The boiler pressure is lost from 1 bar to zero in approx 3 hours with system off. joints have been checked but no visible leaks. What could cause this, could a small seepage of water that does not produce a wet area cause this amount of water/pressure loss?
The loss in pressure could be only 2 things if as you say it happens without the heating on. 1 - pressure relief valve is letting by, you can usually find this going through the wall behind the boiler,under the flue, you will need to go outside to look and see if it is dripping. easy fix, just replace. 2 - system leak, i suspect some of your radiator pipework goes under the floor, if you have floorboards downstairs, you will never see a water stain. you can try leak sealant but this leak sounds to big to be cured in this way. it is quite difficult and expensive to trace this type of leak. the only option left if leak seal dosn't do it is to take up the floor. one test u can do to tell if u have a system leak is to re-pressurise than close the flow and return under the boiler, if the gauge shows no drop the leak is in your pipwork and you will have to re-check.
if your sure theres no leaks, look at the pressure release valve, they often give up, the fact your losing pressure so fast is my concern, have you tried adding some leak seal to the system ? i would add 2, because your losing pressure so quick. sorry cant be of more help but easier when the systems in front of you !!!
the water would leave a wet spot.... you must be missing a fitting somewhere... you say no visible leaks.... try using a water bottle with soap.... just like looking for a leak in a tire....
