I didn't write this but it may help:
The head tank generates a static head on the system and the point at
which the cold feed joins the circulation is known as the neutral
point simply because this is the point on the system where the
pressure is always a constant value.
In a traditional system the cold feed is on the boiler return with the
open vent on the boiler flow. A pump generates the pressure to
circulate the water around the system to overcome resistance to flow.
It follows therefore that as water circulates the pump generated
pressure will fall as it overcomes resistance.
Going back to our neutral point where we have a known pressure
anywhere between the cold feed and pump suction will be at a pressure
lower than the static head. Between the pump discharge and cold feed
will be at a pressure above the static head.
So on a traditional system where the pump is on the boiler flow, only
a small part of the system will be below the static head, but when the
pump is on the return the majority of the system is below the static
head. A pump generates a higher pressure than the static head in a lot
of cases, hence pumping over when an open vent is incorrectly sited.
This then brings the situation where a pump is on the return and parts
of the system will be at less than atmospheric pressure, because the
pump generated suction is greater than the static head, thereby
drawing air into the system if there is a leak on that part of the
system.
Hope you followed this, and are convinced. If not a simple experiment
will prove it if you can find a system where the pump is on the return
as described above. Alll you do is try to vent the radiators when the
pump is running and some, if not all, will not discharge water, but
will instead draw air into the system.
