We have an oil-fired water baseboard heating system. We have 2 zones, one for the 1st floor and most of the basement and one for the second floor. We have one pipe from the furnace that supplies hot water to the second floor. From this single pipe each radiator has a supply line and a return line. My problem is on the last radiator in the chain. This radiator needs bleeding (ie. removal of air) at least once per week. Anyone have an idea why this is happening? Also what do we have to do to fix it?