AJH wrote:

Yup, that's the problem. I understand OV as Open Vent, expand CF
(combined flow?)


CF = Cold feed. The water won't pump-over into the feed & expansion
tank unless the differential pressure head between CF & OV exceeds the
height of the OV above the water level in the tank.

Pumping-over problems can usually be avoided by having the CF & OV
close to each other, so that there is no significant differential
presure between the two connections.