"David Hansen" wrote

Pumping the return usually deals with this. Beware of difficulties
with connections though.
Make sure you know the score if you consider pumped return arrangement.
I inherited a system with pumped return and could not get it to perform.
With pump speed sufficient to heat the house in a reasonable time, there was
a serious issue with pump over into the header tank.
If you are intending to stick with a vented system rather than going sealed,
would recommend you opt for pump in the flow with make up water connection
close to vent.

Phil