Most of the time the pressure relief valve discharges due the expansion vessel losing its charge on the air side. To confirm this check the overflow pipe isn't dripping the top the pressure up to 1bar when cold turn on the heating and see if the pressure guage goes up to 3bar. If it does then the problem is the expansion vessel it can be recharge by the means of a foot pump and is usually carried out as part of a service if you can find a reputable heating engineer. Most heating engineers ignore this as part of a service
SE gas man
February 2009
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