Slugging
What is slugging in HVAC, and why does it matter?
Slugging is liquid refrigerant reaching the compressor. Here is what the patent literature says about causes, damage and prevention, and where the evidence runs out.
By Artifact Editors, Editorial team
Published · 3 min read
In this article
What slugging means in HVAC
The short answer: in HVAC, slugging is the failure where liquid refrigerant enters the compressor during startup. The compressor tries to squeeze liquid, which it cannot compress, and that can damage it Patent 10,967,699, on reducing slugging in vehicle HVAC compressors.
It is industry jargon rather than a formal standard term. Some patents say liquid can also flood the compressor after the unit has sat off overnight, so it starts in a flooded state Patent 10,627,146, on liquid slugging detection.
The term also covers a wider pattern: liquid moving into vapor lines, and vapor moving into liquid lines and the expansion valve. That can lead to mechanical failure, including premature compressor failure Patent 10,337,777, on controlling air conditioning systems.
Why it damages the compressor
Bottom line: the damage comes from liquid in the compression process and from liquid diluting the oil. The patent literature describes two linked failure modes Patent 6,578,373, on refrigerant floodback.
- Audible hammering or chattering, which can mean rapid damage in larger compressors Patent 6,578,373.
- Quiet wear: liquid that dilutes crankcase oil reduces lubricity and shortens compressor life, even when you hear nothing Patent 6,578,373.
- Pressure build-up from compressing liquid, which can damage discharge valves and connecting rods Patent 5,427,506, on compressor pressure relief.
What causes it
Bottom line: the patents point to refrigerant migrating to the coldest part of the system while the unit is off. That risk is greatest when the compressor sits outside the conditioned space Patent 10,627,146 and Patent 4,918,931, on compressor slugging prevention.
Oversized systems that do not fully boil off low-pressure liquid are also cited as a cause Patent 4,884,412, on compressor slugging protection. We found no field failure rates, so we cannot say how common any of these causes are in practice.
How systems try to prevent it
Bottom line: the fixes are engineering controls, and none of the sources give numeric settings you can apply yourself. The approaches described in the patents are below.
| Approach | What it does | Source |
|---|---|---|
| Pump-down before shutdown | Removes residual vapor from the compressor inlet | [Patent 10,967,699](https://image-ppubs.uspto.gov/dirsearch-public/print/downloadPdf/10967699) |
| Accumulator at the inlet | Holds residual liquid before it reaches the compressor | [Patent 10,967,699](https://image-ppubs.uspto.gov/dirsearch-public/print/downloadPdf/10967699) |
| Intermittent pre-start cycling | Purges liquid; one cited publication used two on/off cycles | [Patent 9,377,014, on intermittent start](https://image-ppubs.uspto.gov/dirsearch-public/print/downloadPdf/9377014) |
| Vibration and sound sensing | Detects slugging in a specific frequency band, since temperature alone can be unreliable | [Patent 4,884,412](https://image-ppubs.uspto.gov/dirsearch-public/print/downloadPdf/4884412) |
The beauty trend with the same name
Bottom line: if you saw this word on a skin care feed, it means something else. Slugging there is coating your face, and sometimes your nails, with petroleum jelly overnight. Cleveland Clinic explains that the jelly's occlusive action is the reason it helps skin hold on to moisture.
The evidence is thinner than the feed suggests. We found no randomized trial or systematic review of overnight slugging. A Journal of the American Academy of Dermatology review covers the history, uses and safety of petroleum jelly, but we could only see its abstract, so we cannot report its findings.
For the skin version, see our slugging topic page.
Frequently asked questions
Is slugging the same as flooding?
The terms are closely related, and sources use them in overlapping ways. One patent treats refrigerant floodback as a detection problem of its own [Patent 6,578,373](https://image-ppubs.uspto.gov/dirsearch-public/print/downloadPdf/6578373), so check the context before you assume they mean the same thing.
Can you hear slugging?
Sometimes. The patents describe a hammering or chattering noise in larger compressors, but they also say liquid can dilute the oil and wear parts with no audible sign. A quiet compressor is not automatically a healthy one.
Does overnight slugging for skin actually work?
The evidence is limited. Petroleum jelly's occlusive action is the stated reason it helps hydration, but we found no randomized trial or systematic review testing overnight slugging itself. Treat the benefit as plausible, not proven.
Sources (10)
- US Patent 10,967,699, "Systems and methods for reducing slugging in HVAC compressor of vehicle"
- US Patent 10,337,777, "Controlling air conditioning systems"
- US Patent 10,627,146, "Liquid slugging detection and protection"
- US Patent 4,918,931, "Compressor slugging prevention method for a refrigeration system"
- US Patent 4,884,412, "Compressor slugging protection device and method therefor"
- US Patent 6,578,373, "Rate of change detector for refrigerant floodback"
- Cleveland Clinic, "Slugging: Does This Skin Care Trend Work?"
- US Patent 9,377,014, "Intermittent start compressor"
- US Patent 5,427,506, "Compressor pressure relief assembly"
- JAAD, "Petroleum jelly: A comprehensive review of its history, uses, and safety" abstract only
