HVAC System Replication: IESVE vs. OpenStudio vs. EnergyPlus and eQUEST

When you're modelling a building with dozens of similar zones served by nearly identical HVAC systems, you face a choice. Replicate everything manually, or use whatever shortcuts your software provides. Unfortunately, none of the major energy modelling platforms agree on what those shortcuts should look like.

IESVE: Multiplexing

In ApacheHVAC, multiplexing is visual compression. You design one HVAC network diagram with all its components, controllers, and connections. Then you draw a box around the portion that repeats across multiple zones and tell the software how many times to replicate it. That single diagram now represents 20, 50, or 200 nearly identical systems.

The interface collapses these into "layers." Each layer connects to a different thermal zone, but they all share the same topology. One fan coil unit on screen, 40 in the simulation. Changes propagate to all layers by default, though you can selectively edit subsets when needed.

The "Assign from Room Group" feature speeds things up. Select a group of similar zones, and IESVE creates a multiplex layer for each one automatically. For buildings where HVAC follows clear patterns, this cuts model setup time by hours.

There are rules. You can't multiplex components that wouldn't make sense to replicate (single chillers serving multiple systems). Controllers inside a multiplex need to point to the same node across layers, which matters more than it sounds when you're troubleshooting over-constrained flow errors.

OpenStudio and EnergyPlus: No Multiplexing

OpenStudio doesn't have multiplexing. If you need 40 fan coil units, you're adding 40 fan coil units. The interface uses a visual loop editor where you drag components onto supply and demand sides. You can drag multiple zones onto a single air loop's splitter, which works for VAV systems serving many zones. But for zone-level systems like PTACs or fan coils, you're doing it all manually.

System templates exist (packaged rooftop units, VAV with reheat, water source heat pumps), but they still result in discrete systems needing individual attention. Some users work around this with API scripts or OpenStudio Measures, but that's code management on top of energy modelling.

EnergyPlus, the simulation engine behind OpenStudio and Better Building, has zone multipliers, but they're not equivalent to IESVE's multiplexing. Zone multipliers are designed for multi-storey buildings with identical floors. You model a typical floor, apply a multiplier, and the simulation multiplies the loads. If you've got a 10-storey building where floors 2 through 9 are identical, model one floor with a multiplier of 8.

This saves geometry and speeds up simulation, but the constraints are strict. Shading must be identical. No interzone surfaces between multiplied zones. Position the multiplied floor halfway between top and ground because exterior convection coefficients change with height.

The critical difference is that you're multiplying thermal zones, not HVAC configurations. Each zone still needs its equipment defined in the model. Five zones with fan coils means five fan coil objects in the IDF file. The multiplier just tells the simulation engine to multiply the loads. You're not compressing HVAC system configuration into a single editable diagram.

eQUEST: Wizard-Based Replication

eQUEST's Design Development wizard asks you to specify "systems per zone," "systems per floor," "systems per shell," or "systems per building." Select "systems per floor" for a packaged rooftop system in a three-storey building, and eQUEST creates three systems automatically.

Less flexible than IESVE's multiplexing, more automated than OpenStudio's manual approach. The wizard handles replication, you end up with multiple discrete systems in the model tree.

Assignment is relatively coarse. The "Zone Groups" feature (screen 14 of the shell editor) lets you specify which zones belong to which system. This works when systems follow predictable patterns like perimeter vs core. When the pattern is irregular, you're reassigning zones to different parent systems in the detailed interface.

Each zone has a parent system field you can change, but you're doing it one zone at a time. The "system per site" option gives you exactly one system. Straightforward, but if you later need multiple systems, you're going back to the wizard or creating new systems manually.

Why This Matters?

For initial setup, IESVE's multiplexing saves hours on large buildings with repetitive systems. OpenStudio requires adding each system individually. EnergyPlus zone multipliers help with truly identical floors, but you're still defining all HVAC systems manually. eQUEST's wizard automates some replication, but expect cleanup work in detailed mode.

For maintenance, IESVE's global editing mode is unmatched. Change fan speed in one multiplex layer, it changes for all of them. In OpenStudio, changing 40 identical systems means editing 40 systems. EnergyPlus zone multipliers don't help because they multiply loads, not configurations. eQUEST requires manual changes to each system unless components happen to be defined once and referenced multiple times.

The flip side is that multiplexing can hide mistakes. Misconfigure the base layer, that error propagates everywhere. You might not notice until you're troubleshooting results and realise 40 systems are wrong the same way. OpenStudio's lack of abstraction means errors are usually obvious because you see them repeated across the interface.

Which Approach Works?

If you're modelling large commercial buildings where every floor has 10 to 20 similar zones with nearly identical systems, IESVE's multiplexing is worth learning. The time savings are real, especially when changes happen late in design.

For smaller buildings, or buildings where each system is meaningfully different, OpenStudio's manual approach isn't particularly painful. The interface is clean, system templates are solid, and transparency helps you understand what's happening in the simulation.

eQUEST is reasonable if you're in the DOE-2 ecosystem and want some automation without multiplexing complexity. Just be ready for detailed interface cleanup.

The real question isn't which approach is objectively better. It's which one matches your workflow and the buildings you typically model.