Cold Crankcase, Cold Start: Diagnosing Compressor Flooding Before It Kills the Compressor

A failed crankcase heater doesn't announce itself with an error code — it shows up weeks later as a locked or grounded compressor, and by then the heater is old news. The fastest field check is simple: a healthy crankcase heater keeps the compressor shell noticeably warmer than ambient during the off-cycle. If the shell is cold to the touch after an hour of downtime in cool weather, stop and check the heater before you ever call the compressor bad. Below is how to tell heater failure from migration, flooding, and slugging — three terms techs use almost interchangeably that actually describe three different failure paths.

Migration, Flooding, and Slugging Are Not the Same Failure

These three terms get blended in the field, but they happen at different times and call for different fixes:

• Migration happens during the off-cycle. Refrigerant vapor naturally travels to the coldest point in the system — usually the compressor — and condenses into the oil. This is the failure a crankcase heater is specifically built to prevent, by keeping the compressor shell warmer than the rest of the system so it's never the cold spot.
• Flooding happens while the compressor is running, when liquid refrigerant returns to the crankcase through the suction line — typically from an oversized TXV, a bad superheat setting, or a failing distributor. A crankcase heater does nothing to prevent this; it only addresses off-cycle migration.
• Slugging is the acute version of flooding — a slug of liquid refrigerant hits the compressor on startup and causes mechanical damage (broken valves, bent connecting rods) almost immediately, versus the slower oil-dilution damage from migration.

This distinction matters because a tech who finds a locked compressor and blames "flooding" when the real cause was a dead crankcase heater will misdiagnose the replacement unit the same way — the heater failure never gets corrected, and the new compressor fails on the same timeline.

How to Field-Diagnose a Failed Crankcase Heater

1. Check for power at the heater circuit during the off-cycle, not the run cycle. Many systems wire the heater through an auxiliary contact on the contactor so it's energized only when the compressor is off — if you check with the compressor running, you'll get a false "no power" reading even on a good heater.
2. Feel the compressor shell after at least 30–60 minutes of off-cycle time in cool ambient conditions. A working heater should leave the lower shell noticeably warm, not just "not cold." Marginal warmth on a system that's been sitting overnight in cold weather points to a partially failed or undersized heater.
3. Check for a tripped breaker or blown fuse with no other obvious cause. Failed band-type and insertion-type crankcase heaters are a frequent, overlooked cause of nuisance electrical trips — techs often chase the compressor windings first and miss the heater entirely.
4. Inspect for physical damage on band-style heaters — cracked or brittle bands, especially on units that have cycled through repeated thermal expansion and contraction over multiple seasons. These heaters live outdoors on the compressor shell and take real thermal stress.
5. If the heater tests good but migration symptoms persist — foaming oil in the sight glass, low oil level, frosted crankcase on startup — check the control logic. A heater with no thermostat or pressure-switch control can be undersized for the system's charge and climate, or miswired so it's energized during the run cycle instead of the off-cycle, which wastes energy without preventing migration.

Fixed vs. Controlled Crankcase Heaters — Why It Matters for Diagnosis

Not every crankcase heater runs constantly during the off-cycle, and knowing which control scheme you're looking at changes what "normal" looks like:

• Constant-on heaters energize any time the compressor is off, controlled only by the compressor contactor's auxiliary contact. Simple, but less efficient — and a common source of nuisance calls when a homeowner notices the outdoor unit "always feels warm."
• Thermostatically controlled heaters cycle based on ambient or compressor temperature, only energizing when migration risk is actually present. A tech unfamiliar with this control scheme can mistake a heater that's correctly cycled off in mild weather for a failed one.
• Pump-down systems avoid the migration problem differently — pumping refrigerant out of the evaporator and suction line before each off-cycle, sometimes paired with a heater for redundancy. If you're troubleshooting a "cold crankcase" complaint on one of these systems, confirm whether pump-down is functioning before condemning the heater.

Compare against the equipment's control documentation before assuming a heater that isn't energized is bad — on many commercial refrigeration platforms, a cold heater during mild ambient conditions is exactly how the control is supposed to behave.

What Changes on A2L (R-454B / R-32) Systems

As R-454B and R-32 systems become the majority of new residential and light-commercial installs in 2026, crankcase heater diagnosis picks up a couple of new wrinkles worth flagging before cold-weather season:

• Refrigerant charge is generally lower on A2L systems than the R-410A systems they replace, which changes the migration math — less total refrigerant means less mass available to migrate and dilute the oil, but it doesn't eliminate the need for a functioning heater on systems that specify one.
• A2L refrigerants are not interchangeable with each other or with R-410A, so never assume a replacement heater or control scheme from an R-410A platform carries over — confirm the refrigerant type on the equipment nameplate and follow that manufacturer's specific crankcase heater and control requirements for the installed refrigerant.
• These are still new platforms in the field. Failure-pattern data for crankcase heaters on A2L compressors is thinner than the decades of R-410A and R-22 field history techs are used to leaning on — when in doubt, defer to the OEM's documented control logic rather than assumptions carried over from older refrigerants.

Common Technician Questions

Does a crankcase heater need to run all summer, or only in cold weather? It depends on the control scheme, not the season. Migration is driven by the compressor being the coldest point in the system — which can happen on a hot day if the indoor coil and suction line stay warmer than a rapidly cooling outdoor unit at dusk. Constant-on heaters run whenever the compressor is off, regardless of season; thermostatically controlled heaters cycle based on actual temperature differential.

Can I just cut out a heater that keeps tripping breakers instead of replacing it? No — this removes the compressor's migration protection entirely and sets up exactly the flooded-start failure the heater exists to prevent. A heater that's tripping breakers should be diagnosed and replaced, not bypassed.

Will a bad crankcase heater cause a compressor to fail immediately, or is the damage gradual? Usually gradual. Migration dilutes oil over repeated off-cycles, progressively degrading lubrication until a startup causes enough foaming and oil loss to damage bearings, valves, or windings. This is part of why heater failures are so often missed — the compressor that finally fails may have had a dead heater for months.

How is flooding during the run cycle diagnosed differently from a heater problem? Flooding during operation points to metering device or superheat problems — check TXV setting, distributor condition, and actual operating superheat rather than the crankcase heater, which has no function once the compressor is running.

Do variable-speed and inverter-driven compressors still need crankcase heaters? Many do, and control logic can differ from fixed-speed platforms — some inverter systems use the compressor's own winding heat management or a low-speed "keep-warm" cycle instead of a traditional resistive heater. Always confirm against the specific platform's service documentation rather than assuming.

See our Crankcase Heater Selection.

This is a diagnostic reference for licensed HVACR technicians. Always confirm control logic, refrigerant type, and heater specifications against the equipment manufacturer's documentation before servicing.