Overview
The Refrigerant Branch Box (RB Unit) in Fujitsu Airstage VRF systems distributes refrigerant flow between the outdoor unit and multiple indoor units.
Each RB box includes onboard LED indicator lamps that provide real-time diagnostic information.
These indicators help technicians identify communication, wiring, or PCB faults quickly without the need for full system disassembly.
1. Indicator Lamp Reference
| Indicator Lamp | Description |
|---|---|
| Power Indicator Lamp (Green) | Confirms power supply and system communication readiness. |
| Error Indicator Lamp (Red) | Indicates wiring errors, communication failures, or internal PCB faults depending on light pattern. |
2. Status Display and Fault Interpretation
| Power Indicator (Green) | Error Indicator (Red) | Status Description / Fault Cause | Recommended Action |
|---|---|---|---|
| Lit (steady ON) | OFF | Normal operation. Power is supplied and no error is present. | No action required. |
| Lit (steady ON) | Lit (steady ON) | Wiring fault: “IN/U” terminal of RB unit and outdoor transmission terminal incorrectly connected. The transmission cable is wired to the wrong terminal. | Power OFF before re-wiring. Connect the communication cable correctly between the outdoor unit and RB “U/U” terminals. Verify polarity (U1/U2). |
| Lit (steady ON) | Flashing | Communication fault detected between RB unit and outdoor main control board. Possible fault with RB communication PCB or main control PCB. | Check communication cable continuity, polarity, and voltage (approx. 30–35 VDC). If wiring is correct, replace the faulty PCB. |
3. Applicable Models
These indications apply to the following Refrigerant Branch Box configurations:
Single Port RB Unit
4-Port RB Unit
8-Port RB Unit
12-Port RB Unit
Each model operates under the same LED fault logic, regardless of the number of ports.
4. Field Diagnostic Notes
Always turn off system power before performing any re-wiring to prevent control board damage.
After re-wiring or board replacement, perform a system reset via the outdoor PCB or control interface.
If LED indicators do not illuminate, verify 24V DC supply from the outdoor unit to the RB terminal block.
Persistent flashing even with correct wiring suggests RB board communication failure — replace the RB PCB.
