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Sorry for the late response. Thank you very much for your explanation and the drawing on where to put the Discal air separator! I'll be sure to ensure that when I have a new boiler installed, the installer is experience with residential boilers and piping. I'll also review the book that you recommended.

whats the normal static fill pressure? and does this system also provide DHW or is it heat only? a boiler system on a with an attached fresh water supply would have a pressure reducing valve installed, is there such a valve on this system or is it just taking in city pressure?

you set the pressure to the required cold static pressure of the system if its a closed loop hydronic heating system . and you can only check or adjust this charge with the tank completely detached from the system

Hi, A couple of thoughts. Try putting a gauge on the drain valves of the tanks and measure the actual pressure. That might help in getting to the truth a bit faster… I would want a drain and waste valve installed between the expansion tank and system, so you can check/charge it easily… The pump is mounted vertically in a way that can trap air and damage the bearings. I'd rotate it to sideways… The pump is pulling water away from an air vent. If pressure falls low enough, it could pull air in and cause problems… The older tank is nearly ten years old. It needs a new anode! Also, it's time now to check the anode on the newer tank.

Yours, Larry

that just looks like a backflow preventer, nothing on the heaters side of the system to reduce pressure from the street? if there is no pressure reducing valve on the heater side of the piping, then you need to set the pressure to roughly your street pressure, and either way you will want a DHW rated expansion tank