Hi i've A Potterton Suprima which is designed as pumped central heating and gravity hot water.The hot water is now not working, the boiler fires up but goes off once the pipe is hot with no flow .There are no valves between boiler and cylinder.I have had it powerflushed and get a flow from hot water pipes but when reconnect to boiler nothing ?. Its been suggested the coil in tank may be scaled up to much for the gravity system and only flows when pumped.Any ideas thanks.