I have a Range Tribune HE 210L unvented indirect system with an Ideal Logic System 18 Boiler. Over the past few months water has been constantly flowing from the overflow pipe outside. (filling a 7ltr container a few times a day) I have had a plumber come and check everything over, service the boiler and replace the PRV and the internal expansion vessel in the boiler, however the problem still continues. The system pressure (theres a gauge on the boiler and on the hot water cylinder) is always re-set to 1-1.5bar as per the guidance in the manuals. However within 10-20 minutes the pressure is back up to 2.8-3bar, with the boiler switched off. If the boiler is turned on, it creeps up to 4bar. There is only 1 filling loop to the system which is disconnected. When the cold main to the system is shut off, the pressure holds at 1-1.5 bar. When it is turned back on, it shoots up to 2.8-3bar again. The diagnosis from the plumber (who seems to be very good) is that the coil inside the cylinder has split. Is this likely? However, having spoken to the manufacturer they said it is very unlikely this is the case and there would be noticeable discolouration to the hot water, which there isn't. Any advice would be appreciated, thanks.