In article ,
Dave writes:
Nearly a year ago I changed the old vented system to a sealed one,
whilst also adding new rads and splitting it into zones. It all works
well BUT the pressure falls VERY slowly, probably from 1 bar to 0.7 bar
over about 6 weeks.
I can't find any dampness around the rad valves or visible joints so
have resigned myself to waiting for the leak to get worse before I can
find it. Are there any suggestions on how to find the leak, or for
anything I can add to seal the leak (probably not a good idea)?

A leak that slow probably evaporates before a drip forms.
You are more likely to find it with the heating off (cold),
and the pressure increased to the hot running pressure.
I suggest you leave this until summer when the heating is
off.

The GlowWorm book suggests a minimum pressure of 1 bar, is this just to
stop local boiling in the heat exchanger?

1 bar will ensure pipework up to 30 feet above the pressure
guage is pressurised, which is enough for most houses. In a
3 or more storey house with the pressure guage in the basement,
you will need higher pressure if the pipework extends more than
30 feet in height above the pressure guage. (The pressure at 33
feet above the pressure guage will be 1 bar less than the guage
reads.)

Also, the boiler probably has a pressure loss detector which
will lockout the boiler if the pressure drops below something
like 1/4 - 1/2 bar.

As nrh said, check the FAQ too.

--
Andrew Gabriel