When I build a return system I usually use the original supply as the return and upsize the new supply. So I'd got with the 3/8" supply and use the existing 5/16" as a return. You really can't go too big on the return, within reason in case some clown want's to mwntion 2" black pipe or some such silliness.


They say there are no such thing as a stupid question.
They say there is always the exception that proves the rule.
Don't be the exception.