■ MEP BIM INSIGHTS — MEP ENGINEERING

VRF vs. VAV vs. DOAS: Choosing the Right HVAC System for Your BIM Project

Three HVAC system types dominate US commercial construction: Variable Refrigerant Flow (VRF), Variable Air Volume (VAV), and Dedicated Outdoor Air Systems (DOAS). Each solves a different problem. Each creates a different coordination challenge in Revit.

The system selection decision is made by the mechanical engineer of record — but it directly affects how much coordination work the BIM model requires, how much plenum space the architect needs to provide, and how complex the Navisworks clash detection process will be.

This article explains how each system works, where it performs best, and what it means for the BIM model.

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VRF: Variable Refrigerant Flow

How it works

A VRF system circulates refrigerant directly between an outdoor condensing unit and multiple indoor fan coil units. The outdoor unit modulates refrigerant flow to each indoor unit independently, allowing simultaneous heating and cooling in different zones from a single outdoor unit.

Best suited for

• Multi-zone commercial: hotels, mixed-use, boutique office
• Buildings with highly variable occupancy patterns by zone
• Retrofit projects with limited plenum space
• Projects requiring individual room temperature control

Advantages

• No ductwork required for the primary distribution system — significantly reduces plenum coordination
• High part-load efficiency (COP 3.5–5.0 in mild climates)
• Heat recovery capability: zones in cooling can transfer heat to zones needing heating
• Compact outdoor units relative to chiller plants

Limitations

• Refrigerant in occupied spaces — requires ASHRAE 15 safety analysis and refrigerant detection in larger systems
• Ventilation must be provided separately (typically paired with DOAS)
• Higher first cost than comparable DX systems
• Performance degrades significantly in extreme cold climates (below -4°F / -20°C for most systems)

BIM implications

VRF systems replace large duct runs with refrigerant piping networks. In Revit, refrigerant piping is modeled in the hydronic piping system with custom parameters for refrigerant type, line sizing (liquid vs. suction/discharge), and pipe insulation. Outdoor unit placement — rooftop, grade, or parking structure — requires early coordination with the structural and architectural models. Elevation changes between outdoor and indoor units are system-critical and must be accurately reflected in the model.

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VAV: Variable Air Volume

How it works

A VAV system uses a central air handling unit (AHU) to supply conditioned air at a constant temperature. Variable air volume terminal units (VAV boxes) in each zone modulate the volume of air delivered based on the zone’s thermostat demand. Reheat coils in terminal units provide zone-level heating.

Best suited for

• Large open-plan commercial offices
• Healthcare facilities requiring strict pressure relationships between spaces
• Education: classrooms and lecture halls with predictable occupancy
• Projects where a central plant (chiller + boiler) is already in place

Advantages

• Proven, well-understood technology — straightforward commissioning and O&M
• Centralized ventilation: easier to control outdoor air rates and filtration
• Single refrigerant location (mechanical room) — no refrigerant in occupied floors
• Scales well to large buildings (500,000+ sq ft)

Limitations

• Requires significant plenum depth for ductwork — typically 18–36 inches depending on system size
• Fan energy at low loads can be significant without high-efficiency VFD controls
• VAV boxes at minimum airflow can create poor air distribution in low-occupancy periods

BIM implications

VAV systems generate the most complex MEP coordination work of the three system types. Supply air, return air, exhaust air, and outside air ductwork all need routing through the same plenum, alongside chilled water and heating hot water piping. In Revit, duct coordination is the primary challenge — maintaining minimum duct sizes, fitting clearances, and access space for VAV box maintenance. Clash detection in Navisworks on a VAV-heavy project typically reveals 2–3x more hard clashes than an equivalent VRF project.

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DOAS: Dedicated Outdoor Air System

How it works

A DOAS is a ventilation-only air handling unit that delivers 100% outdoor air — preconditioned to neutral or slightly dehumidified conditions — directly to occupied spaces. It handles ventilation requirements only. Sensible heating and cooling are handled by a separate system: fan coils, radiant panels, VRF indoor units, or chilled beams.

Best suited for

• High-occupancy spaces: conference centers, auditoriums, gyms
• Healthcare: operating rooms, patient rooms requiring precise humidity control
• Education: classrooms where CO² control is critical
• Any project pursuing LEED or WELL certification with enhanced ventilation credits
• Humid climates where latent load management is critical (ASHRAE Climate Zones 1–3)

Advantages

• Decouples ventilation from thermal comfort — each system optimized independently
• Superior humidity control compared to all-air systems
• Straightforward ASHRAE 62.1 compliance documentation
• Energy recovery on the DOAS unit can significantly reduce heating and cooling loads

Limitations

• Requires two parallel systems — higher first cost and more mechanical room space
• More complex controls integration between DOAS and sensible cooling system
• Duct distribution still required for the ventilation air, even if smaller than a full VAV system

BIM implications

DOAS projects require two parallel systems to be modeled and coordinated: the dedicated OA ductwork network and the sensible system (VRF piping, fan coil units, or chilled beams). The DOAS ductwork is typically smaller than full VAV supply ductwork, but the addition of a second system increases total MEP density in the plenum. In Revit, discipline separation between the DOAS and sensible system is important for schedule accuracy and COBie export.

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Side-by-Side Comparison

Factor	VRF	VAV	DOAS
Primary distribution	Refrigerant piping	Ductwork	OA ductwork + sensible system
Plenum depth required	Low	High	Medium
Ventilation handling	Separate (DOAS)	Integrated	Primary function
Zone control	Excellent	Good	Via sensible system
BIM coordination complexity	Medium	High	Medium–High
ASHRAE 90.1 compliance path	System 6 / System 8	System 5 / System 7	System 5 + DOAS
Best building type	Hotel, mixed-use	Office, healthcare	High-occupancy, healthcare

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How System Selection Affects the BIM Scope

System selection is typically finalized during schematic design — but its impact on the BIM model scope is significant and should be considered when developing the BEP and LOD matrix:

• VRF projects require careful refrigerant piping coordination, outdoor unit structural loading, and verification of elevation limits between outdoor and indoor units. Fewer duct clashes, more piping routing constraints.
• VAV projects require the most intensive clash detection effort. Plenum cross-sections should be modeled and verified at schematic design before ceiling heights are fixed in the architectural documents.
• DOAS projects require two parallel system models. QA/QC should verify that DOAS airflows align with occupancy-based ventilation calculations per ASHRAE 62.1.

How We Model All Three at GEOMETRY-S

Our MEP BIM team has modeled VRF, VAV, and DOAS systems on US commercial projects across climate zones 2 through 6. Each system type has its own Revit template configuration, parameter set, and coordination checklist that we apply at project kickoff.

If you are early in design and the system type has not yet been selected, we can model schematic-level geometry for two system options in parallel — giving the design team a BIM-based comparison of plenum impact before the decision is finalized.