Worcester HW Then cool...

[Nigel28CDI]

My Worcester 28CDI gives hot water immediately, but then it goes cool, before getting hot again, and so on. The boiler sparks-up when the HW is demanded, but just before it switches off for its cycle the pressure gauge bounces around +/- .3 the normal 1 bar. Has this anything to do with it ?
It thought that this pressure was only the CH.

 

[ChrisR]

It's probably sludge in the boiler water, blocking up the secondary heat exchanger. Can be cleaned but recurs quickly if the system is full of the stuff.
Try a search (it only works on one word at a time) on
secondary
dhwhe
sludge