if it is open vented system the towel rail flow and return pipes might have been connected at the wrong place which does result in dragging air from the open vents
The most common reason for bleeding required only on one rad is due to the flow rate through that rad being greater than the rest and that the system doesn't have enough inhibitor in it. Add another lt of inhibitor and the problem will normally be reduced substantially
On a system, open vented or otherwise if correctly plumbed, will not result in air being drawn in. Incorrect plumbing often leads to pumping over and air being released somewhere in the system.
On a correctly plumbed system, both vent and feed will be at same prressure when pump running.
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