Can you confirm or dismiss my amateur diagnosis?

[whitty]

My vokeira compact 24 pressure keeps rising till prv blows after about 3 hours. The filling loop is disconnected, It does it hot or cold. I think I recharged the vessel right, had to drain fair bit of water from rads to do it, set it at 1 bar. There didn't seem to be any water come out of the valve. I left the gauge on it when I recharged the rads and the pressure stays the same as on the boiler gauge and has stayed the same when I fired the system up. I have tried it with dhw cold supply shut off and it did not rise hardly. IS MY HEAT EXCHANGER LEAKING INTERNALLY? Am I allowed to change it myself, do I need any specialist tools and how much do they cost??? Be gentle with me P.s could it have been caused by high pressure mains feed?

 

[chrishutt]

This is all explained in the FAQs. There are only two explanations for rising pressures. 1 - the expansion vessel has failed or lost its charge, in which case pressure would rise on heating and fall on cooling, or 2 - there is a leak from mains side to heating side, in which case pressure rise is pretty constant. So which is it?

Having read your post again, I see that the pressure rise stops when you turn off the mains. So it's a leak from mains to heating side. This is usually via the filling loop, but if that possibility is eliminated then it's likely to be the secondary (DHW) heat exchanger.

 

[whitty]

Thanks, Yeah my old loop was very tempremental so I renewed it and took the hose off after pressurising system but it still did it I'm afraid. Any idea of cost?

 

[chrishutt]

Looks like your boiler has a bi-thermic heat exchanger, meaning the DHW function is combined with the main heat exchanger. That makes it a whole lot more complicated to change, especially since it takes you into Corgi territory. I guess the heat exchanger itself is going to be around £150 and the labour to do it - maybe another £100, although I really don't know how difficult it is on your boiler.

Happy Christmas.