I have a combi system (Baxi 224) in a rented 1st floor maisonette that is losing pressure and by losing I mean dropping to 0 overnight. I first had a problem three weeks ago but assumed that it just needed topping up (was low but not 0). Yesterday the system failed ( 0 pressure) so I filed and reset - same thing this morning. I asked downstairs if there was any evidence of leaks in the ceiling (none visible) and none were visible in/around the boiler or radiators plus the pipe for the PRV didn't look as though it had extracted any water. After I had filled this morning, I waited for 5 minutes or so with the heating on and the pressure remained constant. The tenant will use the hot water later and will check to see if this affects the pressure. Not sure if there is anything else I can check - any ideas?