two double raidators not heating on bottom half but top is hot

don't take any advice from a builder, get the system powerflushed properly,new valves,and balanced! nuff said

Hi, Have you checked that all the rad valve's are fully open, you may have a single pipe flow and return section in your pipe layout, which would cause some of the rad's not to work, can't really see how it can be anything else if you really have drained and flushed the system. If it isn't that then maybe the flow and return in this section is very weak and bypassing the lower rad's. Wouldn't have thought it could be an air lock as this is normally in the higher rad's and as you have said, you have drained and refilled. Maybe you have a small rad, somewhere hardly used, cloakroom, utility room, with the valve's turned off, this shouldn't really cause a problem but sometime's a plummer may have cut a corner and or just had a bad day and not realised he had done it, it happen's. Hope this help's you. Kind regard's Paul