I've seen this term used throughout this forum and elsewhere in GT discussions and have not yet determined what distinguishes a 'flow center' from a circulator pump. 

I have seen references to items that, in addition to pumping, have a configuration that simplifies purging and are non-pressurized, but it's unclear to me if these are really distinguishing characteristics as I've seen things called 'Flow Centers' without these characteristics.

Tom

Flow center provides mounting point for circ pump(s), isolation valves

And they're not a required item for a water loop.  For example, I don't have one.  You need purge ports, and you need pumps (for a water loop system), but you don't have to have them all together as one unit.  You can go 'a la cart.'

Best regards,

Bill

Flow Center: a term not needed for a simpler DX system.