Hello --

Hot water boiler question here. We have a boiler that serves the second and third floor of our duplex, and the third floor loop has gotten air locked 3 times over the past couple of months. Each time I isolated the loop, bleed the system by bypassing the fill valve and had luck getting it to work after that. It worked for a few weeks and locked up again -- this most recent bleeding resulted in almost all air coming out when I bled it. There doesn't seem to be any noticeable leaks -- I am looking for some ideas as to what would cause air to enter the system...

Is it possible that the expansion tank diaphragm could be damaged an causing this intrusion of air? Being a closed system, I just can't figure out any other spot that would result in the displacement of water with air.

Any help is greatly appreciated!