T
TheGasEngineer
Oops forgot it's sealed. Actually there is negative and positive head even in a sealed system. I think, it's just half the pump head; negative up stream and positive down stream. If there's vent it can be negative to the value of the whole pump head if the vent is down stream and likewise positive the whole pump head if the vent is upstream. In the former case this occurs if it's a small static head and is the cause if ingress of air which "mysteriously" builds up in radiators suffering the negative head.... Also negative and positive pressures do not apply to a sealed system!
There is another possibilty - air locks. I get quite a lot of work from doing small re-pipes and fitting vents. Even in sealed systems, an inverted pipe loop without a vent can stop any system working.
Trace all pipe runs. Once it starts going up hill start looking for the high point ie just where it starts going down hill again. There must be a vent at each high point.
A "good one" I find is a wall-mounted boiler with flow and return on the top. The pipes come up from the floor, go over the boiler and drop down to the F&R couplings creating a couple of lovely air locks.