thescruff wrote on Jan 9th, 2018, 1:24am:Back too one of my earlier post.
You have a header tank for the heating, hold a jar of water under the vent, turn the pump on and off a few times, does it drink the water or blow bubbles.
Can you post a pic of the neutral point.
OK.
Switch the pump off and air bubbles come out.
Pump back on and the water gets sucked into the vent pipe.
I'm not sure what I was expecting....part of me was thinking nothing hence the 'neutral point'.
EDIT: OK, think I understand what's happening and it is as expected. We've got a body of water circulating by virtue of the pump running. When the pump stops, the momentum of that water pushed up the vent pipe a bit ...pushing out air in the downspout of the vent pipe (the bit hanging over the header tank).
When the pump starts up again there is a momentary low pressure at the neutral point as the pump starts sucking and the inertia of the water in the CH circuit takes a tad to get moving again. So that sucks water into the vent pipe.
Ergo the pump is running the right way round.
Do I get a gold star ?