Those new to designing or troubleshooting hydronic systems often misunderstand how the speed of water flowing through a heat emitter affects heat output. More than once I’ve heard someone reason that if water flows through baseboard too fast, there isn’t enough time for the BTUs to “jump off.” Apparently BTUs are smarter than Tom Cruise in Mission Impossible. They know when the “water train” they’re riding is moving too fast to safely jump off, and therefore stay put and get swept out the other end of the baseboard. This reasoning implies that slowing down the flow rate through the baseboard would increase heat output. This, however, is not the case.
More Speed = More Heat: Heat moves from a fluid to a solid (such as baseboard) by convection. The rate of heat transfer depends strongly on the speed of the fluid along the surface. The faster the fluid moves, the faster heat transfers.