A/C: Mold/Bacteria Protection of A/C Coil

A/C: Mold/Bacteria Protection of A/C Coil

A/C:Here’s the problem with your A/C – Cold gas is pumped through an A/C coil. A coil is made up of copper tubing with aluminum fins. The cold gas travels through the copper coil transmitting its low temperature to the aluminum fins, which are now chilled.

Return air from the house is filtered and blown through the A/C coil fins, which are chilled. The filtered return air is chilled by passing over, under, and around the chilled aluminum fins and copper tubing. The chilled fins condense the moisture out of the return air, much the same as a chilled glass of water. The moisture drips off the fins down to a collector or drip pan, where it is drained away as condensate wastewater.

Here is a cross section of a clean A/C coil. Note that the aluminum fins transfer cold. Meanwhile, the copper tubes carry chilled gas.
The problem comes to play when the filtered, often not so filtered, return air deposits mold spores and bacteria on the moist coil surfaces. (Most A/C filters will not filter mold spores and bacteria and actually act as a breeding ground for mold and bacteria.) As the A/C system cycles on and off, the air conditioner gets damp, cold, and warm. This wet, dark environment is a perfect breeding ground for mold and bacteria.

Many forms of mold love this atmosphere including Listeria, a bacterium that loves ice bins and air conditioner systems. Listeria is known for its ability to cause large outbreaks of food poisoning in restaurants.

Mold and bacteria buildup on an A/C coil will give you the following indoor air and other A/C problems:

• mold odors;

• airborne mold;

• increased allergy risks;

• increased mold colonization of environment;

• increase in airborne bacteria and associated risk;

• decrease in air conditioner airflow;

• decrease in air conditioner efficiency;

• reduced equipment lifespan; and

• higher electric bills.

What to do?

Glad you asked.

UVC germicidal lamps are rapidly becoming very popular as an easy fix for the air conditioner coil mold problem. This is one of the most prevalent causes of the mold smell you get when you enter an air conditioned sick building. UVC (254nm) lamps are basically similar to sun lamps and are typically only effective on microbials that pass by within a few inches of the lamp or areas where the light is shining directly on for extended periods of time, such as the air conditioner coil.

Excerpted from the article “Shedding Light on Germicidial Ultraviolet” in the June 30, 2003, issue of The NEWS:

“‘The biggest questions from contractors are on placement,’ [Robin] Pharo [product manager of Aprilaire’s UV Products] said. For instance, should UVGs be installed in the return or supply?

A small air conditioner coil with one UV bulb mounted in the center. Only one half of this coil has UV protection as the outer sections have no UV bulbs.
“For airstream coverage, the study ‘Defining the Effectiveness of UV Lamps Installed in Circulating Air Ductwork,’ from the Air-Conditioning & Refrigeration Technology Institute (ARTI), recommends placement in the return side, with six lamps for optimum effectiveness, Pharo pointed out.

“Due to space and financial constraints, however, ‘Most homeowners won’t have that many lamps installed. So we recommend the concurrent installation of a really good filter, with the UV lamp placed over the indoor coil,’ Pharo said. ‘Air conditioning systems are great inventions, but the moist environment (at the coils) creates a microbial breeding ground.’

“Additionally, when UV lamps are shining directly on the coils, they are hitting a stationary target. When moving targets (VOCs and microbes) pass UV lights, the more sensitive microbes may be damaged, but the hardier ones will pass unharmed.”

UV lights, for instance, have been found to be better applied to shine on the indoor coil, not to try to clean the air stream, particularly in residential and light commercial applications; air stream use requires intense UV saturation.

UVC lights on an air conditioner coil are like the sun shining on a rock by a stream. No mold or mildew will grow on the sunny rock, unlike a shaded rock.

The main advantages to UVC lights are low cost, easy installation, and effectiveness on suppressing mold/bacteria growth on the coil that has the light shining on it.

UVC lights installed correctly can effectively control mold and bacteria growths on the air conditioner coil. They have little to no effect on airborne bacteria, viruses, odors, or VOCs unless very large commercial, heavy-duty UV systems are utilized.

The key to good UV/air conditioner coil mold control is the installation. A small air conditioner coil will require a minimum of three UV lights. The geometry of the light and the intensity will dictate the effectiveness. Remember, only the surface the light is shining on will be treated, and only up to 6 inches to 8 inches in distance from the bulb.


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