Pressure guage on Worcester 28i Combi Boiler

[citylee]

I have a Worcester 28i Combi boiler and lately the pressure guage keeps dropping to zero when the heating is off. I have a small tap at the side which allows me to add water to increase the pressure to between 1 - 1.5 bar (as instructed at the time of installation). I can find no leaks anywhere on the system and if left alone on zero the pressure increases to approx 3 bar when the heating is on. I have noticed that on the occasions the pressure dropped to zero and I added water to bring the resting pressure up to 1 - 1.5 bar then when the heating came on the pressure increased to near 4 bar and I had water coming out of the outside pressure pipe
Any help greatly appreciated

 

[gremlin16]

Check the expansion vessel. It may need recharging or it may be u/s

 

[SteveZodiac]

I had a similar problem with my 28cdi losing pressure which turned out to be a combination of problems, one of which was the expansion vessel. It was slowly filling with water, which made the pressure fluctuate to the point where it was zero when cold but reached over 3 bar when hot. Had it replaced which cured the problem. It could be as simple as the car-type valve which is used to charge the vessel leaking air which might be fixable, or a blockage in the pipe to the vessel. But if the diaphragm inside the vessel is punctured the whole thing will have to be replaced.