By Jeffrey C. May —
Many of you have no doubt seen UV lamps in HVAC systems. There is a lot of hype regarding these lamps, partly because installers and manufacturers often recommend such lamps as a way to improve air quality and to keep a system clean. I am going to play devil’s advocate regarding use of such lamps in residential mechanical systems.
Let’s start with 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.
UV-C Light as a Disinfectant
The idea of using UV-C light to disinfect surfaces and air is not new. Studies were done in the 1930’s in classrooms in which shielded UV-C lamps were installed at room ceilings (to avoid human exposure) and found to reduce the spread of German measles. More recent studies have also shown that (shielded) ceiling UV-C lamps can help disinfect the air in hospital rooms.
The lamps in HVAC systems are typically the germicidal UV-C type.
Are UV-C lamps useful in disinfecting the air moving through a residential HVAC system?
The answer to this question is “no,” because air moves through the HVAC system too quickly – as quickly as 10 feet per second – too fast for the lamps to have much of a germicidal effect. (In a Trane’s position paper on UV-C on the Trane website, they estimate that a single UV-C lamp in a duct would have to be about a half-mile long to be effective.) It therefore seems misleading that most germicidal lamps for residential use are sold on the premise that they will improve air quality by destroying microbes in the air.
The principal effective use of UV-C is surface and not air disinfection, but not all installations further this objective. One of the lamps I saw was installed with the housing perpendicular to the air flow, preventing both airflow across the lamp and surface irradiation.
UV lamp incorrectly installed perpendicular to airflow – May Indoor Air Investigations LLC
Other UV lamps were far too small, and some were producing noticeable amounts of ozone gas (which smells like “fresh outdoor air,” but which is a pollutant in its own right).
A photocatalytic (UV-C) oxidizing unit that produced ozone – May Indoor Air Investigations LLC
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 as noted is not the case.
Proper commercial UV-C installations contain several long lamps, installed so that the entire incoming side of the coil is irradiated in such a fashion as to supply adequate energy to destroy any microorganisms on the coil; the condensate pan should also be irradiated.
Can UV-C Light Cause Damage to System Components?
In one mechanical system that I inspected several years ago, the UV lamp eradiated the filter and had destroyed the glue holding the fiberglass fibers together. The UV lamp was installed in front of the cooling coil but just after the filter; thus the coil and filter were both exposed by the radiation. The company has since included a shield on the UV lamp that prevents the filter from being eradiated.
A MERV-16 filter destroyed by UV-C – May Indoor Air Investigations LLC
Human exposure to UV-C can cause burns, blindness and 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-C literally cooks organisms. Fortunately, the ozone layer removes nearly all of the UV-C radiation from sunlight.
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.
Read the full article in the digital edition of Healthy Indoors Magazine at: https://hi.healthyindoors.com/i/1243389-hi-april-2020/35
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