GENERAL Assist Technical Reference

VRF Oil Recovery, Preheat & Defrost Control

Special operation logic and operating conditions
Technical reference covering oil recovery operation, compressor preheat and heating defrost control.
These functions are automatic system-control operations. They are used to return refrigerant oil to the outdoor unit, protect the compressor during start-up, and remove frost from the outdoor heat exchanger during heating operation.

1. Oil Recovery Operation

Purpose

Refrigerant lubricant can accumulate in indoor units and connecting pipework. As the amount increases, compressor operating time can increase and the compressor may be damaged due to insufficient lubrication. Oil recovery returns oil to the outdoor unit at defined intervals.

1.1 Simple oil recovery — cooling only

Start conditionSuction temperature − low-pressure saturation temperature ≥ 10°C continuously for 30 minutes.
End conditionSuction temperature − low-pressure saturation temperature ≤ 7°C.
Indoor unit operationExpansion valve of the operating indoor unit is gradually opened.
Outdoor unit operationNormal cooling operation.

1.2 Cooling oil recovery

Start conditionCompressor accumulated operating time since the last cooling oil recovery exceeds 3 hours. First oil recovery: 1 hour.
End condition30 seconds have elapsed since start and suction temperature − low-pressure saturation temperature ≤ 5°C at all outdoor units, or 6 minutes have elapsed since start.
CompressorAll inverter compressors operate together with any constant-speed compressors that were operating before oil recovery. Inverter speed varies with operating state.
EEV openingIndoor/outdoor unit EEVs are controlled by pulse as in normal operation.
Fan speedIndoor/outdoor fan speed is controlled as in normal operation.

1.3 Heating oil recovery

Start conditionCompressor accumulated operating time since the last heating oil recovery exceeds 8 hours. First oil recovery: 1 hour.
End conditionAfter 4 minutes have elapsed.
CompressorAll inverter compressors operate together with any constant-speed compressors that were operating before oil recovery. Inverter speed varies with operating state.
EEV openingIndoor/outdoor unit EEVs are controlled by pulse as in normal operation.
Fan speedIndoor/outdoor fan speed is controlled as in normal operation.
Controller indication: During oil recovery, the oil-recovery indication appears on wired and central remote controllers, and an indication also appears on the simple remote controller. Indoor-unit operation LEDs flash slowly.

2. Preheat Operation

Preheat operation is intended to protect against start-up failure by preventing refrigerant from soaking into the compressor oil.

Crankcase heater ON30 minutes after the installed compressors have stopped. The heater is also ON when power is first turned on.
Crankcase heater OFFInstalled compressors operating.
Temperature controlThe source states that preheat operation does not control according to temperature.
Inverter compressorDetermined by inverter-compressor ON/OFF state.
Constant-speed compressorDetermined by constant-speed compressor ON/OFF state.

3. Defrost Operation Control

3.1 Defrost start conditions

Defrost can start when accumulated heating operation time is 40 minutes or longer. Accumulated heating operation time is reset at the end of cooling operation or defrost operation.

In addition to the 40-minute accumulated-heating requirement, the outdoor unit must satisfy Condition 1 or Condition 2 below.

Condition 1

Heat-exchanger temperature ≤ -2°C for an accumulated operating time of 180 minutes or longer. Where indoor-unit connection capacity is ≤ 90% at a 1-outdoor-unit connection, the source specifies 75 minutes.

Condition 2

After all of the following have been satisfied, the condition based on heat-exchanger temperature being below the defrost-start judgement temperature and heat-exchanger liquid-temperature drop must accumulate for 10 minutes:

  • Accumulated heating operation time ≥ 30 minutes.
  • 10 minutes have elapsed since the outdoor unit started.
  • 5 minutes have elapsed since oil recovery.

3.2 Defrost start judgement temperature

Defrost start judgement temperature = 0.8 × outdoor temperature − 11.6
  • The judgement temperature is limited to the range -27.6°C to -6°C.
  • If the calculated result is below -27.6°C, use -27.6°C.
  • If the calculated result is above -6°C, use -6°C.

3.3 Defrost end conditions

Defrost ends when either of the following conditions is met:

  1. At all outdoor units, heat-exchanger liquid temperature ≥ the defrost end judgement temperature; or
  2. 10 minutes have elapsed from the start of defrost. Where indoor-unit connection capacity is 90% or less, the source specifies 15 minutes.
Defrost end judgement temperature = 0.39 × outdoor temperature + 12.7
  • The judgement temperature is limited to the range 5°C to 12°C.
  • If the calculated result is below 5°C, use 5°C.
  • If the calculated result is above 12°C, use 12°C.

4. Operating State During Defrost

ComponentState during defrost
Indoor unit EEVOpen
Indoor unit fanStop
Outdoor unit fanStop
CompressorIn operation

Step 1 — Defrosting

All compressors operate and all outdoor units are defrosting.

Outdoor unitInverter compressorConstant-speed compressorStatus
MasterOperatingOperatingDefrosting
Slave 1OperatingOperatingDefrosting
Slave 2OperatingOperatingDefrosting

Step 2 — Individual outdoor units complete defrost

An outdoor unit that satisfies the defrost-end judgement temperature stops its constant-speed compressor and operates its inverter compressor at the lowest speed (30 rps) while other outdoor units continue defrosting.

Example unitInverter compressorConstant-speed compressorStatus
Master30 rpsStoppedDefrost end
Slave 1OperatingOperatingDefrosting
Slave 230 rpsStoppedDefrost end

Step 3 — Defrost complete

When defrosting has ended on all outdoor units, all outdoor units stop. Start-rotation execution is then performed and the system restarts.

Service note: Defrost and oil-recovery operations can temporarily change compressor speed, fan operation, valve position and indoor-unit behaviour. These changes are part of normal automatic control when the relevant conditions are satisfied.