Fujitsu General Australia

Overview

The Outside Air Handling Unit (OAU) operates as a dedicated 100% fresh air indoor unit in a Fujitsu VRF system.
Its control strategy focuses on maintaining stable discharge air temperature, managing refrigerant flow through the EEV (Electronic Expansion Valve) and SV21 (Solenoid Valve), and ensuring operational protection in varying ambient conditions

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1. Refrigerant Control and Key Components

The OAU uses the following major components to regulate temperature and refrigerant flow:

• EEV21 (Electronic Expansion Valve): Modulates refrigerant flow (operating range 45–1400 pulses).

• SV21 (Solenoid Valve): Opens or closes to isolate or enable refrigerant passage during certain operating conditions.

• Temperature Sensors (TH21–TH25): Monitor superheat, subcooling, and discharge air temperature.

• Check Valve and Capillary Tube: Prevent refrigerant backflow and reduce noise caused by pressure fluctuations

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2. Discharge Air Temperature Control

Cooling Mode

• Superheat Control:
  Controlled by TH22 and TH24, maintaining ΔT = 2–8°C.
  When discharge air temperature drops 5°C below setpoint, the thermostat switches off.

• EEV21: Modulates to maintain superheat.

• SV21: Closed during cooling operation.

• Temperature Range: Outdoor −7°C to +43°C; set temperature 14–25°C

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Heating Mode

• Subcool Control:
  Based on TH22 and High Pressure Sensor (HPS), maintaining ΔT = 4–50°C.
  The thermostat switches off if discharge air temperature exceeds setpoint +5°C for more than 5 minutes.

• EEV21: Adjusts to regulate refrigerant flow and subcooling.

• SV21: Closes when all indoor units stop (thermostat off).

• Temperature Range: Set temperature 17–28°C

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Auto Mode

• Automatically switches between heating and cooling when the outdoor air temperature changes relative to the setpoint ±5°C.

• Available only if OAU is configured as the master indoor unit

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3. Fan Motor Control

• Fan operates at constant speed (single speed per static pressure setting).

• Speed cannot be toggled between high/medium/low; only static pressure configuration affects speed.

• Rated airflow speeds:

  • 5HP: 1170 rpm (@185 Pa)

  • 8HP: 1230/1190 rpm (@200 Pa)

  • 10HP: 1080 rpm (@200 Pa)

• Includes resonance avoidance control — the fan speed varies slightly to prevent vibration resonance during continuous operation

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4. Protective Functions

• Refrigerant Liquid Pooling Prevention:
  During heating, if TH22 − TH21 ≤ 2 °C, the EEV gradually opens to prevent refrigerant pooling in the coil.

• Cool Air Intake Prevention:
  If outdoor air temperature ≤ −7 °C, the fan stops to avoid intake of excessively cold air.
  (No fault is displayed; the system simply halts airflow.)

• Static Pressure Protection:
  If static pressure is incorrectly set and the fan overheats, the unit automatically reduces static pressure by two steps (≈ 20 Pa) after two protection activations

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5. Oil Recovery and Defrost Operation

• Follows the same sequence as standard VRF indoor units (AR/AU/AS types).

• SV21 remains closed during both oil recovery and defrost to maintain refrigerant isolation from the outdoor circuit

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6. Operating Summary

Conclusion

The Outside Air Handling Unit (OAU) integrates seamlessly into the VRF system, automatically regulating refrigerant and airflow based on mode and ambient conditions.
Its control logic ensures:

• Stable discharge air temperature via EEV modulation.

• Protective shutdowns against low ambient or liquid accumulation.

• Consistent airflow through fixed-speed fan operation.

• Full compatibility with VRF oil recovery and defrost routines.

These functions together maintain reliable 100% fresh air delivery and prevent performance degradation under varying environmental conditions.