Sounds very much like your expansion vessel is NG. This is the large red disc item inside the rear of the boiler. Inside it is a strong diaphragm, one side is pressurised and the other allows the expanding hot water from the heating side to fill the vessel, then as the heating cools the pressurised side pushes the cooling waster back into the system. If the diaphragm fails then the water is pushed out via the overflow outlet usually not noticed, but look for traces of water around the external overflow pipe outside. There is a real easy way to DIY check this with no gas risk etc. Remove the large upper front panel from the boiler, and on the top (normally RHD side) of the expansion vessel is a valve, same as a car type with a cap on it. Remove the cap and for a second press the pin in the centre of the valve down, only thing that should come out is compressed air. If you have water coming out then the vessel is NG and the internal diaphragm is ruptured. This will then need a gas fitter, to either replace, or as I did for ease, add another external expansion vessel, took the gas fitter less than an hour to do. They just tapped into the heating pipes right by the boiler and added a small expansion tank. Long term the inside of the original tank may corroded through and begin to drip, mines still OK after 5 years so not much concern there. Cost for the whole repair was less than £100 as I say 5 years ago.
Dokka Watson, February 2015
