Its possible that the expansion vessel has lost its pressure, ill try and explain. The system is sealed (no water in , no water out, unless done so manually) When the heating system heats up hence the water expands thus increasing the pressure. To allow for this expansion we have the expansion vessel (metal cylinder with an innertube inside) The hot expanding water enters the cylinder and sits around the inner tube which has arround 1.2 bar of air inside. when the water cools the inner tube pushes the water back into the boilers water circuit. hence regulating the system pressure.
If the inner tube has no air or is burst, the system pressure increases with heat, has nowhere to go, so reliefs itself through the Pressure Relief Valve (PRV) When the system cools "We have lost pressure"
There is a screader valve on the vessel (like a car tyre valve) push this in, if Water comes out replace vessel, if nothing comes out inflate to 1.2 bar using a car foot pump.
Sorry about the story but hope this helps
Garry
January 2009
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