A bit more information might help diagnose the problemThanks.
How old is the heating system?
A sketch with rough elevations and pipe sizes would be useful.
It sounds like the problem is on the secondary side, but just to be sure, is the system fully-pumped with 3-port valve or two 2-ports, or pumped rads, gravity HW? Cylinder stat and room-stat? Might even be a Primatic HW cylinder.
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