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Deep Technical Chapters · Compressor & Pressure Path
Floodback
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Liquid refrigerant returning to the compressor. Not the same as poor oil return — understand the difference.
Floodback is liquid refrigerant returning to the compressor. It damages valves, dilutes oil, and can destroy the compressor. It is not the same as poor oil return, though both involve the suction side.
Causes
- Overcharge
- TXV overfeeding or bulb too warm
- Evaporator airflow too low
- Low load
- Head pressure too low (low ambient)
Floodback vs poor oil return
Floodback involves liquid refrigerant. Poor oil return involves compressor oil failing to return. They share symptoms (noise, oil issues) but have different causes and fixes. See Oil Return, Risers & Traps.
Diagnosis at a glance
Symptoms of failure
- •Cold crankcase / slugging
- •Knocking on start
- •Diluted oil
- •Valve damage
- •Low or zero superheat
Measurements to take
- •Superheat (low/zero is the warning)
- •Suction line temperature
- •Crankcase temperature
What can mislead you
- •Calling floodback noise "a worn compressor"
How to prove the fault
- •Low/zero superheat + cold crankcase = floodback
Common mistakes & callbacks
- •Treating floodback as an oil-return problem
