psi, head and air

CC.Rob

Recent conversion from psi->head(ft) in another thread has me wondering. Backstory: My two zone hotwater system has significant air problems at "normal" pressures say 15-19 psig. A longish loop with lots of ups/downs and elbows on the second floor seems to be the culprit (TEL = ~325 ft). Air in this loop is generally always trapped in the top of the system, but some apparently makes it down to the vertical return manifold and gets trapped, causes cavitation in the circulator, degrades system performance, etc. I presume flow velocity is insufficient to get into/through the boiler. (Yes yes I know we're not pumping away but way back when I didn't know that, and the system as piped presently is very efficiently done and repiping would amount to major surgery and if possible I don't want to go there, yet.)

However, bumping up the psig to around 24-26 appears to really quiet things down. Significantly reduced air noise upstairs, apparently no air accumulating in the vertical return manifold, etc.

Why is that? Possible reasons I'm thinking of include: 1) at higher psig, air is not allowed to come out of solution, 2) at higher psig, although system resistance (head)increases, the air is better entrained in what I presume to be sluggish flow on the long loop and makes it through the boiler to the spirovent.

Using these data and observations, is it possible to back-calculate the head of the long loop at "air free" psig? (e.g., psig(noair) - psig(air) = head but I'm pretty sure that's not the right way to do it)Standard calculations for this loop suggest 10.1 ft of head at 4 gpm, but I wonder if it's higher.

Thanks for any insight.