So during the summer I drained down my gravity fed heating system to do some work on it. Installed a slightly smaller radiator so I moved a leg. I cleaned the system with x800 and refiled with an inhibitor after draining and refilling with water. Now during the winter I realised that with the downstairs radiators the flows are hot but returns cold. To try and combat this I balanced the system but no luck. I have also just brought and installed a new grundfos alpha 2 pump but the problem remains. I suspect a blockage and only just have realised why this might be the case. The kitchen radiator contains the only drain off valve located downstairs. However the 3 radiators that are not working do not contain a drain off valve and are on a separate loop downstairs. So I suspect when I used the sentinel x800 it moved all the sludge from upstairs to the downstairs radiators. But when I was draining the water because the drain off is on a different loop I didn't actually remove the sludge. I suspect it is still stuck and causing the issue. Just wondering would this potentially be the issue ? I have tried everything else.