if your pressure gauge is rising quickly when you call for heat and then judders and the pressure drops you need to check the pressure in your expansion vessel, if this is below the pressure setting on the label on the expansion vessel you need to recharge it by draining a small amount of water from your heating system until your pressure gauge reads 0 then attach a foot pump or bike pump to test nipple on the expansion vessel and pump it up until your pressure gauge on the front of your boiler reads 1 bar or to if you have a gauge on the pump to the recommended setting upon doing this the boiler still does the same thing check the pressure with a tyre gauge again and if its dropped you need a new expansion vesseel, if the pressure is ok it could be a couple of things broken diverter valve , seized pump scaled heat exchanger, also it may sound daft but check that the filling loop hasnt be left turned on and is over pressurising the system causing the blow off to activate
mike
December 2005
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