Heating and Air Conditioning Fix
How To Fix Air Conditioning Cooling Coil Drip Pan Problems
When humid air passes over chilled cooling coils, water condenses and drips through
the coils into a collection pan, from which it continuously drains. Problems with these
systems may occur when this water collects and becomes stagnant either on the coils or in
the drip pan. When standing water is present, a biofilm will develop. This biofilm is
composed of bacteria and fungi that are embedded in a slimy matrix. Other organisms such as
amoebae and algae may also occupy this comfortable growth site, feeding off the accumulated
Because this is a slimy layer, one might think that the organisms are unlikely to ever
become airborne. This is not true. Organisms are released into the water, and the drops
falling from the cooling coil, or wind from the fans create bubbles, each of which contains
some of these organisms. The bubbles actually scavenge particles (including bacteria and
spores) so that the concentration in the bubbles is higher than that in the water itself.
Once in the air, these bubbles dry down into droplet nuclei and are readily transported
downstream into the ventilation system. In addition to these particles, the organisms
growing in the biofilms produce volatile organic compounds (odors) that are readily carried
with the ventilation air into the occupied space.
Another problem that water in drip pans may cause results from the locally high relative
humidity near the drip pan, and the fact that the fast moving air stream may pick up liquid
droplets that impact onto downstream surfaces. The combination of high local humidity and
deposition of droplets may be enough to allow fungal growth on the surfaces.
What can be done about these problems?
First, all air conditioning coil drip pans should drain continuously, and should never contain standing
water. This means that the drain must be the lowest point in the drip pan, and be connected
to drain plumbing.
Second, systems should be operated such that the coils are continuously washed by
water so that biofilms are slow to develop. There is some evidence that germicidal
ultraviolet light will reduce the chances of growth on cooling coils.
Third, porous insulation should be avoided close to cooling coils. It should be
noted, however, that water droplets will not travel far before evaporating, so that only the
first few feet of ventilation system surfaces are at risk of becoming wet.
Fourth, biocides will not fix the problem of non-draining drip pans. Organisms
embedded in biofilms are relatively resistant to biocides, and continue to grow and produce
odoriferous compounds. Also, biocides are likely to enter the ventilation air and be
delivered to occupant breathing zones.
The Coil Dripping Pan Problem information is taken from:
"Section 2 Cooling Coil Drip
Pans", Environmental Reporter, EMLab. DEC 03/JAN 04
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