In article ,
Dave writes:

Sorry, I don't understand that explanation. The system is sealed,
therefore it's all at the same pressure;

It's not at the same pressure. The weight of water increases
the pressure linearly as you go lower down the system. It
increases by 1 bar every 33 feet (and conversely decreases
by 1 bar every 33 feet above any given point). The pressure
guage only tells you what the pressure is at the height of
the guage itself, but you can work out the pressure at every
other point in the system by knowing its height above or below
the pressure guage.

(The pump can generate a differential pressure on top of this
of a couple of feet or so at different points in the system,
but that's rather insignificant in the scale of things).

how does atmospheric pressure
(32 ft of water) affect anything?

Well, you probably don't want the water at lower pressure than
atmospheric at any point in the system.

--
Andrew Gabriel