I have a gas fired boiler that heats water, uses 1 circulator pump to pump water to 2 zones (each with its own thermostat), using copper pipe to baseboards with fins. I am having difficulty bleeding the air out. One zone heats fine, the other not at all. I think there may be some kind of air block. I tried forcing water into the system, and when I opened the small bleeder valve I heard air being sucked into the system. I expected air to be escaping from the system, hopefully being displaced by water. When the system wants to suck air in, am I contributing to an air block? Should I allow as much air in as it wants, or should I keep the bleeder valve closed when I realize it is trying to suck air in? I do not know if the suction is part of the bleeding process? Thank you.

You should never draw air into the system. Does your boiler have a temp'/pressure gauge? It should read 12 psi cold and about 18 psi hot. There is a pressure reducing valve that keeps pressure on the system connected to your domestic water supply. This part could be bad, have a plugged filter screen, improperly adjusted or the valve just before it is closed. (Usually just a simple gate or ball valve)
Are there bleeders at the radiators? They can be located on either end of the radiators.
I'm guessing the vapor lock is in the second floor zone?

i work at a condo we have 62 units. We have baseboard heat hot water we are having a problem with 1unit with heat it is hot up to zone value .we bled it pass zone value we get hot water for atime but buy the time we get done bleeding the rest of the system its coldwater we put a new zone value in . What else should we do .should we just keep bleeding it thank [email protected]