Wonky heating supply/return lines

[Tim McCahill]

Hi all - I own a four-story apartment building. At some point, probably when the building was turned from a single-family to a multi-family and the utilities were separated, new supply and return lines were run from the boilers in the basement to baseboard heaters in the apartments. These lines zigzag and turn up or down at sharp angles; not one has a straight run up or across. Related or not, the system is always full of air - I purge the boilers weekly and within a couple of hours the pipes and fintubes are running noisy. I would like to update the system and would benefit from any insights or experience the community has replacing or improving an old or efficient heating system. Many thanks!