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Here’s the problem. Cold gas is pumped through an air conditioner 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 air conditioner 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. |
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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 air conditioner filters will not filter mold spores and bacteria and actually act as a breeding ground for mold and bacteria.) As the air conditioner 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. |
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“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.” |
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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. |
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UV has the ability to kill surface bacteria that the bulb is shining directly on at a distance of usually less than 6 to 8 inches. Some UVC light companies state their UV systems can kill 99.9 percent of MRSA bacteria on a single pass and then reference an EPA study. This is very misleading as the test was conducted on a test unit of five UV high-energy lamps, each 50 inches long in a reflective tunnel, which burns 1,100 watts of electricity. This is the equivalent of running a hair dryer in your air conditioner 24 hours a day. This obviously generates a lot of heat in addition to burning a lot of electricity, and the units cost thousands of dollars. |
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Tables of CT values have been published, and are well-known. To properly determine what values to use, look in one of those tables and pick a microbe, then read the corresponding CT value, and design the system around that CT value. (Note of caution: Unprotected UVC lamps were used for this article. In practice, protected lamps should always be used as UV lamps contain mercury, a hazardous heavy metal known to cause health problems. A broken lamp in an HVAC system could permanently contaminate the system.) Typical HVAC home systems operate around 2,000 cfm for a 2,000-square-foot home. For example, 2,000 cfm will have an exit velocity of 500 feet per minute, or 8.33 feet per second. From this value, the proper residence time can be evaluated for specific CT values. |
