period two
Chilled-Water System Design
notes
Primary-Secondary System Rules
▲The bypass pipe should be free of restrictions
▲Load terminals should use two-way modulating control valves
Figure 52
Obviously, in order to achieve variable flow at the distribution pump, the load terminals must be configured to vary the system flow. This requires the use of a modulating two-way control valve for each coil. Typically, no three-way valves or wild coils need to be used in a primary-secondary system. Their use decreases the energy savings potential from the variable-flow distribution pump. In fact, ASHRAE/IESNA Standard 90.1–1999 (Section 6.3.4.1) requires the use of modulating two-way control valves, and thus variable water flow, in most systems. Primary-secondary systems comply with this requirement.
Some systems, however, will use one three-way valve at the load terminal furthest from the distribution pump, to ensure that cold water is immediately availability to all terminals in the system.
