Your heating and cooling system is an important ally in the battle for healthy indoor air quality. Mechanical filtration with high-efficiency, whole-house filter arrays installed in the ductwork can remove most airborne particulates. While this method effectively captures inorganic particles like dust, living microorganisms trapped by the filter may remain active. In fact, the filter media often serves as a breeding ground for mold, bacteria and viruses that continue to disperse spores into the airflow, tainting the air throughout the home with allergens and sources of disease. Ultraviolet, or UV, light is commonly used to disinfect surfaces in operating rooms as well as to purify water. When utilized in combination with high-efficiency mechanical filtration or used to treat specific trouble spots in the system, UV lamps installed in HVAC systems may provide enhanced air purification.
The Power Of Ultraviolet Light
Dating back to its use against the tuberculosis bacterium in the early 20th century, UV light has been known for its powerful germicidal properties. UV light in the C wavelength band inactivates microorganisms by penetrating their DNA and destroying their ability to replicate. C-band UV light is produced by special fluorescent lamps and has been shown to kill airborne viruses, bacteria, mycoplasma and fungi such as mold spores. HVAC installations of UV lamps generally fall into three categories: standalone in-duct lamps that expose the airflow to UV light as it moves through the duct, lamps incorporated into a whole-house filter array installed in the duct that also mechanically filters the airflow and dedicated lamps to treat specific surfaces in the HVAC system such as air-conditioning coils.
Standalone In-Duct Lamps
Standalone installations are accomplished by cutting small circular access holes in the ductwork to accommodate the lamp tubes. The power supply is mounted outside the duct. An AC power source must be provided to energize the lamps. A typical installation consists of one or two high-intensity UV lamps that protrude into the return duct, oriented perpendicular to the airflow. Many lamp tubes incorporate parabolic reflectors that disperse the light to ensure that the entire volume of duct airflow is exposed to UV treatment. Effectiveness of the array depends on the number of lamps, the size of the duct, the air velocity in the duct and the nature and amount of contaminants in the airflow.
Whole-House UV Arrays
Whole-house filter units that include UV lamps utilize a high-efficiency mechanical filtration stage consisting of pleated cotton or polyester filter media. These units typically offer a minimum efficiency reporting value filter rating of up to 16, providing mechanical removal of airborne particulates down to a size of 0.3 microns. A UV lamp array inside the unit exposes the filter media and airflow to continuous germicidal light treatment to destroy organic pathogens. Some whole-house units also integrate a titanium oxide grid that causes a catalytic reaction when exposed to UV light. The catalytic process breaks down chemical fumes in the airflow such as volatile organic compounds like formaldehyde that may be emitted by building materials.
Air Conditioner Coil Arrays
UV lamps that treat stationary surfaces in the HVAC system are usually aimed at central air-conditioner evaporator coils. The moist environment inside these coils caused by condensation is a perfect breeding ground for mold. Spores sucked into the ductwork infiltrate the coils and form a film of mold growth inside that dramatically decreases the efficiency of heat extraction by the coil, as well as serving as a launching pad for millions of additional spores. These recirculate throughout the ductwork, infecting other areas and contaminating the air in living spaces. In a typical installation, UV lamps are mounted on both sides and in the center of the standard A-shaped air conditioning coil, carefully positioned to ensure that UV light penetrates the small air passages in the coil.