A plummer replaced my old oil boiler with a new Crown oil bolier last week. I bled the fin radiators on saturday while runing the furnace and circulator for 45 minutes. There was air in the radiators on this mono flow heating system. I got all the air out from all radiators. I noticed the last radiator on the heat line never got hot even when all the air was bled out. I repeated the same procedure today with the same results. It seems that the pipe leading to the last fin radiator is hot but the return pipe is cold. The boiler temperature on the aquastat was 175-180 degrees with the pressure about 35 to 40 lbs pressure but the new 30 lb presure relief valve never blew off. What is causing the last radiator on line NOT to become hot? Should I be removing some water from the large metal expansion tank?