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Open vented system must have vent from flow on boiler before any other connection.
Virtually all open vent systems (except gravity to cylinder) I know of in my area are done with the vent off the flow at cylinder and feed off the cyl return.
Pump is at boiler. They usually work fine, but some factors make it likely to pump over if cylinder is close to boiler and pump on a non zoned system, or where many rads and zones turn off.