In an earlier contribution to this discussion,
Milleniumaire wrote:

I checked the pressure on the expansion vessel with the boiler "quiet"
and this showed 2.25 bar. According to the technical data in the
manual:

Expansion vessel charge pressu 3 bar
Expansion valve setting: 6 bar

On the expansion vessel itself (this is a large silver container at
the front of the boiler) it shows that the vessel should be set at 1
bar.

I'm not sure where most of those figures came from - they sound like garbage
to me!

Please explain what you mean by "I checked the pressure on the expansion
vessel".

I suspect that you are talking about water pressure which is *not* what is
required.

The expansion vessel has a synthetic rubber diaphragm inside with air on one
side and water on the other. The air pressure has to be set with zero
pressure on the water side. It is checked using a car-type tyre pressure
guage on the built-in schrader valve. The air pressure typically needs to be
between 0.7 and 1.0 bar - which is probably where the 1 bar message on the
vessel itself comes from. It it's too low, you pump air in with a car tyre
pump.

Once you've got the air charge pressure correct, you can *then* pressurise
the water system - introducing water via the filling loop. The water
pressure when the system is cold should be very slightly higher than the air
charge pressure - so that it just starts to compress the air in the
expansion vessel. When the system gets hot, the water expands - compressing
the air in the expansion vessel further. The vessel provides resilience to
allow the water volume to increase without the pressure rising to too high a
level. A water pressure of 2 bar when the system is hot is about typical. If
the pressure rises above 3 bar - which it can easily do if the expansion
vessel is faulty or not correctly charged with air - the safety valve
operates and lets water out. Then when the system cools, the pressure falls
to zero - which *ain't* what you want!

I've no idea where your information came from stating a charge pressure of 3
bar and blow-off pressure of 6 bar - unless you've got a highly atypical
system. I don't think I'd want those pressures in a domestic situation!
--
Cheers,
Roger
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