Water pressure drops in boiler

13 May 2008
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United Kingdom
I have a Vaillant gas boiler that has a water pressure gauge and a Megaflow storage tank that also has a water pressure gauge installed near it.

My problem is that the water pressure gradually drops from 1.5 bar to zero over the course of about 3 weeks - I can re-introduce the pressure by turning a water tap that builds up the pressure (both gauges at boiler and tank read the same) but it always goes down gradually.

I cannot see any leaks and recently replace the Pressure Relief Valve in the Megaflow.

What could be causing the drop in pressure?

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