Hydraulic Separator physical placement

That will depend upon your Hydro-Sep. Alot of them have an air vent installed at the top for convenience of getting the air out of the hydro-sep and that's all. So in essence, yes, I would install an air separator

If your Hydro-Sep has a collision media inside, then you have an air separator already built in and do not need to add another.

Being below the boiler is irrelevant because all of the water/air has to pass thru it.

Dave H.

I would not include a second air separator after spending the money on a hydraulic separator. I love those things but they are always massive and difficult to place in the system. In fact, where space is an issue, I don't use them at all. But they do a great job of eliminated both air and dirt and I've never made any effort to place one below the boiler and I don't know where that directive comes from.