Depends on model, but the expansion vessel is a pancake metal container, with a schraeder valve fixed to it (car tyre type), you will need to drain the vessel of water before charging with air, and have the drain point open whilst pumping with air, to allow any accumulated water to be drained away, however if you here gurgling sounds of air being forced through the water ways, you will have to repalce the vessel due to a failed diaphragm.
Vessels on newer vaillants are accesible (right hand side) older models have the vessel mounted behind the boiler (between boiler and the wall) the air valve is usually accesible from the top.
www.elanar.co.uk, October 2009
