Hi, I've had this problem for a while. I have a fully pumped system with 3 way divertor valve. I have hot water and hot radiators upstairs. All radiators downstairs are cold (6 rads). A new pump was fitted, I have flushed the system with Sentinel X800 and X400. Also power flushed system. Recently drained down system again, flushed all radiators and opened all valves which flowed freely with hot on oneside and luke warm on other. Fitted a magnetic Boiler Mag filter, refilled system and still hot radiators upstairs but cold downstairs. If there is a solid plug of magnetite in pipes between upstairs and down stairs, how come the water flows freely from both hot and cold feed, but as soon as radiators refilled no heat?