Just bleed off some water from a radiator. You will see the pressure drop. If it doesn't then you need to check the mains water inlet valves. One maybe leaking water ( at mains pressure ) into the system. There should be 2 so that to top up ( low pessure ) you need to open up both. Don't forget to turn off BOTH after the pressure is correct.
Marathon
December 2010
bleed water from rad air vent
brian
December 2010
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