Verified answer

Its the water pressure valve that is gone. Most probably the spring inside is worn and no longer holding the pressure in the boiler. The valve causing this is the one the overflow pipe is connected to. It most probably has a red cap on it. If you can turn the water off to your boiler, you can fit a new one your self. You may need to remove a side panel and bottom panel to do so. Replace it with the same one on the boiler as they are presset to a pressure suitable for your boiler.

Vokera Mynute 20e

Pressure relief valve , right angle valve with a red top , sometimes letting some water out manually lets grit out that may be causing a problem

link to pic

http://uk.images.search.yahoo.com/images/view?back...