Anyone here know anything about them?, i have a gravity fed system but since having a new radiator fitted seem unable to have all radiators on, if i turn the ones off upstairs the ones downstairs heat up, i can then turn the ones on upstairs and while the central heating system is on the radiators downstairs stay hot, but when turned off have to go through the same process, This is an unpumped system, i have also bled each radiator fully. Any ideas? (apart from getting a plumber out)!