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Modeling Multiple VRF ODUs with Different Capacities & Efficiencies for a Single Zone Group

asked 2025-06-13 04:55:54 -0500

Anurag_G's avatar

updated 2025-06-13 22:06:39 -0500

Hello everyone,

In DesignBuilder, when modeling multiple VRF outdoor units (ODUs) serving the same VRF zone group, the software requires summing their capacities into a single equivalent ODU. However:

How are part-load efficiencies accounted for if the ODUs have different capacities and performance curves?

Since VRF systems operate at varying loads, do the combined efficiency curves (COP, EER, IPLV) adjust based on the mix of ODUs?

Or does the simulation assume a single averaged efficiency curve?

Is there a way to explicitly define multiple ODUs (with different sizes and performance data) rather than combining them into one?

If not, what is the recommended approach for ensuring accurate efficiency modeling?

Does EnergyPlus (after IDF export) allow more detailed VRF ODU configurations?

Or does it also follow the same "summed capacity" simplification?

This is important for accurately simulating real-world systems where multiple ODUs of different sizes serve the same zones (e.g., a mix of 10HP and 15HP units).

Thank you for your guidanceC:\fakepath\VRF system arrangement-Group wise.png

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answered 2025-06-13 15:07:58 -0500

Greg Estep's avatar

Hi Anurag, why don't you dedicate the zone groups per VRF OU?

If you want multiple VRF OU to serve the same zone (not zone group), then I would break up the zone into multiple zones, and the dedicate the VRF OUs to the associated zones. You could use virtual partitions and ensure interzone airflow for mixing.

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Hi Greg, thanks for your response. I have attached the image of VRF schematic for better understanding in the forum. The layout you see is for one of the many groups. As you can see, all the zones(from ground to second) are being served by 3 ODUs. Also, the treated fresh air systems are served by 3 ODUs. I am having difficulty to model this scenario in design builder. Can you help?

Anurag_G's avatar Anurag_G  ( 2025-06-13 22:10:03 -0500 )edit

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Asked: 2025-06-13 04:55:54 -0500

Seen: 119 times

Last updated: Jun 13