Should a water distribution manifold consist of a circular loop?
The circular loop would only be for the hot water side. It is called a recirculating line and it is in place so when you turn the hot water on the line will not have to purge the cold water first, the hot water will be there at the ready in seconds. It is more expensive in energy costs since the hot water is always moving through the pipes and loosing a little heat which would need to be maintained.