Look for leak on the pipe from the Pressure Relief Valve, this is the 15mm or 22mm copper pipe on your outside wall. Also if expansion vessel is in need of recharge you will see pressure on gauge ramp up quickly when you run boiler on central heating.
Expansion vessel needs to be recharged with boiler at zero pressure, ie close both flow and return isolation valves, open up the drain cocks( 10mm spanner) then check recharge vessel.
Andy
July 2009
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