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Hi,This is risky business in the blind, not responsible for damages on your end. Click 'accept' when satisfied.. or we can get another expert.
If the units pressure relief is allowing that, and not releasing, then its common with that unit OR the pressure relief valve has failed.
Do this, find the pressure relief valve inside the unit and lift its manual release lever as a test.. do this when the boiler is at 3.5 bar..get back to me with the results.