1. Overview
The Fujitsu AIRSTAGE VR-II System is a 3-pipe heat recovery VRF (Variable Refrigerant Flow) system.
It builds upon the proven V-II architecture but adds enhanced heat recovery control, oil management reliability, and system efficiency optimization.
This training module explains:
Why the VR-II uses a 3-pipe refrigerant cycle
The principles of heat recovery operation
The high-reliability control methods employed in the VR-II series
2. Why a 3-Pipe Refrigerant Cycle?
A. Concept and Industry Rationale
The 3-pipe system is the most widely used heat recovery configuration globally due to its:
Simplicity in design and installation
Reliable performance across diverse conditions
Ease of service and reduced installer error
Competitors such as Daikin (BS units) and Mitsubishi (BC controllers) use equivalent 3-pipe logic.
Fujitsu’s VR-II maintains this architecture, ensuring familiarity for consultants, contractors, and designers while offering distinct design improvements.
B. Advantages of the VR-II 3-Pipe System
| Feature | Benefit |
|---|---|
| Independent Cooling & Heating Circuits | Enables simultaneous operation in different zones |
| Simplified RB Unit Design | Smaller, lighter, fewer brazing points |
| Flexible Control Box Placement | Easier to install in tight or irregular spaces |
| Reduced Risk of Installation Mistakes | Fewer connection points and clearer flow direction |
C. Comparison with 2-Pipe Systems
| Aspect | 2-Pipe System | 3-Pipe System (VR-II) |
|---|---|---|
| Operation | Cooling or heating only | Simultaneous cooling and heating |
| Energy Use | Inefficient switching losses | Heat energy recovery across zones |
| Control Complexity | Higher (requires changeover) | Stable and continuous |
| Comfort | Temperature swings during mode change | Steady temperature control |
3. Heat Recovery Operation
A. Basic Principle
During simultaneous operation:
Heating indoor units act as condensers, releasing heat into the refrigerant.
Cooling indoor units act as evaporators, absorbing that same heat.
The RB (Refrigerant Branch) Unit channels refrigerant to each indoor unit as required.
This allows waste heat from cooling zones to be reused in heating zones, increasing total system efficiency.
B. Operation Logic
Mainly Cooling Mode
The compressor adjusts speed to match low-pressure (cooling) demand.
The outdoor expansion valve regulates liquid pressure to maintain stable evaporation.
Mainly Heating Mode
Compressor capacity is modulated according to high-pressure (heating) load.
Liquid refrigerant flow is adjusted through the outdoor expansion valve for balance.
This dynamic balance ensures optimum refrigerant flow to all connected indoor units while minimizing energy loss
4. RB Unit Function — Cooling/Heating Switching
The RB Unit (Refrigerant Branch Unit) acts as the “brain” for zone control:
Directs high-pressure gas, low-pressure gas, and liquid refrigerant to each indoor unit.
Uses internal 3-way valves to route refrigerant depending on whether each zone is in cooling or heating mode.
Maintains the same specification design as the V-II series but with enhanced reliability and reduced size.
This modular branching design:
Simplifies system layout.
Reduces field brazing.
Provides quiet and stable switching between operational modes.
5. High-Reliability Control Methods
A. Oil Management Strategy
Oil recovery and refrigerant circulation reliability are key to VRF performance.
VR-II employs multiple measures for safe oil distribution and recovery:
Oil Separation and Return Circuit — Based on V-II design, ensuring consistent lubrication.
Automatic Oil Recovery Operation — Runs every 3 hours during normal operation.
Accumulator Enlargement — Prevents oil accumulation in specific outdoor units.
Receiver Tank Integration — Manages excess refrigerant volume and prevents liquid backflow.
Superheat and Subcool Control — Maintains stable refrigerant state through thermistor feedback (TH7–TH10).
B. Oil Recovery During Various Modes
| Unit State | Oil Recovery Method |
|---|---|
| Cooling Mode | Oil is returned via the gas line while cooling continues; fans remain active. |
| Heating Mode | Fans are temporarily stopped to prevent cool air discharge during recovery. |
| Stopped Indoor Units | Oil is drawn back using refrigerant circulation through inactive circuits. |
| Outdoor Units | Compressor RPM adjusted for controlled refrigerant velocity; termination judged by suction temperature. |
Result: Full oil recovery achieved in minimal time while maintaining occupant comfort and compressor safety
6. Reliability Enhancements in the VR-II System
| Reliability Feature | Function |
|---|---|
| Enlarged Oil Accumulator | Prevents oil pooling and uneven distribution |
| Liquid Backflow Prevention | Avoids refrigerant migration when units are off |
| Automatic Oil Recovery Control | Scheduled every 3 hours; adapts to real-time conditions |
| Compressor Control Optimization | Balances operation between multiple compressors |
| Superheat/Condensing Temperature Sensors | Real-time detection for refrigerant stability |
| Simplified 3-Pipe Design | Reduces mechanical stress and error points |
7. Summary and Key Takeaways
3-Pipe Heat Recovery System — Standardized, reliable, and easier to install than 2-pipe alternatives.
Heat Recovery Operation — Efficient energy exchange between heating and cooling zones.
Oil Management & Reliability Controls — Ensure long-term stability, compressor protection, and consistent operation.
Conclusion
✔ Fujitsu General’s VR-II system adopts the 3-pipe cycle as an industry-standard solution.
✔ The design provides high reliability, simplified installation, and effective heat recovery.
✔ Oil is recovered efficiently and automatically, protecting system components while minimizing service needs.