The most likely cause of this (pressure fluctuation) is due to the expansion vessel inside your boiler being low on air pressure. This is a two part device - one side has air pressure at around 10 psi, the other side has water at usual pressure, and the two are separated by a rubber diaphragm. Perhaps you'd like a heating engineer to check this for you? Failing that, if you have a car tyre pressure gauge and foot pump, you can check the air pressure inside the vessel and adjust it accordingly, using the tyre type valve thats there to see. If you press the valve with a matchstick or something though to test and water appears, the diaphragm has packed up.
John
April 2008
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