This sounds unhealthy, the pressure should never do that, it should never be operated at low pressures as the water in the heat exchanger could boil. Top up the pressure using the filling loop. If the pressure shoots up when the central heating heats up you need to recharge the expansion vessel. Do it now, if you have a car tyre pump and access to the top of the big red expansion vessel at the back of the boiler, do it yourself, google it Its time you got it serviced.
c#
February 2011
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