My Setup: Electric boiler, heating HW / CH - I have a gravity-fed hot tank and header tank in loft. My Problem: When turning on the HW for a downstairs shower, after between 12-24 hours, the header overflow starts trickling with water. The ball valve is operating fine, but the expansion pipe for the cylinder leads to the header tank. I have tried really hard, but can't work out why the overflow is, er, overflowing. Pressure? Air intake? Leak anywhere? The pressure is on for me to tile the shower, so would really appreciate any help in solving this one!