The pressure guage on the boiler keeps going down which would suggest a leak. However when the system is checked a leak cannot be found and all the radiators are full.
When water is introduced into the system it retains the pressure when the heating is not on and only after the heating has been on and gone off starts to lose pressure.
The guage goes down to just under 1.5 bars and we get irregular hot water. When the heating comes on the pressure can rise to 2.5 - 3.0 bar.

Ron Davis., November 2007

Hi ron sounds like your diaphram in the expansion tank has failed, without any room to expand pressure will increase and discharge through your pressure releife pipe outside your building. check outside for any sighns of water,

dave from tranmere, November 2007