As with most things hydronic, there are multiple approaches, and the “best” approach for each installation has to consider cost, aesthetics, access to the existing piping, available wall space and the goal of how the overall system will operate based on existing or newly created zones.
When you look at the big picture of hydronic heating on a worldwide basis, one of the most evident trends is migration toward low-temperature distribution systems.
Although not specifically intended as heat emitters, the copper tubing connecting the components of many hydronic systems certainly does release heat to its surroundings, compromising the system's ability to deliver heat precisely where and when it's needed.