Help please. Already had one plumber out and problem persists. central heating system keeps draining. Starts as cold radiators then you can hear bubbling or water flowing in pipes then boiler cuts out. IF I reprime the system by letting all air out and increase pressure to 3 bar the boiler cuts in again. 4-6 hours later back to square one. Hot water system based on a large Megaflow tank. Any ideas. Wife and 6 cold kids causing me strife!