©2023, Jeffrey C. May
Installers and manufacturers often recommend UV lamps as a way to improve air quality and to keep an HVAC system clean, but I am going to play devil’s advocate regarding use of such lamps in residential mechanical systems.
Some of the Science Involved
UV is a part of the continuous electromagnetic energy spectrum that extends from radio waves and infrared (heat) to visible light, UV, X-rays, and gamma rays. UV waves are more energetic than visible light but not necessarily detected by the human eye.
UV light has been divided into three categories: UV-A, UV-B, and UV-C. Typical black lights produce UV-A, which is not particularly harmful. UV-B is more energetic; these rays (from the sun
and sun lamps) cause tanning and are associated with skin cancer. UV-C is more energetic still and is referred to as “germicidal,” because it can destroy the chemical bonds in molecules. The lamps in HVAC systems are typically the germicidal UV-C type.
UV-C Light as a Disinfectant
The location of the lamps can be wrong when the goal is to disinfect surfaces. Since nearly all microbial contamination on coils occurs on the “front” (where incoming air impacts and the dust accumulates and provides the nutrients), irradiating the front of the coil makes sense. Irradiating the back of the coil is nearly useless, because installation at this location is based on the theory that the air will be disinfected, which does not occur.
And why don’t UV lights disinfect the air in a residential HVAC system? The answer is that air can move through such a system as quickly as 10 feet per second – too fast for the lamps to have much of a germicidal effect.
And lastly, some UV lights I saw were producing noticeable amounts of ozone gas, which smells like “fresh outdoor air” but which is a pollutant in its own right – in fact, one of the major pollutants
in smog.
Be Careful
Human exposure to UV-C can cause burns, blindness, and even cancer. Prolonged exposure to UV-C destroys the bonds in many organic compounds, including the DNA that provides the genetic identity of most living things. UV lamps in HVAC systems should never be looked at directly, so it is very important when observing the interior of an HVAC system that any UV lamp be shut down. Even brief exposure can cause eye damage.
Filtration and Not Irradiation
I once inspected a very large home with a hydro-air system and seven air handlers. Each air handler contained a UV lamp. Three of the air handlers were in a spacious, unfinished attic area off the second floor. The odor of ozone in that area was very strong. I asked the owners to turn off the UV lamps so I could open up each unit without worrying about exposure to the lamps.
Despite the owners’ substantial investments in the lamps, the filters in the air handlers were completely inadequate. The return plenums contained exposed fibrous lining material that was full of moldy dust, and there was visible mold in all of the blower cabinets as well as on the blower blades. So much for the benefits of UV lamps!
What’s the point of filtration?
I once spoke at a conference for HVAC technicians. I asked members of the audience to raise their hands if they thought that the purpose of filters was to “cleanse” the air. Almost all the attendees raised their hands, much to my surprise and, I must add, dismay.
As I’ve told my clients numerous times and mentioned in many articles, the most important function of a filter is to prevent the accumulation of biodegradable dust on coils and not to “clean the air” (although air cleaning can certainly occur in the rare case of a clean distribution system). If filtration is inadequate, microbial growth can occur within the system. By-products of this growth are spread throughout the building on airflows and present a health hazard to building occupants.
Electronic filters aren’t effective because most people don’t follow manufacturer’s maintenance directions. I always recommend pleated media filters with a MERV rating of at least an 8; a rating of 11 is preferable for families with allergies or asthma. If a return plenum must be adjusted for a filter of this size, so be it. The goal of improved indoor air quality is worth the effort and expense.
Like the familiar real-estate mantra “location, location, location,” the HVAC mantra should be “filtration, filtration, filtration!” This is what keeps the coil clean and thus prevents microbial contamination.
