Having not been used for a few weeks, when I turned it on our central heating it fired up but after 15 minutes it tripped the RCCD. If you immediately reset the RCCD it immediately trips again. However, if you leave it off for half an hour or more, it fires up but trips after 15 minutes. I have replaced the RCCD and checked the system with everything else disconnected. There is clearly something connected with the central heating system that is causing the RCCD to trip. Could the initial delay in tripping suggest an earth leak that takes time to build up, or is something in the system, the pump or valve, overheating? Any advice would be much appreciated!!