The expansion vessel, as fitted to combi systems is actually in two halves - one tank separated by a rubber diaphragm. The boiler side is full of water, the other side is air pressurised to about 10 psi. You'll see a schraeder valve there for testing the pressure and topping up. If you press the valve, and water is present, the diaphragm has failed.
John
April 2009
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