Modelling Primary/Secondary Chilled Water Loops and Headered Pump Configurations

This article explains how to accurately model primary/secondary CHW plant loops, with a specific focus on correct pump placement and configuration. It clarifies when to use standard loop pumps versus headered (parallel) pumps and provides guidance for systems that require dedicated chiller pumps or multiple pumps operating as a group. References and best practices are included to support detailed modelling workflows.

Primary/Secondary Chilled Water Loop Modelling

The Primary/Secondary Chilled Water Plant Loop is available to model the primary and secondary pumps of a CHW loop. 

The primary/secondary chilled water plant loop uses the EnergyPlus Common Pipe Simulation mechanism to represent a chilled water circuit comprising both primary and secondary pumped sub-loops.

Note: In this configuration, the primary pump must be located on the first (inlet) branch of the half-loop as shown in the image below.

For more details, see: EnergyPlus Plant Application Guide and Engineering Reference Summary of Pump Rules. 

Modelling Dedicated Chiller Pumps

If your project requires a dedicated pump at each chiller, do not use the common-pipe setup. Instead, model each chiller and associated pump on its own plant loop, connecting to the secondary loop with a  Fluid-to-fluid HX, as demonstrated in Detailed HVAC System No. 8.

Modelling Headered Pump Configurations

When you need to model multiple identical pumps operating in parallel as a group to provide all the flow for the plant loop, replace the Pump:VariableSpeed and Pump:ConstantSpeed objects that are available inside DB with their headered equivalents using scripting.

• HeaderedPumps:ConstantSpeed
• HeaderedPumps:VariableSpeed

See also the sample script: headeredPumps.cs