combi boiler expansion vessel

what would the outcome of having an expansion vessel with no pressure in it?

You'll see the water pressure start to rise when the boiler is on.

The air in the expansion vessel is there to compensate for the expansion of the hot water. Pressure in the vessel should be 1bar (15psi) but check the manual.

Air pressure in the vessel, when water pressure is zero should be enough to push the air out of the highest bleed point. ie equal to the static pressure. This means with a two storey house the highest point will be less than 16 feet, which is equal to 1/2 bar. A higher pressure might be needed on some older gas boilers, but the higher the pressure, the less expansion capacity from a given size vessel.

If there's no expansion space left, the radiators and boiler have to take the strain. This will continue until they have a union meeting and decide it's not their job, then they'll leak the story to the press.

Not quite , if there is no air the pressure will rise to ver 3 bar then the expansion valve will dump the system pressure outside and when the system cools you are left with no pressure in the system which is kind of irritating as you will have to keep represuring it.

Not quite ,..

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appears to be contradicted by

.....if there is no air the pressure will rise to ver 3 bar.....

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This is the problem in the longer term, with a repeated pressure change of 45psi, in the end something will suffer from fatigue, then it leaks. 45 psi assumes the PRV is not stuck, if it is anything over 100psi is possible (and I do know of a valve that did not un-stick until nearly 140 psi).