I've had the above model boiler in my house for a number of years, and last winter I noticed that after every use of the central heating the system would completely depressurise. For one reason or another I didn't get round to looking at it, but this year its become an annoyance so I've investigated. After talking to friends in the know they told me to look for a red safety valve, as it may be there's leak from it (I've checked every joint in the entire house and I'm certain there are no leaks). Last night I made a discovery - I found the valve and noticed that if you turned it one way, it drained the system (I heard the water splash onto the path outside) so I concluded that while cooling after use, the water is leaking out past the safety valve. To prove that fact I put a dry cloth under the outgoing pipe to see what happened - what actually happens it after about 15 minutes of use, the pressure rises to about 3.5 bar and the valve opens, emptying a good proportion of the water out. Once switched off, the remaining water slowly drips from the system past the safety valve. Now, obviously a safety valve is a necessity, but is there any way I can increase its resistance to stop this happening? Its in a awkward place to get at so I can't tell if its adjustable - all I know is its a red plastic twistable cap which opens the valve if twisted anti-clockwise. Any ideas?