Notice the unexpected benefit that emerges from these reports:

When the Iolani School in Honolulu specified ultraviolet light in the "C" band (UVC light) to protect students against potential allergy and asthma symptoms that might be linked to mold in the air handlers, they were surprised to discover that the coils looked "factory-clean" just a few weeks later. By providing continuous cleaning of coil and drain pan areas, the UVC devices enabled school administrators to eliminate an $8,000-a-year coil cleaning program.

When Southern California Air Conditioning Distributors (SCACD), the world's largest independent Carrier distributor, installed UVC lights in an administrative facility over three decades old to control mold and nonspecific odors, they found that the lights also rejuvenated coils and allowed greater heat transfer, resulting in an impressive 30% increase in cooling capacity. This translated into annual cost savings of more than 20%.

And when Central & South West Corporation (CSW), a Dallas-based electric utility, installed ultraviolet "UVC" lights in the air handlers of their headquarters building to control indoor air quality, they received an unexpected dividend. Not only did the lights perform the desired IAQ function, they also made it possible for CSW to keep the 500,000-sq-ft building cool using three chillers instead of the customary four (a 300-plus kW power reduction) - even during two summers of record-breaking heat.

UVC energy - a relatively new tool for IAQ control in hvac systems - is proving to offer advantages beyond the enhancement of air quality. Common to the above cases and thousands of others, a properly designed and installed UVC system can deliver even more in energy, maintenance, and operational benefits. To that end, UVC may be considered as a new type of cost-saving maintenance tool: one that can bring back and/or maintain air-conditioning cooling coils to "like new" condition - not just a product that makes biologically sick buildings well.

A UVC installation at Central & South West Corp. (Dallas) is credited with helping this utility achieve a significant decrease in energy usage. CSW was able to cool a 500,000-sq-ft building using three chillers instead of the customary four, even during two summers of recordbreaking heat.

Track Record

The UVC wavelength is just below the more familiar UVA & UVB in a light spectrographic scale. It has proven its safety and effectiveness in countless applications, killing harmful microorganisms of all types for more than 70 years. Worldwide, it is best known for disinfecting drinking water, where it is was first introduced in France in 1908 and is now used by most other countries except the United States. These more common or conventional UVC tubes, however, suffer drastic output losses and thus "killing power" when exposed to cold and/or moving air. As a result, the hvac industry has not used them for microbial control, and especially not as a tool for hvac upkeep, coil cleaning, or potential energy savings.

Ever since the World Health Organization, through its research contributors, concluded that the majority of poor IAQ issues were bioaerosol-based, and that cooling coils and drain pans were significant sinks and amplifiers, they have earnestly sought solutions. But some techniques used to clean hvac components involve controversial surfactants; solvents; and/or acids that are toxic, flammable, and detrimental to coil life (erosion). Coil cleaning can also be a significant expense in a preventive maintenance program.

The top row shows mold and bacteria samples from an a/c system at CSW. The bottom row of samples were taken after 48 hrs of exposure to UVC energy.

High-Output UVC: A New Way To Keep Coils Clean

In the 1990s, the industry response to these bioaerosol problems was a new generation of UVC devices - products that broke the conventional UVC tube temperature barrier in that, for the first time, they provided peak output under hvac conditions. This breakthrough allowed one of nature's best tools to be used indoors.

Output of these newer devices is so much greater than conventional tubes in cold and moving air that they can be installed just downstream of a cooling coil, which also encompasses the drain pan and other damp locations of an air-handling system. Here, the high output UVC devices not only "zap" both coil and drain pan mold and bacteria (to eliminate their toxins, VOC and spore production, and "allergens"), they also remove ordinary coil and drain pan debris for significant maintenance and energy savings.

High-output UVC devices, which provide this continuous form of source control, are thus emerging as a practical and effective solution. What the sun does outdoors, UVC does indoors: providing the same regenerative and restorative effects to hvac surfaces and recirculated air to continuously improve air quality and system performance. It is especially effective, for the first time, in eradicating mold and other common allergens right at the source.

But by cleaning and keeping coils and drain pans clean, UVC's best effect may be the reduction or elimination of coil-, drain pan-, and plenum-cleaning programs for significant cost savings. This decreases a maintenance staff's exposure to biocidal agents or cleaning compounds. Moreover, coils and drain pans no longer seed the rest of the system, ductwork, or occupied spaces with mold or bacteria. Everything stays cleaner, safer, and free of the musty odor sometimes characterized as "dirty sock syndrome."

On new systems, UVC lights are placed either up or downstream of coils to bathe the source in UVC energy. On existing systems, best results are obtained when UVC devices are located downstream of, and facing, the coils. In this location, the user can expect the highest degree of efficacy in eradicating surface (coil and plenum), drain pan, and/or airborne microbes.

In most cases, it is not necessary to clean coils prior to installing the devices. Users report that UVC will degrade all of the organic material on and within a coil in a typical time frame of 30 days or less. The depth of the coil, fins per inch, water activity, and the type and amount of contamination are all determining factors. Now let's look at a couple of additional advantages UVC devices can provide.

General IAQ Control

UVC devices are widely used to control mold and mildew in hvac systems. They also reduce the spread of cold and flu viruses and other airborne-transmitted diseases, and they fight unpleasant odors. They also provide a recognized control strategy for reducing the development of Legionella or the spread of tuberculosis in health care facilities, shelters, prisons, etc.

The reduction in absenteeism also promises to be significant. Researchers at the Lawrence Berkeley National Laboratory (Berkeley, CA) have estimated that U.S. companies could save up to $58 billion a year by preventing so-called "Sick Building Syndrome" and an additional $200 billion by improving worker productivity through better indoor air. They also found that the financial benefits are many times greater than the costs of making the necessary improvements to achieve them.

Payback & Energy Savings

Though these new high-output UVC devices were originally developed for IAQ control, they reward users with significant capacity and energy-saving benefits. Properly sized and positioned as mentioned above, they successfully break down the biofilm and dirt that foul heat transfer surfaces - effectively returning components and thus heat transfer to optimum conditions. This phenomenon provides significant improvements in coil pressure drop and heat transfer efficiency over existing conditions, for predictable energy reductions.

Those who utilize variable frequency drives (vfd's) will rapidly see the operational benefits, and those who have been considering vfd's may now have more incentive to use them. Users of constant-speed fan systems would need to watch their fan rpm and damper settings, as many will be able to revert back to the original "as built" specs to save energy. The end benefits are capital reductions in power consumption and maintenance and capital improvements in human resources and productivity. It's not uncommon to see energy savings of 20% typically, and often 30% overall.

When combined energy and maintenance savings are taken into account, a typical UVC light system can pay for itself in as little as three to six months. Table 1 shows an example of typical first-year cost savings that were achieved from a UVC light installation in an actual small, commercial a/c system.

Based on readings taken just before an installation and one month after, the table shows a reduction in pressure drop across the cooling coil of almost 30% and a corresponding airflow increase of from 7,000 to 8,000 cfm. The leaving air wetbulb temperature also dropped, providing a greater differential between the air entering and leaving readings (enthalpy). This combination of factors resulted in a significant increase in net cooling capacity.

Based on around-the-clock operation and an energy cost of $0.08 per kWh, energy savings for the first year were $4,443. By eliminating an $800 coil-cleaning program, total first-year savings are $5,243. After deducting installation expenses and energy required to operate the lights, net first-year savings were an impressive $3,259. Future savings will be even greater because the one-time installation cost is eliminated; only the tubes will need to be replaced. Though not shown here, one can also factor in additional cost savings from reduced absenteeism and higher productivity.

Payback in the Table 1 example was about four months, but system improvements were evident long before that. In fact, many users report noticeable enhancements in air quality and/or operating performance in anywhere from 48 hours to two weeks.

Selecting The Right Device

As a result of the documented success of the new devices, a variety of products are arriving on the market. To be safe and sure you receive the same performance as reported above, here are some key questions to review with a supplier:

Has the manufacturer commissioned independent testing to demonstrate output under hvac conditions or to prove output claims in general? A recognized laboratory should have tested the device in a moving airstream of at least 400 fpm and 45°F in accordance with the general provisions of the Illuminating Engineering Society of North America's 1981 Applications Volume.

What level of output or intensity does the proposed device deliver? As previously noted, it takes a very high-intensity UVC device to work efficiently in a/c applications. To avoid having to use too many fixtures, output per inch of arc length should be no less than 10µW/cm2 at one meter when tested under the above conditions.

What sizes and configurations are available? Given the spectrum of air conditioning systems in today's buildings, UVC selection is not "one-size-fits-all." Ask if the manufacturer offers different fixtures, tube lengths, and mounting configurations to suit varying application needs.

Will the supplier offer help in sizing the system correctly? If a UVC system is underdesigned and not equipped with independently tested fixtures, the devices won't perform as expected; if overdesigned, unnecessary expenses will occur. As a general rule of thumb, when using high-output UVC devices for IAQ, improved heat transfer, and reduced maintenance and odors, about 24 in. of UVC tube length will be required for every four sq ft of coil face area. For infectious disease applications, where more intense "kill" rates are required, the ratio will be higher. The supplier should always calculate the number of fixtures required in these critical applications.

What is the recommended location for installation of the UVC device? As noted previously, evaporator (cooling) coils and drain pan areas are the correct locations for UVC installation, as these are the areas of highest contamination. Some devices are designed, instead, for use in the return air ducts. A drawback of such installations is that they allow contaminated air to pass through the occupied space before UVC treatment occurs.

What are the materials of construction? Desirable features include stainless steel housings with high spectral aluminum reflectors, high power factor, and conversion efficiency electronic power supplies and thick-wall quartz tubes. What are the anticipated changeout cycles? Properly designed "high-output" tubes last 9,000 hrs, or one year (about four times longer than some conventional UVC tubes). Older-style magnetic power supplies have trouble maintaining output or even staying lit in cold or moving air, and the blackened tubes show it in less than 3,000 hrs. Cold-cathode tubes were abandoned some two decades ago due to lack of output by today's standards. In short, the entire lighting world is going to rapid-start electronic power supplies and hot-cathode tubes for these and other reasons.

What certifications does the device carry? Most importantly, is the entire device UL-approved and listed for damp locations? Do the materials of construction meet the environmental conditions, application intent, and/or installation methodology? Has it earned any industry awards or other accolades? Does the device have a proven track record? Look for a customer list that is both sizable and recognizable. Ask for references and documentation on results achieved in actual installations. There are many ways that results can be meaningfully documented (i.e., through Petri dish sampling of coil and drain pan surfaces, through "before" and "after" energy and operational cost comparisons, and through anecdotal reports from building occupants or operators). A reliable device will also be referenced in educational materials and peer-reviewed scientific works. Documented experience from government agencies or electric utilities may also be helpful.

Does the manufacturer offer technical and engineering support? Seek out a company experienced both in hvac technology and the microbiological aspects of UVC. This is especially important with health care, food, and other infestation applications of a critical nature.


Though UVC is an old, well-established technology worldwide, its successful use in hvac systems is relatively recent. Still, in just a short time, new, high-output "UVC for hvac" devices have demonstrated an impressive ability to keep coils, drain pans, and other areas clean for improved IAQ, reduced energy, and simplified hvac system maintenance. As a result, some industry experts predict that use of UVC in hvacr applications will shortly be as common as air filters. Until then, hvacr system designers and operators can enjoy the many benefits of UVC technology now by using the guidelines in this article to help make informed decisions. ES