Off-Cycle Refrigerant Migration → Liquid Slug on Startup
A field reference for suction accumulator faults — what causes it, how to confirm it on the unit, how to repair it, and what fails next if you leave it. Written for working HVAC & refrigeration technicians.
How to confirm it on site
- Crankcase heater terminal voltage: 24V/120V/240V nominal — no voltage = fault
- Heater surface temp: OFF state should be ambient + 10–15°C (normal)
- Abnormal compressor noise immediately after startup (knocking)
- Discharge pressure spikes high then crashes at startup
- Oil sight glass shows foam right after startup (boil-out signature)
Root causes
- Crankcase heater faulty / missing (essential outdoors)
- Long off periods (overnight, weekends, seasonal)
- Wide ambient swings (cold-climate outdoor unit)
- Refrigerant overcharge (more refrigerant available to migrate)
- No pump-down sequence (liquid line not isolated before stop)
If you leave it unrepaired
- When compressor OFF, its body cools below ambient → refrigerant migrates into compressor body
- Oil dissolves refrigerant → on startup pressure drop boils it out → foam forms
- Right after startup: liquid slug → discharge valves / rods damaged immediately
- Repeated long term: cumulative compressor damage (burnout sometimes within 1 year)
- Volume can exceed accumulator capacity (compressor body holds extra)
How to fix it
- Inspect crankcase heater: terminal voltage / resistance / surface temp → replace if faulty
- Pre-warm 30 min before startup after long off period (heater on, then start)
- Implement pump-down sequence: close liquid-line solenoid, equalize at suction, then stop
- Review accumulator capacity (should hold 50–70% of system charge as liquid)
- Prevention: schedule heater operation check before/after each season
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What causes off-cycle refrigerant migration → liquid slug on startup?
Crankcase heater faulty / missing (essential outdoors) Long off periods (overnight, weekends, seasonal) Wide ambient swings (cold-climate outdoor unit) Refrigerant overcharge (more refrigerant available to migrate) No pump-down sequence (liquid line not isolated before stop)
How do I diagnose off-cycle refrigerant migration → liquid slug on startup on site?
Crankcase heater terminal voltage: 24V/120V/240V nominal — no voltage = fault Heater surface temp: OFF state should be ambient + 10–15°C (normal) Abnormal compressor noise immediately after startup (knocking) Discharge pressure spikes high then crashes at startup Oil sight glass shows foam right after startup (boil-out signature)
How do I fix off-cycle refrigerant migration → liquid slug on startup?
Inspect crankcase heater: terminal voltage / resistance / surface temp → replace if faulty Pre-warm 30 min before startup after long off period (heater on, then start) Implement pump-down sequence: close liquid-line solenoid, equalize at suction, then stop Review accumulator capacity (should hold 50–70% of system charge as liquid) Prevention: schedule heater operation check before/after each season
What happens if off-cycle refrigerant migration → liquid slug on startup is left unrepaired?
When compressor OFF, its body cools below ambient → refrigerant migrates into compressor body Oil dissolves refrigerant → on startup pressure drop boils it out → foam forms Right after startup: liquid slug → discharge valves / rods damaged immediately Repeated long term: cumulative compressor damage (burnout sometimes within 1 year) Volume can exceed accumulator capacity (compressor body holds extra)