Mysterious boiler pressure loss

[Bowsaint]

Earlier this week, our boiler - in a sealed system - started losing pressure rapidly (ie 1.4 bar to 0 bar in an hour).

We have had intermittent pressure issues in the past but whilst I had to repressurise after a boiler service in November, prior to that pressure hadn’t dropped since May. Which seems like a normal timeframe.

We realised there was a quite significantly leaking downstairs cast iron radiator and our plumber immediately isolated the rad and took it out of the system. He did a couple of checks after this and suggested the pressure appeared stable.

At the same time we carried out some radiator bleeding and a couple of the rads had quite a bit of air in them which led to a couple of bedroom radiators stopping working. I then bled another couple of radiators which had a lot of air in it and all rads started working again. Subsequently, as expected, the pressure dropped and I repressurised.

After only 2-3 hours the boiler pressure dropped again from 1.4 bar to 0.4 and the safety cut out kicked in.

What could the cause of this be? It feels really unlikely there would be 2 leaks? Certainly none others are visible.

Any insights hugely appreciated!

 

[Stuckinarut]

Check the prv for passing through the safety discharge pipe (copper pipe that returns back to the wall externally). Could be failed, grit on seating and or expansion related issue. For the latter part, if you pressurise the system to approx 1 bar, and after the heating being on for 10-15 mins observe the pressure gauge, if it’s high, then likely flat or failed expansion vessel.