Mandate you can only get this secondary circulation if the water has a circuit to flow through. If you common ALL the rad returns water cannot 'find' its way into heating system as the water already there has no where to go.
This problem occurs because the "3 Tee Rule" has been broken. i.e. Never connect a flow to a radiator or a return from a radiator into the pipework between the boiler and the tees feeding the boiler. If this advise is ignored then reverse circulation will occur.
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