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I have a unvented hot water system & when the hot water is turned on the pressure is as good as it was before a new expansion vessel was fitted, however after 5 > 10 seconds it drops to a level which is Ok but not brilliant. The expansion vessel pressure is set at 2.1 bar (as was the old one) which is the pressure recommendation on the hot water tank, the reducing valve is also set at 2.1bar. Could the reducing valve be faulty ? or could a air lock be causing the problem ?

Hi rbtisa
The pressure control valve would seem to be the issue but do check all valves which may have been shut for the EV replacement are fully open.
Regards S

Thanks for the reply S
All valves are fully open so I might replace the reducing valve with one which is fitted with a gauge, at least that way I'll know if the tank is at the correct pressure.