I have always had issues with my open-vented CH (posted here in 2009). Radiators take ages to heat up, particularly two large radiators downstairs. Eventually heat gets to them, but the return is luke warm. I suspect an airlock on the return. Boiler recycles frequently. I gravity flushed and the water was clear. I have renewed the pump. I have thoroughly mains flushed, radiator by radiator, and then all radiators together. On turning off the mains, water was still exiting the drain for a while. I closed the drain and opened the fill, and some water rushed up the fill pipe. This suggests to me that there is still a large, compressed air lock after the mains flush. (Of course, I have bled all the radiators!) Any suggestions?