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Balance Valve Location: Supply or Return Side of the Coil?
Whenever possible, balance valves should be placed on the return side of coils to reduce air and noise problems.
ASHRAE states that "water velocity noise is not caused by water but by free air, sharp pressure drops, turbulence, or a combination of these, which in turn cause cavitation or flashing of water into steam" (1997 ASHRAE Fundamentals Handbook, page 33.4). Compared with placement on the supply side, balance valves on the return side will reduce the amount of free air in the coil.
At a given temperature, the amount of air in water depends on pressure. If the water pressure is reduced, air is released. This phenomenon is experienced by a diver if he quickly decreases the pressure on his body by ascending quickly to the surface. A rapid decrease in pressure will cause gases to come out of solution of blood and tissue, causing pain and possible damage. This is known as "the bends".
Balance valves placed on the return side will result in higher water pressures within the coil, which means that more air will remain in solution and out of the coil (see figures below). In each figure, the coil pressure drop is 20 ft.
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Note that the flow balancing effect of the valve is the same in either case. That is, the overall circuit pressure drop is the same, which will result in the same flow through the coil.