I am going to put together some knowledge sharing of points I feel are never really taught much or discussed, leaving many installers unsure of what to do, but feel silly sometimes to ask, but in my opinion there is no such thing as a stupid question, but not asking and doing something wrong is stupid.
The point of no pressure change in a heating system is the point where the expansion vessel is connected, basically at this point the pump cannot change the pressure to create circulation, this is similar to the neutral point on an open vented system and in reality we should be following the same principle as we used to with the neutral point on open vented systems, with the neutral point on an open vented system the open vent and cold feed were always on the suction side of the pump, so the pump was always pumping away from the neutral point, this is the same for a sealed system and the point of no pressure change, you need to always pump away from the point of no pressure change (Expansion vessel connection)
Now before I get more into this, I would like to emphasis this information is out there for all to see, this is not something I have invented myself, there is a great book out there written by Dan Holohan called "Pumping Away" where I gained my knowledge on this information.
Now within a sealed heating system, for the pump to create circulation the pump will create pressure differential rather than pressure, now at the point of the expansion vessel connection to the system because the pump can't add water to, or remove water from the expansion vessel the pump cannot change the pressure within the expansion vessel, this is why its called the point of no pressure change, the circulator will respond to the expansion vessels location, raising or lowering the differential pressure based on that location, if you pump away from the expansion vessel, the circulator will add its differential pressure to the systems static fill pressure, if however if you pump towards the expansion vessel the circulator will remove its differential pressure from the system static fill pressure, and if the pump's differential pressure exceeds the system's static fill pressure, the pressure at the pump's suction will be below atmospheric and air will enter the system, you then have problems with the system.
Here is a diagram from Caleffi that may assist in understanding this principle, unfortunately its in psi, as its from USA, as I could not find anything in metric covering this information, why is that?
If you are interested in reading further the Book, "Pumping Away" is available HERE
I hope this proves useful to some, if you have any questions please ask away and let me know if there is any subjects you would like me to cover