Without model number. general. Might be a filter on HW inlet to boiler, could be scale build up in heat exchanger, I assume water is same with boiler off. SO a blockage in boiler somewhere. check for filter 1st, would be near to the cold inlet. otherwise blockage could be in heat Exch. could possibly be descalled, but might need replacing, especially as pressure goes up so much when going via exchanger. Without model No. Difficult
SD
December 2012
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