A drop in the cold main pressure can cause a boiler not to fill because the non-return valve on the filling loop would not open. This valve may also be stuck or need renewing. If water is getting past this valve and into the system (you would probably be able to hear it), then the pressure guage could need replacing. It is worth having a look outside to see if there is any water discharging from the 'blow-off' pipe from the boiler. There may be other checks required to the expansion vessel as well which could be causing zero pressure. There may be leaks on the radiators, valves and pipes (check floor covering and ceilings for wet patches).
Ed
January 2009
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