Hi Anne, If the pressure is dropping a lot more than normal then you must be losing water from the system, it could be caused by a minor leak on the c.h. system, normally from a radiator valve which gets turned on & off a lot, Check all the valves to see if theey are wet. Secondly and most probable is the expansion vessel has lost it's air charge, this would cause water to escape via the safety valve because it can't expand into the vessel and this would cause loss of pressure in the system. The vessel should be checked by a "Gas Safe" engineer and the air pressure recharged if it has been lost. You can check the exhaust pipe from the safety valve outside to see if it is discharging any water when the boiler is on, this will confirm the expansion vessel is at fault.
Menditman, January 2010
