make sure the filling loop is closed and disconnect it to be sure. if it's happening even with the heating off with the filling loop off then the main heat exchanger will be burst internally and need replaced.
Ian
September 2009
As your friendly boiler repair man to fit a new pressure relief valve (PRV) - once these things have been opened, for one reason or another, they rarely seal well again. Cost around a tenner for the valve.
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