I recent had a e 18 error on the Keston c55. ( return overheat ) so I drained down the system and refilled it making sure to vent it thoroughly ( including the heat exchanger ) to eliminate any air from the system. I also replaced the flow and return thermistors and fired up the boiler everything was ok until yesterday now it is saying e19 ( flow overheat ) I tried bleeding the heat exchange but no air or water is coming out. The system is filled by an f and e tank so it can't be empty.
thoughts?
thoughts?