One popular solution involves the use of ultraviolet C-band energy, commonly called UVC lights. When they shine on surfaces, these lights destroy mold, viruses, bacteria and other containments that collect there. They vaporize the organisms, effectively removing them from the air.
The same type of lighting used in HVAC systems to improve indoor air quality and control infectious diseases is sometimes used as an alternative to conventional duct-cleaning methods. UVC lights offer contractors an effective, nonpolluting and permanent way to clean ductwork of microbial contamination.
For best results, it is important to select a UVC device engineered to deliver high output in HVAC systems. It should provide output of at least 10 microwatts per square meter at 1 meter, in a 400 feet-per-minute 45°F airstream.
In most cases, direct cleansing of the ductwork by UVC light is not needed. A more effective approach is to go to the source of the problem: the air-conditioning coils. That's what officials at Orlando, Fla.-based Florida Hospital discovered.
Hospital discoveries"Since installing UVC in the air handlers, we have observed that the devices also help to eliminate mold buildup on duct surfaces, even though there are no lights installed in the ducts themselves," said Firouz Keikavousi, a mechanical engineer in charge of facilities maintenance at the hospital.
"This phenomenon occurs because the coil - not the ductwork - is the source of mold growth in the system," he added. "Fresh inoculation coming off the coil typically migrates downstream and some of it settles in the ductwork - a process that is self-perpetuating.
"But when UVC energy is used to destroy mold and microbial growth at the coil, the food source is eliminated and the chain is broken. As a result, the contamination that has already built up on duct surfaces will eventually decay away, a process that may take months".
However, there were times when UVC alone would not clean the ducts completely, Keikavousi said.
"Where duct surfaces are badly contaminated, it isn't always possible to wait for the effects of UVC. Therefore, in those areas where buildup was particularly heavy, we performed duct cleaning as a precaution prior to installing UVC lights," he said. "We have not experienced any recurrence of duct contamination since adopting UVC, nor do we anticipate any future duct-cleaning requirements."
Keikavousi expects the use of the lights will save the hospital thousands of dollars in future cleaning costs.
Installation tipsExperts say UVC lights should typically be installed downstream of and facing the coil, or on the return-air side, if installing it downstream is too difficult. This method allows coil surfaces to be continuously bathed in germicidal light, killing the microorganisms that grow and multiply there, not only on the visible part of the fins, but also on the surfaces within the coil, where the greatest amount of mold activity occurs.
The coil's total surface area may be three to five times that of the ductwork, but contamination on the coil tends to be less visible. When you inspect the inside of a duct and it looks moldy, many people become alarmed, not realizing that the coil is the real culprit. There is much evidence that infestation in ductwork, while it may look scary, is not a major contributor to IAQ problems. The movement of air along interior duct walls is actually very slight, so the amount of contamination coming off the ducts and into the occupied space is not usually significant.
Where duct surfaces are heavily infested, a one-time cleaning may be deemed necessary, as Florida Hospital found. Another nonchemical approach is to install supplemental UVC lights in the ductwork. Remember: If you irradiate the ducts without irradiating the coil, you are treating the symptoms and not the disease.
A typical installation requires making a 1-inch penetration into the duct wall and running the light across the width of the duct. The UVC energy rapidly "cooks off" mold and other microbes, leaving ductwork free of organic buildup, with no other surface cleaning necessary.
Selection criteriaWhen selecting UVC lights for coil - and duct-cleaning applications, make sure the UVC-energy output is at the recommended level. For duct-cleaning applications, select a design where only the lamp or "tube" will penetrate into the ductwork and the power supply will be external. This will ensure minimum intrusion into the ductwork. The tube should ideally be long enough to extend across the width of the duct.
UVC devices provide line-of-sight irradiation only, with a typical effective range of about 10 feet in a ducted system. This range is related to lamp size and output, and may vary from manufacturer to manufacturer. Be sure to compare the total installed cost, which is more important than the number of lights needed.
When you install UVC to keep coils and ductwork clean, it is essentially a permanent fix: There is little to do except change the tubes about once a year, or when a radiometer indicates that the output has dropped below specified levels.
Buildings that use UVC devices report going for years without cleaning coils, ducts, drain pans or plenums. As a result, service crews and building occupants are no longer exposed to the potentially harmful chemicals associated with cleaning.
(This article was supplied by Steril-Aire Inc., a Burbank, Calif.-based manufacturer of IAQ products.)