What Is a Chilled Water System?

A chilled water (CHW) HVAC system uses chilled water — typically cooled to 44–45°F supply / 54–55°F return — as the heat transfer medium between a central chiller plant and air handling units distributed throughout a building. Chilled water systems are the dominant HVAC strategy for large commercial buildings (over ~50,000 sq ft) because they are more energy-efficient than direct expansion (DX) systems at scale, centralize maintenance, and allow flexibility in building layout.

Chiller Plant Components

Chillers: The refrigeration machines that remove heat from the chilled water. Centrifugal chillers (most common for large plants above 200 tons) operate by spinning refrigerant vapor through an impeller. Screw chillers are efficient at part-load and suited for medium-size plants. Absorption chillers use heat (steam or hot water) rather than electricity and are used where waste heat or natural gas is available.

Cooling towers: Reject heat from the condenser water loop to the atmosphere through evaporation. Counter-flow induced-draft towers are standard for commercial buildings. Tower sizing is based on condenser water flow rate and the required approach temperature (how close the leaving water temperature can get to the ambient wet-bulb temperature).

Condenser water pumps: Circulate water between the chiller condenser and the cooling tower. Standard design is 3 GPM per ton of cooling at a 10°F ΔT (85°F supply to tower, 95°F return from tower).

Chilled water pumps: Circulate chilled water from the chiller evaporator to the AHUs. Standard design is 2.4 GPM per ton at a 10°F ΔT (44°F supply, 54°F return). Variable speed pumps (VFD-driven) are required by ASHRAE 90.1 for systems over a certain size.

Primary-Secondary vs. Variable Primary Flow

Primary-secondary (decoupled) systems: Separate pump circuits for the chiller (primary) and distribution (secondary), connected through a common pipe (decoupler or bypass). Primary pumps run at constant speed; secondary pumps vary speed to match building load. This was the standard design for decades.

Variable Primary Flow (VPF): A single pump circuit where chilled water flow through the chiller varies directly with building load. VPF systems save pump energy but require minimum flow protection for the chiller (typically 25–40% of design flow). Modern chiller controls make VPF reliable and it has largely replaced primary-secondary in new designs.

Air Handling Unit Design

Each AHU contains a chilled water cooling coil, a heating coil (hot water or steam), filters, and a supply fan. The cooling coil is sized to handle the peak zone load plus the ventilation load. Coil selection involves:

  • Entering air conditions (mixed air temperature and humidity)
  • Leaving air conditions (supply air temperature setpoint, typically 55°F)
  • Chilled water supply and return temperatures
  • Air velocity through the coil face (typically 450–550 FPM)

The coil valve controls chilled water flow through the coil in response to the AHU's discharge air temperature sensor. Two-way valves (which shut off flow at zero load) are preferred over three-way valves for variable flow systems.

Chiller Sizing and Redundancy

The total cooling plant capacity should match the peak building load plus a redundancy allowance. Many large facilities use N+1 chiller configurations — if one chiller fails, the remaining units can handle the building load at moderate outdoor temperatures. The building peak load is calculated by summing the peak loads of all zones, applying a diversity factor (not all zones peak simultaneously), and adding the ventilation load.