On Fri, 24 Feb 2017 15:34:11 +0000 (GMT), Dave Plowman (News) wrote:

The whole system is under pressure either from gravity or from a
filling loop. There is not really a "suck" side to the pump - it
is a
"circulator rather than a positive displacement pump

Then air could never find its way in?

With open vented with even a little bit of pump over it will.
Sealed system is trickier to get air in but the chances are it's not
air but hydrogen released when the iron grabs the oxygen from a water
molecule.

--
Cheers
Dave.