Conventional (at least it was 25 years ago) pumped CH and gravity DHW system with F&E tank in loft (two storey house), cylinder on first floor and boiler on ground floor (25 year old Glow-Worm, very reliable), immediately below the cylinder, so very short vertical feed to cylinder. I drained the system last week to fit new radiator valves, filled up again and had some difficulty getting CH to circulate, but eventually managed it. At first the DHW worked fine, but now there is no circulation in the primary circuit from the boiler through the heat exchanger in the cylinder. 28mm pipework to/from boiler, feed from F&E tank tee'd into the return pipe just after cylinder, and vent tee'd into the flow pipe just before cylider. Can't believe the heat exchanger can be blocked as I understand it just a coil of pipe (presumably 28mm like the pipework but maybe its smaller?) Can't think of anything else, though, as the system couldn't be simpler. Any ideas?