Huge problem with water pressure after expansion vessel replacement

We had a new gas boiler (Buderus 500) fitted last year and it's worked very well. We HAVE had a history of leaks but they've all been found and repaired. The pressure never held at 1 bar, it always dropped to 0.5 but stayed steady there. We'd top it up every month or so it would, eventually, drop back to 0.5 but it took weeks to do it.

Recently, the pressure started dropping below 0.5 which immediately made us suspect a leak. Topping it up like normal didn't help as the pressure then dropped below 0.5 within day.

A detection company came round and spent three hours searching but, ultimately, could not say with 100% certainty that there was a leak. There was a patch of floor upstairs where it showed the heat spread being "slightly wider" than he would expect but he wasn't sure it was a leak. However, he did say that the expansion vessel was full of water and needed replacing.

So, we had a guy replace the expansion vessel and PRV but now the pressure stays at 1 bar for an hour before suddenly and rapidly dropping to ZERO! We've NEVER had it drop all the way down to zero before. The standing pressure always keeps it at 0.3

The plumber who replaced the expansion vessel is certain it's a leak because we have closed the two valves that isolate the boiler from the rest of the house and the pressure does not drop. As soon as we open the valves, the pressure drops slightly before rising as the water heats and then this rapid drop to zero occurs within two hours.

What is even stranger is that if we top it up again the pressure gauge rises incredibly quickly back to 1 bar. I mean, in like 1 second. Hardly any water gets added to the system. I would expect that if the pressure is at zero we'd have lost a LOT more water than that and it would take much longer to fill the system back up again.

Does anyone have ANY ideas at what this could be? It's sub-zero temperatures and we, essentially, have a totally broken system.

One other thing to mention - next to the PRV there is another valve with a small black cap (bit like a bicycle valve cap). Ever since the new boiler was installed that cap was tightly closed (no idea why) but I understand that it's meant to be slightly open. Several weeks ago (when I found this out) I loosened the cap but, unfortunately by too much, and dropped it behind the boiler. So, for a few days, the cap was off until I could finally get a replacement one. Not sure what damage (if any) that might have caused.....

 

Sounds like he's not set the charge pressure correctly on the expansion vessel - he's just lobbed it in there and crossed his fingers. Either that or the EV supply pipework is blocked.

You're going to need to post a photo of your black cap device. Lots of things have black caps...

Also, get that leak fixed. Use some Miracle Seal if you don't want to take floors up

 

This is the black cap thing

https://i.imgur.com/h3Rx13Q.jpg

There's another pressure gauge next the expansion vessel which stays steady at 1 bar

I'm not sure Miracle Seal would work. Whoever built the house used cast iron pipes and didn't insulate them before putting the concrete on top. Previous leaks have been rather large (as in 2-3cm) corroded holes

 

What country are you in?

The Buderus 500 (a combi or system boiler) was a boiler made for the UK market around 2007/8 AFAIR...they were withdrawn a considerable time ago.

As mentioned the expansion vessel was probably installed without the internal pressure being checked (it's sounds like it's way to high).
It looks as though the vessel is installed on a wall manifold so the vessel connection would normally be 3/4 BSP thread and therefore unlikely to block.

Is that safety valve plumbed in...I can't see a connection.

The black cap fits on the auto air vent. Inside is a small float chamber with a needle valve. Any air accumulating in the manifold is bled off automatically. However, these AAVs often leak or block.
Once the initial air has been bled some installers tighten down the cap to prevent future leakage.

Time for a re-pipe I'd guess. Can you isolate the pipework buried in the concrete?

 

Ok, maybe I have the model wrong. Definitely a Buderus though and it's brand new from 2017/2018. Got someone coming round today to take a look thankfully!

For the safety valve, the outlet is just open, no pipe attached. It always has been like that.