Hi, I've had an ongoing problem with 4 upstairs radiators not getting as hot as they should. They do get warm along the top but the bottom is always cold, the return pipe tends to be hot but the flow pipe is cold so I assume it is heating via convection rather than flow? I've tried isolating single radiators, changing the pump and even powerflushing but nothing has helped. Recently I tested the the flow from the pipes into a bucket. The return seemed ok but the flow pipe barely dribbled. I tried draining down, removing the TRV and and attaching a small pump to the pipe but there was resistance stopping the pump. I guess the pipe are truly blocked? The system is open vented with with 18 radiators piped with 10mm plastic (with 10mm copper to downstairs). The boiler is a Trianco Oil and I do have a Magnaclean fitted. I'm thinking about draining the system and injecting a small amount of DS40 directly into one of the blocked legs to see if this will clear the blockage. Is this safe to do and will it help or is there better things to try first? Thanks in advance.