I have Worcester Bosch combi boiler.....532i. Very occasionally I have to re-pressure it a little.
Today I noticed a small leak. The pressure needle was at 3.5 and in the red.
Dave had a quick google. We have run some water off and bled a radiator and the pressure is now back in the green, the leak has stopped, but the pressure is still a little higher than it normally is.
What might have caused this and what else could we do to reduce the pressure?
We had a similar problem with pressure rising whilst the boiler was on, then dropping back down when the boiler went off.
As already mentioned, it's probably the expansion vessel. We called a plumber out to sort our's out, but if you're feeling ambitious, have a look here:
https://www.doityourself.com/stry/boiler-pressure-keeps-rising