I'm struggling to understand the interplay between the pump that pumps water and how the location of the feed expansion tank affects whether radiators work. In my system most of the radiators work okay, but he two on the second floor just below the feed expansion tank do not,I'm told that because there is not enough head/height difference between them. But the boiler has a pump that pumps the hot water around, and if you put liquid in a pipe it will just fill it so I don't understand the significance of the expansion tank ( I do get it for hot water system regarding showers).
Assuming it is issue is there way to get radiators on the 2nd floor working properly without investing in a sealed system. Could I install a second pump just to pump water to these two radiators ?