Biasi Prisma 28SE DHW Problem

[panky]

I've read through many threads on here but can't find the same issue I'm having so hopefully some one can help me.
A couple of days ago I took the DHW heat exchanger out to give it a flush (hot water temperature was erratic). Put it back together and now the hot water will only come on when the heating is running.
I've checked the diaphragm and the operation of the diverter valve and the pin on the end pushes out when the hot tap is opened, the relay on the PCB clicks but the fan stays off and boiler doesn't fire up, also tried operating the micro switch on the end of the DV with a coctail stick but no joy, still the same click of the relay. When the heating is on the DHW is great and the heating works better than it ever has.
I've fired up the CH with the hot tap open and checked the heating feed pipe and it doesn't get hot until the tap is switched off.
One other thing that has me puzzled in that the CH pressure gradually creeps up, even though I've bled some water off and settles out at about 2.5bar - probably the setting of the relief vave.
Help please

 

[panky]

I've checked the voltage across the DHW contacts on the main PCB which read 12v with no flow from the tap and open circuit with flow so it appears the diverter valve is working OK. Is my next step a replacement PCB? Would like some advice if possible before I start spending please

 

[panky]

Thanks to all those that have read this post. Just to close it off after much head scratching and diagram perusing I popped the cover off the R1 relay and gave it a squirt with contact cleaner and what do you know everything works as it should. I like a cheap fix.
Next job is to replace the mains top-up valve and see if it cures the pressure increase in the system.

 

[panky]

Disconnected the filling link but the boiler pressure kept rising, turned out to be the DHW exchanger leaking between plates. A new one cured the problem.

 

[ollski]

You've had a. Yay week panky, that's why I never clean plates hx's....for a chargeable job it's just not worth it

 

[panky]

Thanks ollski I think you're right, for £30 it's worth changing it. The old one lasted about 20 years so can't complain.
Found a couple of dry joints on the PCB connections for the R1 relay too, but I think I'll be changing that soon as the contacts seem to be burning away.