Our boiler has always needed a bit of topping up on a regular basis,
but recently it seems to be getting worse - in the morning the
pressure can sometimes be so low that it refuses to fire up. However,
if I re-pressurise it (just enough to take it out of the red zone), it
registers about 2 bar when hot and will happily stay there all day, so
if there's a leak why is it only when it's cold? I can't imagine any
leaky joint is going to expand with heat and seal itself quickly
enough to keep the pressure up.

--
Halmyre

Halmyre wrote:
Our boiler has always needed a bit of topping up on a regular basis,
but recently it seems to be getting worse - in the morning the
pressure can sometimes be so low that it refuses to fire up. However,
if I re-pressurise it (just enough to take it out of the red zone), it
registers about 2 bar when hot and will happily stay there all day, so
if there's a leak why is it only when it's cold? I can't imagine any
leaky joint is going to expand with heat and seal itself quickly
enough to keep the pressure up.

--
Halmyre

I'd say your first course of action is to determine if the pressure loss
is via the Pressure Relief Valve, or a leak in the pipework.

Pressure it up to the middle of the green when cold ( around 1 bar ).

Use electrical tape to tape a clear polly bag over the pressure relief
vent ( on the outside wall, behind the boiler, probably a 15mm copper
pipe turned back on itself to point at the wall ), so the pipe is
completely sealed into the bag.

Let the system run for a day.
After the pressure loss, is there water in the bag?

If yes, it indicates the system went over-pressure when hot, causing the
PRV to vent. This indicates a failed expansion vessel.

--
Ron