We have an oil fired heating system in our home with 5 radiators downstairs and 4 upstairs..yesterday I bled the downstairs ones only to end up with all but two rads going full heat and the other two ‘freezing’…I investigated the attic header tank and discovered the tank empty due to a stuck ballcock…fixed this and re bled ……still the same . The two that are cold sre downstairs and are the last two to be heated , they appear to get their feed from upstairs!? anyone help a cooling Irishman !!
A gravity system has one motive force, gravity. If the feed is upstairs, then warm water needs to come down, which is against physics. It is the falling column of cold water that moves the warmed water upward. A circulator overcomes the problem.
The block to the heating system is the introduction of air into a location that traps air in the path. Look for such a location and eliminate the air. Better yet, eliminate the air trap.
If the feed to these two radiators is as you describe, a system would take a long time to overcome the piping problem. The velocity of water in the feed would need to increase to induce a circulation. This can take time.
take off the rads that are not working put a bucket under the rad valves and open them 1 at a time till hot water comes through then when hot water comes through refit rad, turn on one side and bleed rad, then open other tap and hey presto !!!