Field-Ready · Lesson 4.3
Chilled Water Systems
One chiller, two pumps, and differential pressure control. The mirror image of the hot water system.
What Is a Chilled Water System
A chilled water system is the mirror image of the hot water system. The chiller replaces the boiler; the process moves heat out of the building instead of into it. The control architecture is nearly identical: prove the pump, enable the plant, maintain a setpoint, control VFDs to differential pressure.
What the chiller actually does
A chiller is a refrigeration machine. The refrigerant circuit absorbs heat from the chilled water on the evaporator side, compresses it, and rejects it through a condenser, either to outdoor air (air-cooled chiller) or to condenser water flowing to a cooling tower (water-cooled chiller). The chilled water leaves the evaporator at 44 to 48°F and returns at 54 to 58°F, warmer from absorbing heat at the building's coils.
OAT lockout
Unlike a boiler, a chiller cannot run in very cold weather. Most chillers lock out below 40 to 50°F outdoor air temperature (the exact threshold depends on the chiller type and manufacturer). Running a chiller when it is too cold outside can damage the compressor or trip the chiller on low suction pressure. The BAS sequence must check OAT before enabling the chiller and must lock it out when conditions are not safe.
Chilled water supply temperature reset
On a hot summer day at full load, the chiller must deliver 44°F water. On a mild spring day when the building only needs modest cooling, delivering 48 to 54°F water is sufficient, and the chiller runs at significantly better efficiency because the compressor lift (the temperature difference it must overcome) is smaller. CHWST reset is a setpoint that moves upward when loads are lower, saving energy without sacrificing comfort.
Same two pumps, same VFD differential pressure logic as the hot water system. The points are named differently, the setpoints are reversed in direction, and the lockout logic is new. Everything else maps directly to what you learned in Lesson 4.2.