it may be few things, are you sure you have turned the filling loop off.
It may be the PRV pressure relief valve is faulty, as it is not closing and letting water through.
Expansion vessel may need charging, but this can be checked out by just draining the bolier only, and then checking with the pressure guage like a cycle pump and see if you get reading between 0.75-1bar.
siraz
December 2009
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