Heating Pressure keeps Falling

[claireq]

I have Oil central Heating with High pressure but at the moment the pressure keeps falling and we have to keep topping up the system. I know I do not have any leak in the house or joints under the house. Can anyone tell me if my problem could be a leak coming from the boiler in the garage to the house or could something else be causing the problem????????????

 

[DGOBS]

If you suffering high pressure, you may well be losing water everytime the pressure exceeds 3bar (through your safety valve) then water cools, pressure drops back down to below where it started.

Have your expansion vessel pressure checked and set, and check safety valve is not now dripping (does this sometimes after a blowoff and drips forever)