[canoebuilder]

A closed Loop system, in Layman terms is simply a boiler system that can is filled with water and or anti-freeze (as many people in the North does) and it is isolated from the other plumbing from the house with either valves, backfloe preventers or physicaly cut pipes.
I run 2 boilers in my home, one a large wood fired boiler, used in winter and the other a smaller gas fired boiler, used when we go away for a few days or more.
I have on the water feed line, from the potable water lines this set up....
1 an isolation valve, which after boiler is full I close to insure against over filling if ATF fails. I then check water pressure regularly
2 a D9 Watts back flow preventer
3 a ATF set for 19 lbs
4 another valve....if ever need to replace one of these parts can shut off valves and replace without draining heat lines.

Both boilers have gauges and both read 19 lbs when cold, but I manually hold ATF open to attain 22 lbs, this aids getting water to upstairs.

When either boiler heats the water in thier boilers the pressure will raise in both boilers to 28 lbs...BOTH THE SAME.

I know of no way to check the valves unless they are checked on the same system with the same pressure and it sounds like this is just what you have now and one is not the same as the other.?.?

If I have your set up, in my minds eye correct?

Gerry