My central heating system has been losing pressure for approx 1 year (An old Vaillant 24Kw combi boiler).
I have been topping up periodically.
I have recently had a Vaillant Eco Plus 837 fitted and still losing pressure (Approx 1 bar in 3 days).
Only 1 radiator is losing heat (bathroom, no thermostat), i have bled this twice in 3 days then the system loses pressure. I have topped it up to 1.4 bar. There are no signs of water damage to any of the ceilings. :(