if its pouring out pr pipe outside cos pressure is constantly rising even when off, due to mains cold entering sealed system side then there are two possabilities really, filling loop passing even though it appears to be turned off (disconnect it to make sure its not passing) or the plate exchanger is pin holed and allowing mains cold into system side and raising the pressure that way.
they are available from heating parts companies depending on your area cant tell you who, they are razor sharp and quite fiddly to change and require draining of boiler and bit system knowledge.
iain
October 2008
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