I have forced hot water baseboard heating. Had to replace the boiler last month (woe is me) with an Energy Kinetics System 2000. Since the replacement, we have had air in the system such that we are hearing a loud sloshing sound in the top floor when the circulator activates. The installation guys have been back twice to purge but to no avail. They suspected that we may lack water pressure sufficient to really purge, so I filled a tub with water and we used a pony pump the second time. Even though immediately after the operation there was no audible air, that night it was back!
The loop in question is part of an addition that was put on the house two years ago, and the plumber who did it used PECS rather than the copper that is used elsewhere. The Energy Kinetics guys think that the differing ID of the PECS might be to blame, but everything went well two years ago after the addition, so I don't buy it. They are implying the only solution now is to inert a bleed valve on the top floor.