How to Select Two-Way and Three-Way Valves for HVAC Systems

nexoradesign.net
Aug 1
2 min read

Control valves are among the most systems. Although a valve may appear to be a relatively small item compared with chillers, boilers, pumps, AHUs, and cooling towers, incorrect valve selection can cause unstable temperature control, excessive pumping energy, noise, poor chilled-water ΔT, insufficient coil capacity, and difficult system balancing.

One of the first decisions an HVAC designer must make is whether a terminal unit or coil should use a two-way control valve or a three-way control valve.

The decision should not be based simply on pipe size or the type of equipment. It should be based primarily on the hydraulic configuration of the system, the required control philosophy, pump operation, minimum-flow requirements, and how the system is expected to behave at part load.

ASHRAE describes two-way and three-way valves as both capable of varying the flow through a load. Their fundamental difference is what happens to the overall distribution-system flow: a two-way valve produces a variable-flow response, whereas a three-way arrangement generally maintains approximately constant distribution flow by bypassing water around the load. . Why Control-Valve Selection Matters. (How to Select Two-Way and Three-Way Valves for HVAC Systems)

Consider a chilled-water AHU.

When the space cooling load decreases, less chilled water is required through the cooling coil. The control system therefore has to reduce the heat-transfer capacity of the coil.

This can be achieved in two main ways: