The development of copious amounts of flaky, white powder from indoor coils has been a
recognized industry issue for a couple decades now. The first known occurrences were reported
on residential heatpump systems that used refrigerant as the heat transfer fluid. Commonalities
between reported cases were coils constructed with bare aluminum fins in an air cooling/
dehumidifying application. The problem would surface during the heating season of the year when
the dehumidifying coil’s fin surface was dry of condensate and within the first couple years of
operation of the equipment.
Large commercial equipment applications, while still rare, have also been reported as well as
chilled water systems in a variety of light commercial and terminal products. As additional
occurrences have been identified over the last 10 to 20 years, it has now been shown to happen
to any type of dehumidifying coil with bare aluminum fins operating in a system that also supplies
forced air heating during the winter season.
The incident rates have been low over the years and until recently limited to coastal areas mostly
regionalized in the southern United States. With millions of aluminum fin coils shipped in the
reported time period, problem jobs only number in the dozens. The phenomenon has been
reported across additional geographical areas in the Southeastern and Midwestern United States,
Europe and Asia. But, the primary regions have continued to be hot and humid climates or milder
climates that had severe cooling seasons prior to development of the problem. Cases have not
been reported in dry, arid climates or on coils that are in heating only applications utilizing hot
water, hot glycol solution or steam. There are no documented cases with coils that have nonaluminum
fins (copper, steel, etc.) or with coils that have fin surface coatings (corrosion resistant
System design does not seem to be an important contributing factor. A variety of applications have
demonstrated the capability to produce the white flakes such as coils using either chilled water,
chilled glycol solution or volatile refrigerant; 100 percent return air, 100 percent outside air or
mixed outside and return air.
Once the coil has been installed and operated for a few cooling/heating seasons its vulnerability
is drastically reduced. The phenomenon has only been reported within the first one or two heating
seasons and has yet to be reported during a cooling season. Theories suggest that the coils might
still be producing the white flakes during the cooling season, but the condensate on the fins formed
from dehumidifying the airstream prevents the powder from becoming airborne and thus from
See the attached Engineering Bulletin for the complete details for aluminum fin surface oxidation on air-cooling coils.
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