Flooded Start Vs Migration Vs Low Charge Cold Compressor Decision Tree

Why this matters

A cold compressor with liquid refrigerant in the crankcase is one of the fastest ways to destroy an otherwise healthy compressor: liquid does not compress, oil washes out and dilutes, bearings lose their film, and a slugging start can break valves or rods. Three conditions present similarly at start: a flooded start from liquid that condensed in the crankcase during a cold off-cycle, refrigerant migration where vapor migrated to the cold compressor oil over a long off period and absorbed into it, and a separate low-charge condition confused with the symptoms when the start behavior is misread. The remedies diverge sharply: a crankcase heater fix, a pump-down or off-cycle control change, or a charge and leak repair. Treating migration as low charge by adding refrigerant makes the next cold start worse. This tree separates the three.

Symptom presentation

The complaint centers on the start after a long or cold off period. Flooded start and migration both produce a violent, knocking, slugging start: loud mechanical noise for the first seconds, foaming visible in a crankcase sight glass if equipped, sometimes a momentary loss of oil pressure where monitored, and a compressor that smooths out after the liquid boils off. The sump is cold before the start. With low charge, the start is not a liquid slug; instead the running system shows low suction saturation, high superheat, bubbles in the liquid sight glass, and inadequate cooling, with no slug noise at start.

Quick checks before diagnosing

Feel the compressor body and crankcase before a start after a long off period. A sump colder than ambient, or sweating, points toward refrigerant collecting in the oil. A cold or absent crankcase heater on a unit that needs one is a strong migration clue.

Listen to the actual start. A liquid slug is a distinct hard knock and foaming that clears in seconds; a low-charge start is quiet but the running system reads starved.

Verify crankcase-heater operation by measuring heater current during the off cycle. Confirm the off-cycle control strategy (continuous-run heater, pump-down, or none) against what the equipment and line-set length require.

Isolation tree

Branch 1: flooded start.

• Liquid condensed in the crankcase during a cold off-cycle because the compressor was the coldest point in the system. Start produces a slug. The fix is keeping the crankcase warmer than the rest of the system during off cycles: confirm the crankcase heater is present, sized correctly, energized during the off cycle, and on long enough before a start (a cold unit returning to power after a long outage needs heater run time first). Replace a failed heater; add one where the application (long line set, low ambient, outdoor compressor) requires it.

Branch 2: refrigerant migration.

• Over a long off period, refrigerant vapor migrated to and absorbed into the cold compressor oil even without bulk liquid condensing, lowering oil viscosity; at start the dissolved refrigerant flashes and foams, washing the bearings. Distinguished from flooded start mainly by the long idle time and gradual absorption. The remedies overlap: maintain a warm crankcase via the heater, and consider a pump-down off-cycle control that isolates refrigerant in the condenser so little remains to migrate to the compressor.

Branch 3: distinguish a true low-charge condition.

• No slug noise at start, but the running system reads low suction saturation, high superheat, low subcool, and bubbles in the liquid sight glass with inadequate cooling: this is undercharge or a leak, not a cold-compressor liquid problem. A crankcase heater does nothing here. Find the leak, repair, evacuate, and weigh in the OEM charge. Do not respond to a slugging cold start by removing charge as if it were an overcharge.

Branch 4: oil dilution confirmation.

• On equipment with oil monitoring, low oil pressure or a loss-of-charge trip shortly after a cold start with foaming confirms oil dilution from liquid or migration rather than a charge-quantity error. Confirm oil level and condition; severe repeated slugging may have already diluted or pumped out oil and damaged the compressor, warranting mechanical inspection.

Branch 5: off-cycle and control strategy.

• The system has no crankcase heater and no pump-down on an application that needs one (outdoor unit, long line set, cold ambient, large charge relative to compressor displacement): the design itself permits migration. The remedy is proper off-cycle control: crankcase heater plus, where the OEM indicates, a pump-down cycle that stores refrigerant away from the compressor during off periods.

Confirming the diagnosis

After adding or repairing the crankcase heater or off-cycle control, allow the heater to run for the OEM-specified time, then start the compressor and confirm a quiet, slug-free start with no foaming and stable oil pressure where monitored. After a charge-and-leak repair, confirm the running system reads correct suction saturation, superheat, and subcool against the OEM table with proper cooling.

Verify across a cold off-cycle: let the unit sit off long enough to recreate the condition, confirm the crankcase stays warm, and confirm the next start is clean.

Remediation summary

References

• Copeland Application Engineering Bulletin AE4-1351 - refrigerant migration, flooded starts, crankcase heaters, and pump-down control.
• ASHRAE Handbook - Refrigeration (compressor oil, refrigerant solubility, and migration).
• EPA 40 CFR Part 82 Subpart F (Section 608) - recovery and recharge requirements for charge repairs.
• AHRI Standard 210/240 - Performance Rating of Unitary Air-Conditioning and Air-Source Heat Pump Equipment.
• ACCA Manual S - Residential Equipment Selection (compressor application and off-cycle control).