Single vs. double pipe loop providing hydronic heat to radiators.

My hydronic heat system has one large loop around the basement perimeter from which two tees feed each radiator on the upper floors.
An HVAC friend of mine suggested that the one single large loop is the old way and the modern way is to have two loops (one supply and one return) in the basement, and that this new way is more efficient.
Perhaps this alternate way may provide more even temperature distribution? But how would you regulate water flow to each radiator in an even manner? The single loop way I'm familiar with is to use venturi tees to encourage circulation through its respective radiator. What's typically installed for similar type systems these days?
Anyone have an opinion to offer?
-Anthony
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A reverse return system is one of the most effective, in most cases. The first unit supplied becomes the first unit to return, & starts the return loop, the last unit supplied is the end of the supply line & is the last on the return circuit. These systems are typically self balancing & require no "veturis" etc... for flow control as long as the units in each room are sized reasonably for the sq.ft. of the room (a 20 strip of baseboard will deliver the same btu's in a 100 sq ft bedroom as it will in a 300 sqft living room) (& yes I know air & BB delta T will cause variations in heat delivery... so don't...)
The inherent problem with a single loop system like yours, is that the lower water temps delivered to each of the down stream spaces, as the return from each previous unit cools the supply to the next, lowers the heat output capacity of the space heating unit. Individual control valves or circulators will help resolve this issue & is probably much more cost effective than repiping the entire system. Note also, some series type systems were sized with larger output capacity farther downstream to account for the cooler water temps delivered. If a new system is being installed, reverse return is one of the best options, in retrofit... maybe not.
goodluck geothermaljones stpaulmn

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