when the boiler is runing how high is the pressure geting on the boiler,if you have no leaks it could be the pressure relif valve letting by(check the prv pipe outside to see if its been driping)or the expansion vessel might need recharging the 105 uses the expansion vessel for the preheat on hot water, has the hot water been ok,the pressure guage should be 1 bar cold and 2.5 max hot
jim
January 2009
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