Modeling Floor-Wise TFA Rooms in DesignBuilder with VRF Groups (DOAS + ODU Connection)

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We are modeling a VRF system in DesignBuilder with the following setup:

Three VRF zone groups, each with a VRF outdoor unit (ODU) connected to indoor units (IDUs: hi-wall, cassette, ducted).Treated Fresh Air (TFA) rooms (one per floor) also connected directly to the ODU (via DX-treated outdoor air units).

Design Constraints:

Each floor has a separate TFA room supplying fresh air via ducts to its zone.

The TFA units are part of the VRF system (connected to the same ODU as IDUs).

Challenges in DesignBuilder:

When defining a VRF zone group, all floors merge into a single zone, making it impossible to:

1.Assign floor-wise TFA rooms within the group.

1. Model DOAS systems connected directly to the ODU (in addition to IDUs).

Question: What’s the best workflow to:

1. Maintain per-floor TFA room ventilation while using VRF zone groups?

2. Accurately represent DOAS units connected to the ODU (not just IDUs)?

Attempted Solutions:

Option 1: Sum all floors’ fresh air and assign a single DOAS to the VRF group (loses floor-wise control).

Option 2: Model TFA rooms as separate zones with standalone DOAS (but breaks ODU connection)