VRF Systems
Summary
VRF systems connect one or more inverter-driven outdoor units to many indoor fan coils through refrigerant piping, with heat pump systems running all zones in one mode and heat recovery systems heating and cooling zones at the same time through branch controllers. They give efficient, individually controlled zones with small pipes instead of large ducts, but refrigerant charge limits in small rooms, piping routes, ventilation air, and condensate must be planned with the architecture.
Cols
Terminology
- Outdoor unit (condensing unit): inverter compressors and coil; air-source or water-source
- Indoor unit: ducted, ceiling cassette, wall-mounted, floor console, or concealed unit
- Heat pump VRF: two-pipe system; all indoor units heat or cool at once
- Heat recovery VRF: three-pipe or two-pipe system with branch controllers allowing simultaneous heating and cooling
- Branch controller / BC box / branch selector: device distributing refrigerant to zones
- Refrigerant concentration limit: maximum refrigerant per occupied volume if the whole charge leaks
- A2L refrigerant: mildly flammable low-GWP refrigerant such as R-32 or R-454B
- DOAS: dedicated outdoor air system providing ventilation separately
- Water-source VRF: outdoor units replaced by water-cooled units on a condenser water loop
Standards
Ratings
- Piping length, vertical separation, and capacity ratio limits are manufacturer-specific
- Total system charge limited so that a full leak into the smallest occupied space served stays below the refrigerant's concentration limit
- ASHRAE 15-2022 added requirements for safe use of A2L refrigerants, including leak detection and air circulation
Test standards
- AHRI 1230: VRF multi-split efficiency ratings
- ASHRAE 15 and 34: refrigeration safety and refrigerant classification
- UL 60335-2-40: equipment safety including A2L refrigerants
- ASHRAE 62.1: ventilation (usually via DOAS)
Components
- Outdoor units: roof or grade mounted, modular combinations
- Indoor units: multiple styles per system
- Branch controllers: heat recovery systems
- Refrigerant piping: insulated copper with brazed joints
- Controls: individual zone controllers and central management
- Leak detection: sensors and alarms where required for charge limits or A2L
Detailing
- Check refrigerant charge against the smallest rooms served; add leak detection or split systems where needed
- Route refrigerant piping through accessible chases; coordinate with fire-rated penetrations
- Provide condensate drains or pumps for each indoor unit
- Provide ventilation air by DOAS or ERV; VRF alone does not ventilate
- Locate branch controllers in accessible ceilings away from quiet rooms
- Provide service clearances and snow or wind protection for outdoor units
Uses
Specific uses
- Offices with many perimeter and interior zones
- Hotels and dormitories
- Multifamily housing
- Schools and renovations where ductwork space is limited
- Buildings needing simultaneous heating and cooling (heat recovery)
Pros and cons
Pros
- High part-load efficiency and individual zone control
- Small refrigerant piping instead of large ducts
- Heat recovery shifts heat between zones
- Modular outdoor units and phased installation
Cons
- Refrigerant inside occupied spaces; charge limits and leak risk
- Proprietary controls and components
- Separate ventilation system required
- Higher first cost for heat recovery systems
Manufacturing and cost
Construction
- Outdoor units set on roofs or pads; piping run, brazed, pressure-tested, and evacuated
- Indoor units hung, ducted where needed, wired, and commissioned by manufacturer-trained technicians
Main manufacturers and suppliers
- Trane and Mitsubishi Electric (CITY MULTI)
- Daikin
- LG
- Samsung HVAC
- Carrier
Trade associations: ASHRAE and AHRI.
Sources
- MEP Academy: VRF heat recovery vs. VRF heat pump, https://mepacademy.com/vrf-heat-recovery-vs-vrf-heat-pump/
- Trane: VRF and A2L white paper, https://elibrary.tranetechnologies.com/public/commercial-hvac/Literature/White Paper/VRF-WPR001A-EN_10292025.pdf
- Trane: Mitsubishi Electric CITY MULTI indoor units and branch controllers, https://www.trane.com/commercial/north-america/us/en/products-systems/ductless-solutions/n-generation/indoor-units-branch-controllers.html
- ICC: 2024 I-Code A2L related changes, https://iccsafe.org/wp-content/uploads/2024_I-Code_A2L_Related_Changes_FINAL_06.14.2024.pdf
- Sweets: Division 23 HVAC (23 81 29), https://sweets.construction.com/BrowseByDivision/Heating, Ventilating, and Air-Conditioning (HVAC)
- Wikimedia Commons: Anteni Sin Vrv 1.jpg, by kallerna (CC BY-SA 4.0), https://commons.wikimedia.org/wiki/File:Anteni_Sin_Vrv_1.jpg