I'm currently trying to deal with a sealed central heating system in which the pressure is dropping by 0.2–0.3 bar per day (this is obviously far too fast, because the intended pressure for this system is 1 bar). Although I intend to call in a plumber to fix the issue, it seems important to find out the location of the problem first, so that the plumber can access the appropriate place to fix it (e.g. if the problem is under the floor, there's a need to move floorboards to provide access).
One common cause of this appears to be a malfunction in the pressure release valve. However, on one occasion after a loss of pressure, the circulating pump in the system was running continuously and (based on the sound it was making) failing to pump water; this was almost certainly because the water level had fallen below the location of the pump. The pressure release valve is higher than the pump is; does that mean that the pressure release valve can't be responsible (on the basis that the pump should be underwater even if the pressure release valve were opened entirely), or is it possible that the water in the system could fail to reach the pump despite the only issue being with the pressure release valve?
The other possible issue is a leak, but I haven't been able to locate the leak, if indeed there is one. Is there any technique that can be used to determine where the leak is? One thing I tried was to leave the system off for several days and then attempt to bleed an upstairs radiator – on the basis that a leak below the radiator should make it impossible to bleed due to the lack of pressure – but air still seems to be coming out. Would that test automatically rule out a leak below the radiator, or might it still be bleedable even with a leak below it?
One piece of information would be helpful would be the approximate rate (in terms of volume/time) at which I can expect the water to be leaking out; this would aid both in identifying a leak (because it would predict what the leak would look like), and in determining how long to wait before testing to see if the radiators were still bleedable.