Lets logically look at this,you lower the system pressure to 1.5bar,your top-up hose and associated v/vs are shut, the only way mains water can interact with the system pressure is when the internal heating unit exchanges its heat to the domestic hot water this occurs in the flat plate heat exchanger regardless if you have a 3 way valve or a dual pump system in your boiler which swaps the boiler's heated water from its furnace to either CH or DHW. The flat plate exchanger can leak and cause the mains water to leak thro' and over pressurise the system the flat plate is over£100 so I think that hasn't been replaced. When in CH mode the pressure should not significantly rise, turn off the cold feed as an extra check. However if you then go to Hot water demand from your tap check and see if the pressure is now on the rise,turning back on the cold feed first, you can decide your next action on the results.
Guytawe
June 2010
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