Outside Air Handling Unit (OAU) Control Mechanism
How the Fujitsu VRF OAU controls discharge air temperature, refrigerant flow, airflow and system protection.
Overview
The Outside Air Handling Unit (OAU) operates as a dedicated 100% fresh-air indoor unit within a Fujitsu VRF system. Its control strategy maintains stable discharge air temperature, manages refrigerant flow through EEV21 and SV21, and protects the unit under varying ambient and operating conditions.
1. Refrigerant control and key components
The OAU uses the following components to regulate temperature and refrigerant flow:
- EEV21 — Electronic Expansion Valve: Modulates refrigerant flow across an operating range of 45–1400 pulses.
- SV21 — Solenoid Valve: Opens or closes to enable or isolate refrigerant flow under specified operating conditions.
- Temperature sensors TH21–TH25: Monitor conditions used for superheat, subcooling and discharge-air temperature control.
- Check valve and capillary tube: Help prevent refrigerant backflow and reduce noise caused by pressure fluctuations.
2. Discharge-air temperature control
Cooling mode
- Superheat control: TH22 and TH24 are used to maintain a temperature difference of 2–8°C.
- Thermostat-off condition: The thermostat switches off when discharge-air temperature falls 5°C below the setpoint.
- EEV21: Modulates to maintain the required superheat.
- SV21: Remains closed during cooling operation.
- Operating range: Outdoor temperature −7°C to +43°C; set temperature 14–25°C.
Heating mode
- Subcool control: TH22 and the High Pressure Sensor (HPS) are used to maintain a temperature difference of 4–50°C.
- Thermostat-off condition: The thermostat switches off when discharge-air temperature remains more than 5°C above the setpoint for over five minutes.
- EEV21: Adjusts refrigerant flow to regulate subcooling.
- SV21: Closes when all indoor units have stopped and are thermostat off.
- Set-temperature range: 17–28°C.
Auto mode
The OAU automatically changes between heating and cooling when outdoor-air temperature moves relative to the setpoint by approximately ±5°C.
Important: Auto mode is available only when the OAU is configured as the master indoor unit.
3. Fan motor control
- The fan operates at a constant speed determined by the configured static-pressure setting.
- High, medium and low fan-speed selection is not available.
- 5HP: 1170 rpm at 185 Pa.
- 8HP: 1230/1190 rpm at 200 Pa.
- 10HP: 1080 rpm at 200 Pa.
- The unit includes resonance-avoidance control, which makes small fan-speed adjustments to minimise vibration resonance during continuous operation.
4. Protective functions
5. Oil recovery and defrost operation
- Oil recovery and defrost follow the same sequence used by standard VRF indoor units, including AR, AU and AS types.
- SV21 remains closed during oil recovery and defrost to maintain refrigerant isolation from the outdoor circuit.
6. Operating summary
| Function | Control | Target or action | Range / notes |
|---|---|---|---|
| Cooling | EEV21 | Superheat using TH24 and TH22 | 2–8°C |
| Heating | EEV21 and HPS | Subcool control using HPS and TH22 | 4–50°C |
| Fan motor | Fixed speed | RPM determined by static pressure | Constant-speed operation |
| Low-ambient protection | System logic | Stops fan at or below −7°C | No fault displayed |
| Oil recovery / defrost | Standard VRF sequence | SV21 closed | Automatic operation |
Key outcome
Together, these controls support stable discharge-air temperature, reliable 100% fresh-air delivery, consistent fixed-speed airflow and protection against low ambient conditions, refrigerant accumulation and incorrect static-pressure settings.