.
or a combination