Do You Know What Flooding Really Is?

September 1, 2005
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HVACR terminology is often confusing and could be misused by even the most seasoned service veterans. Using terminology precisely is of utmost importance in order to focus on the real problem and efficiently find the remedy.

Clear, concise, and accurate communication between service technicians, part suppliers, customers, and the home shop is rapidly gaining importance as the industry changes and becomes more technically oriented.

Three of the most important service terms are flooding, migration, and slugging. In this column we will look at the term flooding as it is applied to refrigeration and air conditioning compressors. The Oct. 3 column will look at migration and slugging.

Figure 1. Semihermetic compressors often have check valves located on the partition between the crankcase and motor barrel to prevent oil and liquid refrigerant from mixing.

What Is Flooding?

Flooding is what happens when liquid refrigerant enters the compressor's crankcase while the compressor is running. Flooding occurs to a compressor only during the running cycle.

Causes of flooding can include:

  • Wrong TXV setting (no compressor superheat).

  • Electronic TXV thermistors bad or misplaced.

  • Microprocessor programmed wrong for electronic TXV.

  • Malfunctioning microprocessor for electronic TXV.

  • Overcharge of refrigerant.

  • Evaporator fan out.

  • Low load on evaporator.

  • End of cycle (lowest load).

  • Defrost clock or heater out (iced coil).

  • Dirty or blocked evaporator coil.

  • Capillary tube overfeeding.

  • Capillary tube system overcharged.

  • Expansion bulb loose on evaporator outlet.

  • Oversized expansion valve.

  • Flooding after hot gas termination.

  • Heat pump changeover.

  • Defrost termination.

    Liquid refrigerants are heavier than refrigeration oils; liquid refrigerant returning to the compressor will settle under the oil in the bottom of the compressor's crankcase. This liquid refrigerant will gradually be boiled off from the low pressures in the crankcase.

    However, since the liquid refrigerant being boiled off is under the coil in the crankcase, very small oil particles will be entrained in this vaporization process.

    The oil level in the crankcase will drop, robbing mechanical parts of vital lubrication. Often, refrigerant-cooled semihermetic compressors have check valves located on the partition between the crankcase and motor barrel to prevent oil and liquid refrigerant from mixing. (See Figure 1.)

    Air-cooled semihermetic compressors and hermetic compressors are often more prone to flooding. Suction accumulators can help a flooding condition, but if the situation is severe, accumulators can also flood.

    Resulting Damage

    Liquid refrigerant boiling in the crankcase can cause crankcase pressures to become excessively high. These high pressures can cause refrigerant and entrained oil particles to escape around the rings of the pistons during their down stroke.

    Once in the compressor's cylinders, the particles will be pumped by the compressor into the discharge line. The compressor is now pumping oil and refrigerant, robbing the crankcase of lubrication.

    Oil that is in the system and not in the crankcase will coat the inner walls of the tubing and valves, causing lowered system efficiencies. Higher-than-normal crankcase pressures are caused by the higher-density refrigerant-oil mixture pumped through the compressor's cylinders; this will cause high compressor current draw. It may overheat and even trip the compressor. Broken valves can also occur from this phenomenon.

    A telltale sign that a compressor's crankcase is being flooded with refrigerant will be a cold, frosted, or sweaty crankcase.

    You may also see foaming oil in the sight glass with a low oil level. Higher-than-normal current draws also will be present.

    John Tomczyk is a professor of HVACR at Ferris State University, Big Rapids, Mich., and the author of Troubleshooting and Servicing Modern Air Conditioning & Refrigeration Systems, published by ESCO Press. To order, call 800-726-9696. Tomczyk can be reached by e-mail at

    Publication date: 09/05/2005

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    marvin terrell
    June 28, 2008
    I have 2 5 ton new bard wall-hung units that both compressors are cold and sweating all the way up to the out-put discharge line on the compressor.the tempature if the compressor is approx40 degree same as sucton line comming to them .would you have any links that could help me determind how bad the flooding is?these are zr61pfv copeland compressors on these units. thank you very much.


    laura rodriguez
    June 3, 2010
    I have a drip pan loaded with water coming from the unit (carrier. When the unit is on it cools the house but produces lots of water and therefore floods the pan. the external drain appears to be clear. how can i know for sure that it due to a faulty txv valve before spending hundreds of dollars to have it repaired?



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