Vokera 28 combi shower temp pressure problem

I've got a few problems with my Combi Boiler Vokera Linea 28.

It all started a year ago when I noticed water dripping from the overflow. I got a new PRV and fitted it but it still dripped.

Since then I've noticed it seems to build up in pressure and blow the PRV but I never have to add anymore water to the system.

I disconnected the filling pipe from the fill valve incase it was passing but the problems still remains.

I got a service kit for the boiler think it may be diapragm problem and the other day I fitted a new Diapragm, O rings, etc to the 3 way valve and the flow switch.

Still didnt seem to make any difference until I found this forum and discovered through reading the posts that my expansion vessel was empty. I let off all the pressure and used my car foot pump to increase the pressure in the expansion vessel back up to 1 bar (was 0 bar). I then added a little water into the water side to bring it to just over 1 bar and placed a plastic cup onto my overflow pipe to see what came out.

Last night I put the heating on and the pressure gauge barely moved (before it would go to 3 bar and lift the safety) so I was pretty happy I had solved the problem. The plastic cup still collected water but only a tiny amount compared to before (it used to fill it in no time).

This morning I got up to find the plastic cup full as before but the pressure was much the same as yesterday so I assume the PRV is passing slightly, probaly due to a year of constant leaks.

Anyway this brings me to my other problem which has been getting increasing worse in the last few months but which I thought would have been resolved after yesterdays repairs.

My shower (fed from the combi) works fine but every few minutes the temperature drops from nice toatsy warm to almost cold for a second then back to toasty warm again. Today it did it 5 times (was having a long shower). The pressure remains the same throughout, it's just the temp that changes briefly.

When the wife got up and went for a shower I went to watch the boiler to see what was happening. Nothing happened, her shower was fine and the pressure on the combi never moved from 1.5bar, the temp rose to 63C then sat there for the duration and only when she turned off the shower did it jump to 90C then fall rapidly to 50C.

So I've trawled through the posts on here again and am now stuck for things to try.

I'm pretty sure the recent pumping up off my expansion vessel has solved some problems am beat with this shower problem.

any advice would be greatfuly appreciated

thanks

Webbie

ok here's an update on whats happening now.

I put the shower on and went to watch the boiler. It started and worked fine until it reached 61C then the burner cut out and the temp started to drop, then the burner would relight and the temp would steadily rise until it reached 61C then the same thing would happen. It did this 1/2 dozen times before I switched it off. The pressure gauge on the boiler does not rise or fall through any of this

also the plastic cup on the overflow is filling up again and I still have the fill pipe disconnected ? but I still reckon the PRV is passing slighty

It looks to me as if something mechanical inside the boiler is allowing water to get from the Hot Water system through to the Central Heating system ? but could this in turn be giving me the problems with the shower.


any ideas ?

thanks

Webbie