Recently had an issue with my ~5 year old Baxi Platinum HE 24 combi boiler.
Pressure had dropped and needed topping back up 3 times a day to retain pressure and not get any errors.
Got a heating engineer in, issue was diagnosed as a full expansion vessel. That was emptied and system recharged to the correct pressure. It worked great for around 3-4 days then there was a small drop in pressure. I topped it up a little. Then a couple of days later (yesterday) it dropped to 0 overnight. I topped it up and it has again dropped to 0 overnight.
I actually setup a camera to timelapse the pressure gauge and temperature gauge to see what was happening, and it seems to have the behaviour of the expansion vessel being full again.
Heating turns on - pressure instantly drops around 0.25 bar
Heating is on - pressure increases as water heats and expands in the system
Heating turns off - slow decrease in pressure over time
From what I've seen/read this sounds like another full expansion vessel, but why has it filled up so quickly?
There must be another issue elsewhere - any ideas?