Hydronic systems are not that different from ducted air-distribution systems. The duct is sized according to the airflow rate, and often there’s no need for dampers to control the flow to the various branch ducts. In larger, more complex systems, however, the physical dimensions of the duct are insufficient to modulate airflow. Balancing dampers are needed at major branches, and at each diffuser as well, to ensure the proper airflow to each zone.
Dampers cause a high friction loss, though, and the supply-fan motor must work harder to overcome this static loss. That’s why the duct is sized so carefully, and why transitions are made with such deliberate care, to reduce the pressure loss and control the airflow to the greatest extent possible.
The alternative is costly dampers and diffusers, and their inherent energy drain on the system.
Water piping is an analogous system that should be designed with the same amount of attention to detail. Properly-sized branch piping not only saves on space, pipe, and fitting costs but also helps to balance the flow to each point of use. The lazy piping designer sizes the piping generously, then places automatic control valves at strategic locations. These valves, though, increase the static pressure in the loop and cause a higher head—and energy use—at the pump. Without the control valves, there is inadequate pressure at remote locations, increased hydronic pressure on all components of the fluid circuit, and valves that frequently fail under pressure and that have trouble modulating the fluid flow. All this can be avoided if the piping is sized correctly in the first place.
The benefits of good flow control are lower pump energy use, possible downsizing of the pump, and more uniform pressure and flow at the point of use. In a hydronic system, this is usually a heating or cooling coil. If the coil is consistently provided with the design-flow rate, then space-conditioning costs are reduced, and the occupants have a more comfortable environment, with minimal temperature variations from a variable-coil flow rate. Flow-control valves are inexpensive, dependable fittings that are a must for any large hydronic system, especially for large branches off the main.
Even the best-designed hydronic system benefits from the fine-tuning made possible by carefully placed flow-control valves. Still, it’s a mark of good design to have only a few of them at critical points in the hydronic loop.
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Excerpt from Energy Conservation Projects.