It needs to be on the return to the boiler - so that it is filtering water coming back from the rads. Putting it on the cylinder return is pointless.
You would be better off putting it on the flow after the pump and any feed/vent pipes, and before the two zone valves. But this is not ideal.
You would be better off putting it on the flow after the pump and any feed/vent pipes, and before the two zone valves. But this is not ideal.