How do you bleed a boiler? I have a 2 family home, there are 3 boiler lines. 2 lines for 2nd floor. 1 line for 1st floor. The 2 lines upstairs work, but the 1st floor line is cold with no heat. I heard there might be air in the line. Now, all three lines are connected to the same boiler. I guess they all connect somewhere. Do I have to bleed all of them or just the 1 line for the 1st floor?