I think hydraulics of course have a lot to do with it I have under formal engineering direction during my working years installed secondary loops with supply return distance anywhere from 18" to 10 ft. Large diameter stufff They all had pumps though. So far so good my one current circuit is 1-1/4 pipe around 40kbtu. So far so good. I have thermometers on the systems and balancing valve on the closest return port to assure splitting return flow. I have a butterlfly valve that in can create more resistance between the supply and return if need be. . Ihave not installed my largest load which will be 1 12 pipe 130 kbtu glycol system. supplies and returns are on separate sides of the bf valve. First to get hot water is last to return it Currently the balancing valve is at 70 percent open...... no particular reason just where i left it.