hi most likely the expansion vessel needs refilling. its a red thing inside the boiler normally located at the back its a metal capsule with a balloon in it when the water heats up it expands the water in the heating this then causes the balloon to shrink to accommodate the expansion of the water. anyway to fill it u will need to drain the boiler or the heating to 0 bar pressure then on the red tank normally at the top of the tank is a tyre pressure valve as the same as the one you have on your car. get a foot pump and inflate it between 1 - 1.5 bar and it should do the trick. ps if it keeps happening you vessel has had it
kash, November 2012
