Pumping over occurs when water is pumped over the open vent pipe and back into the feed and expansion header tank. This only happens in vented systems.
It can be caused by:
1) An incorrectly placed pump,
2) Your pump speed is too high
3) There is insufficient static head
4) The system is incorrectly configured.
5) System blockage. Further edit required.
This problem should be rectified quickly as oxygenated water will be entering the system resulting in corrosion. The most likely evidence will be pinholes in radiators.
A correctly installed system will create positive pressure in as much of the installation as possible. The diagram below shows correct ‘left to right’ and ‘right to left’ installations.
If you find that the order of your system is different, you should have it checked over by a qualified plumber.