If you have a storage cylinder connected it may be low pressure in the expansion vessel. This should be checked annually and the manual will tell you what pressure is needed. Mine should be 4 bar but was down so I pumped it up again using a car-tyre pump.
Dennis Skinner
February 2009
You need the fill loop installed or opened, if its not installed you will have to call a heating engineer.
JB
September 2008
I can answer this question...
My pressure keeps falling on my Vailant boiler, how do i top it up??????