[ben333]

Well, in a closed loop gravity doesn't matter. So you could even put your pump at the highest point, radiator upside down, block at the bottom, etc. So placement doesn't matter other than filling / draining. Open up paint and draw up your loop as you plan it, then it's easy to think about how easy it will be to fill / drain.

In my loops I've usually had the pump at the lowest point, the T line or res feeding into the pump's inlet and with no drain.