Retrofitting HVAC Systems with UVC to Inactivate Viruses and Other Infectious Pathogens

While the AJIC article attracted the health-care industry, general consumers were awakened to UVC’s benefits after the April 23, 2020, nationally televised U.S. White House Coronavirus Task Force press briefing concerning outdoor UV’s effects on COVID-19. Acting Under Secretary William Bryan of the U.S. Dept. of Homeland Security’s research and development arm, the Science and Technology Directorate (STD), revealed two months of research results on COVID-19 and sunlight UV. STD’s studies concluded that COVID-19 is inactivated within a minute once it’s exposed directly to the sun’s UV rays outdoors.

The press conference also inspired general consumers to check online for the hundreds of peer-reviewed studies and white papers proving UV’s general effectiveness on bacteria and viruses. The sun’s UV rays that reach the Earth’s surface consist of 95 percent UV-A (315 to 400 nanometers) and 5 percent UV-B (280 to 315-nm) and can neutralize SARS-CoV-2 with enough exposure time, according to the STD. In retrospect, UVC (200 to 280 nm) is the shortest and most lethal of the three wavelengths. Many UVC lamp systems operate at 254 nanometers, which sufficiently scrambles DNA and RNA so biological contaminants can’t reproduce or infect. Microbes can’t adapt to UVC because it’s filtered out by the upper atmosphere and doesn’t reach the Earth’s surface, thus they have no previous exposure or mutation to it.

Microbes are disinfected within the airstream as they pass through the HVAC system’s UV light field.

“Things got crazy after that press briefing,” recalls Brian Stacy, president of Trinity UV, which recorded sales increases of more than 300 percent as awareness in UV disinfection technologies grew. The Trinity, Fla.-based contractor specializes in commercial UV installation and retrofits; however the press conference significantly increased Trinity’s residential demand. “People were seeking me out from Internet searches on UV. There were a lot of phone calls with questions. People that were on the fence finally jumped into UV after that. Not a lot of people know about UV, but something like a televised press conference in the middle of a pandemic gets consumers’ attention,” he says.

Specifying UV for Commercial Buildings

Airborne disinfection is typically facilitated by the same UVC light systems that prevent biological contaminant buildup in HVAC units. Microbes are disinfected within the airstream as they pass through the HVAC system’s UV light field. There are UV light system kits that can be onsite custom sized-to-fit any large building air handlers or small enough to fit smaller HVAC systems, such as chilled water room unit ventilator coils or mini-split ductless air conditioning evaporator coils.

Chapter 62 of ASHRAE’s Applications Handbook recommends an irradiance level of 50-100 µW/cm2 which works out to approximately 7.5 lamp watts/sq.ft. of coil surface, but Fresh-Aire UV engineers often recommend sizing applications that deliver a minimum of 400 µW/cm2.

Typically, UVC lamps are positioned inside HVAC units near the evaporator coil. There are also systems designed for inside return or supply ductwork. UVC lamps range up to 60-inches long, and are available with two to three-year replacement choices and are typically spaced from 8- to 40-inches apart depending on level of air and surface disinfection requirements. The lamps are affixed to a simple framework mounted onsite consisting of hardware mounts and 1/2-inch EMT tubing. While it’s important for disinfecting biological contaminants that pass through standard media filters, UVC lamps also maintain the coil and the surrounding HVAC unit interiors free of mold and microbial contamination.

While UV lamp selections might have been rough estimates 20 years ago, today’s advanced software programs help specifiers use exact science for selection, sizing and analysis. Consequently, selection now is more exact and helps specifiers achieve the UVGI industry’s dosage criteria without ineffective under sizing or costly oversizing. Calculation programs also help with other critical factors, such as:

While UV lamp selections might have been rough estimates 20 years ago, today’s advanced software programs help specifiers use exact science for selection, sizing and analysis.
  • Reflectivity—Different materials used for air handlers, plenums and ducts have unique reflectivity that must also be considered. For example, stainless steel is less reflective than galvanized steel and aluminum has the highest reflectivity. Ductwork with internal insulation have little, if any, reflectivity.
  • Target Pathogen–Different pathogens, such as viruses, fungal spores, bacteria, etc., all require unique dosages based on their susceptibility to UVC.
  • Dwell Time and Airflow–How long a pathogen is exposed to HVAC system UVC lamps depends greatly on airflow speed and will dictate the required lamp intensity.
  • Lamp Position–Upstream or downstream placement can affect lamp output.
  • Air Temperature/Relative Humidity–UVC lamps perform better in warmer air, but cooler air discharges might require more lamp output. Pathogen longevity is also dependent on relative humidity. Viruses, for example, survive better in high or low humidity, but not as well in 40 to 60 percent RH, which is a preferred comfort range for most occupied spaces.

While all these variables can be challenging, especially for first-time UV-C specifiers, there are calculation programs, such as BlueCalc, that can help determine a project’s UV output data, such as selected lamp model and parameters, number of lamps, lamp locations, UV power, electrical power requirements, each lamp’s peak irradiance, in-duct irradiance (µW/cm2), microbe inactivation calculations and other considerations mentioned above.

Fan coils, such as those found in hotel, hospital or classrooms, can also be outfitted with UVC lamps for disinfection. Likewise, specialty HVAC systems, such as mini-splits and VRF evaporator coils, can also receive air disinfection, as well as internal parts and surfaces with UV-LED lights, which are small enough to fit the tight confines of these limited space units.

PHOTOS: Fresh-Aire UV

About the Author

Aaron Engel
Aaron Engel is vice president of business development at Fresh-Aire UV, a North American manufacturer of UV disinfection and carbon/PCO-based indoor air quality products. Fresh-Aire UV offers UVC light systems for virtually every type of HVAC system, ice machine and surface disinfection. Engel can be reached at [email protected] or (800) 741-1195.

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