if the pressure rises only when using your central heating then its an expansion vessel problem.
the expansion vessel is half air half water seperated by a diaphragm. as the system water (radiators) gets hot the water obviously expands and the air section is the part that allows this to happen.
if the expansion vessel is undercharged with air then once expansion starts it'll cope for a little while then pressure will rise when it cant cope no longer.
ive had to recharge a few ecotecs and its solved the problem.
the expansion vessel can generally be flat or dead
flat: press pin in and you'll get nothing out, maybe a little air
dead: press pin in and you'll get water
iain
October 2008
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