worcester boiler noise after hot tap use

[theexey]

hi

any help would be greatly appreciated..
i have a worcester 28i junior and it has started to make a rumbling noise and the pressure release valve is opening. this only happens when the central heating is on and then a hot water tap is used. the noise is after the tap is turned off. this is also causing the pressure to spike.
i have checked the expansion vessel and no water came out of the valve but it needed pressurising.
the system pressure is at 1 bar and stays pretty constant exept when it hakes this loud rumble..

thanks

 

[tipintim]

Have you checked the pump is working. Pump over runs after hot water is drawn to desperse the heat from the boiler.

 

[shorticus]

when you say that the expansion vessel needed re-pressurising did you do this with the boiler drained, and also what did you re-pressurise it to?

 

[tipintim]

Does it work ok on heating mode only.
i.e do the radiators all heat up to temperature with out the prv going

 

[theexey]

hi
the expansion vessel was pressurised with the system partially drained so the system pressure was at zero. the e.v. was then pressurised to 0.8 bar.
im not sure how to tell if the pump is over running, this problem only happens when the c.h. is on and then there is demand for hot water.its after a hot tap is turned off thats when it makes the noise. when the hot water is on without the c.h. its fine, also the c.h is working fine as well.