Formicary Corrosion & Biofilm Fouling of Cooling Coils

Formicary Corrosion and Biofilm Fouling of Cooling Coils

By Normand Brais, P.Eng., M.A.Sc., Ph.D.


Air conditioning is responsible for substantial electricity consumption and peak demand in most of the United States. Over the past decade, energy conservation researchers have studied air conditioning more and more. Much of this research has focused on the impact of air flow, duct leakage, and refrigerant charge level on cooling performance.

One area, which has been neglected by researchers, is fouling of evaporator and formicary corrosion. Known commonly as ant’s nest corrosion, champagne leaks, or just simply as formicary, the issue presents as a hard to detect leak within the fin pack of an evaporator coil. This tiny leak or set of leaks results in loss of system refrigerant over time. While the incidence of formicary corrosion is low nationwide, it is more prevalent in warm, humid climates found in southern portions of the United States where slimy biofilms coat the coil fins.


The tunneling action that leads to corrosion is caused by the presence of organic acids mixed with moisture on the copper tube within the fin pack. Two common acids known to cause formicary corrosion are formic and acetic acids. Certain manufacturing oils and lubricants can contain compounds that form these organic acids, but common household items can also breakdown to form formic and acetic acids. These can include building materials like formaldehyde adhesives, foam insulation, and laminates, as well as personal hygiene products like cosmetics, disinfectants, deodorizers, and cleaning solvents. To initiate corrosion, the presence of water is necessary. The corrosion rate is aggravated by the presence of mold biofilm that keeps the fins wet.


Every technician will tell you that every time they look at these coils, they are dirty with mold biofilm. A standard part of routine A/C maintenance and residential commissioning is to clean the evaporator coil with a wire brush and detergent or other cleaning chemistry, and to clean the outdoor coil of leaves and other debris. Over the last 20 years, the simple use of appropriate germicidal UV light can prevent this and keep the coil clean and free of any biofilm buildup.

The presence of mold biofilm on cooling coil acts like a water sponge that keeps the fins constantly wet and sticky, which capture and retain particulates and chemical contaminants, thus enhancing the formic acid corrosion rate. When the biofilm is eliminated with an efficient germicidal UV system, the coil is not permanently wet, does not trap and retains as much contaminants, and consequently its corrosion rate is greatly reduced.

                                                                                     Buildup of mold biofilm in lower right hand corner completely blocks air flow

Coils normally foul due to bioaerosols and other biologically active materials, which are ubiquitous in hot climates. This usually also lead to significant indoor air quality problems that can trigger respiratory allergic responses of building occupants.

                                                                        Fungal biofilm growth on a residential (Left) and commercial (Right) cooling coil

The biofilm that causes evaporator fouling and its accelerated corrosion also impacts on the cooling performance and energy efficiency. Large commercial heating and cooling coils have the same problems and are prone to the same type of fouling and consequential corrosion issues. Their lifespan can also be increased notably by using an appropriate UV germicidal system that prevents mold buildup in hot and humid climate.

The first step to take to reduce the formicary corrosion due is to prevent the formation of a biofilm of molds and fungi with an adequate germicidal UV system. Biofilm-free fins will be less susceptible to retain dusts and potentially corrosive compounds, as well as the moisture that activates the corrosion effect.


Siegel, Jeffrey, I. Walker & M. Sherman. 2002. “Dirty Air Conditioners: Energy Implications of Coil Fouling” Submitted to the 2002 ACEEE Summer Study on Energy Efficiency in Buildings.

Siegel, Jeffrey A. and W.W. Nazaroff. 2002. “Modeling Particle Deposition on HVAC Heat Exchangers.” Submitted to the Indoor Air 2002 conference.

Siegel, Jeffrey and I.S. Walker. 2001. “Deposition of Biological Aerosols on HVAC Heat Exchangers.”

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Disinfecting Air & Reducing Ethylene in Cold Rooms

Disinfecting Air and Reducing Ethylene in Cold Rooms

Mold and bacteria can severely impact the quality of meat, chicken, fish, fruits and vegetables that may be stored or prepared in warehouses and cold rooms. Ethylene off-gassing causes fruits and vegetables to prematurely ripen and aged, dramatically shortening shelf-life.

Sanuvox UV IL-CoilCean systems installed facing the cooling coil are designed to bask the coil and air with ultraviolet energy destroying microorganisms including bacteria, mold and viruses while oxidizing and reducing ethylene off-gassing.

With its high efficiency patented air disinfection systems, Sanuvox offers the right solution when the objective is to destroy airborne bio-chemical contaminants (e.g. bacteria, viruses, mold) that may affect the storage and preparation of fish, chicken and meat, as well as destroy ethylene off-gassing that causes produce to ripen faster.


Multi-IL CoilClean units are installed facing the cooling coils in the fan coil unit. Each IL unit includes a UV-C/UV-V lamp mounted in an anodized aluminum parabolic reflector. The ballast box incorporates LED status lights for providing lamp status and replacement and can be remotely monitored.


The fan coil unit recirculates the air where:
1. The UV-C germicidal section of the UV lamp destroys airborne biological contaminants (viruses, mold, bacteria and spores).
2. The UV-V oxidizing section of the UV lamp reduces ethylene, slowing down the ripening process of vegetables and fruits. Coils remain clean and more energy efficient.

Produce will degrade due to the rotting process that is caused by parasitic fungi and mold. Food deterioration begins with the breakdown of the cellular tissue by enzymatic action that allows the growth of microbes. Germicidal UV (UV-C) is extremely effective at preventing the reproduction of bio-contaminants because UV-C destroys airborne fungi, molds and spores, limiting the contamination spread from one fruit to another. Meat, fish and chicken are especially vulnerable to airborne biocontamination. UV-C sterilizes the air, destroying contaminants as they circulate within the cold room.

Photo-oxidation with UV-V can be used to reduce chemicals that trigger the ripening of fruits and vegetables. The life stages of a plant are influenced by plant hormones. An organic compound involved with ripening is ethylene, a gas created by plants from the amino acid, methionine. Ethylene increases the intracellular levels of certain enzymes in fruit and fresh-cut products, which include:

  • Amylase, which hydrolyzes starch to produce simple sugars.
  • Pectinase, which hydrolyzes pectin, a substance that keeps fruit hard.

UV-V oxidizes and thus neutralizes the ethylene molecules released by the ripening process, slowing down the spread of ripening to the surrounding produce. This oxidation process breaks down ethylene into carbon dioxide and water vapor.
Ethylene C2H4 C2H4+ O* -> CO2 +H2O


Many buildings and facilities can be equipped with the IL-CoilCean systems, like cold storage rooms, groceries, meat, fish and chicken storage, preparation facilities, fruit and vegetable retailers, warehousing and transportation.

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Coil Cleaning in Museums & Archives

Coil Cleaning in Museums and Archives

Air quality can severely impact and deteriorate irreplaceable paintings, documents, drawings, books and journals within a vault, storage area, library and exposition hall. Preserving these fine treasures from the ravages of mold, spores and bacteria are a priority for libraries, archives, museums and collectors. UV systems are designed to destroy airborne mold spores and their associated odors, as well as bacteria that can very well destroy treasures from the past.

Sanuvox UV IL-CoilClean systems are designed to destroy mold and other bio-contaminants on the evaporator coil, which results in spores as well as off-gassing being « blown-off » the evaporator coil and distributed through the facility.

Sanuvox in-duct UV BioWall systems effectively destroy thousands of airborne bio-contaminants, such as mold, bacteria, viruses, chemicals, VOCs and odors.


The Sanuvox IL-Coil Clean system for HVAC coils utilizes a patented technology to focus the maximum UV energy on any surface. The patented anodized aluminum parabolic reflector serves two purposes:
1. Redirects the maximum amount of UV energy produced by the lamp onto the coil surface, requiring less or shorter lamps and fixtures.
2. Protects the UV lamp from fouling.


Prolonged exposure to UV radiation will keep the air conditioning coil clean and free of bio-contaminants, including viruses, fungi, bacteria and bio-film that may grow on the coil. Maintaining a coil free of microbial growth will maximize the efficiency of coil heat transfer and reduce the hours of operation of the compressors, resulting in lower energy costs.


The UV-C wavelength is well documented for its germicidal properties. The effects of ultraviolet radiation on biological contaminants have also been included in the latest ASHRAE Handbooks. Generally, this relationship is similar to the absorption curve of nucleic acid (DNA) the basis of all living organisms. The germicidal destruction rate for any specified bio-contaminant can be greater than 99.9% as the maximum UV intensity produced by the UV lamp is directed onto the coil and each application is sized according to its requirements.


Many buildings and facilities can be installed with either the IL-CoilClean or the BioWall, like libraroes, museums, archives, record rooms, evidence rooms, private collections, or galleries.

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The Myth of HEPA Filtration

The Myth of HEPA Filtration

By Vigilair,

High Efficiency Particulate Air filters (HEPA) are commonly used to achieve a significant dust and particulate control. Hospitals and companies operating clean room fabrication labs use these types of filters to reduce the particulate contamination to acceptable levels. Properly installed and maintained HEPA filters undoubtedly reduce airborne contaminants. But this fact has spawned the myth that environments serviced by HEPA filters are free from contamination.

Like all filters, HEPA filters have an efficiency curve with a minimum in the range of 0.1- 0.3 μm. The particle removal efficiency for 0.3 μm particulate is 99.99%. However, reports1,2,3 show that as many as 50% of installed HEPA filters operate well below their theoretical efficiency due to:

• incorrect installation resulting in air bypassing the filter bank

• damage during the installation or service of the air handler, especially in settings where maintenance staff lack the specific training needed to maintain HEPA filters.

• trapped viable organic matter (e.g. fungi, bacteria, mold) that has grown through the filters In many cases a combination of the above factors compromises the efficiency level of HEPA filters.

Another way to look at HEPA filter protection is to determine the particulate allowed to pass through the filter because of its inherent 0.01% inefficiency. Assume that conservatively the HEPA will be challenged with 10,000 particles in the size range of 0.1-0.3 micron per cubic foot of air every minute (cfm) and that this HEPA filter is rated at 1,000 cfm. This HEPA will allow 10,000 particles to pass through every minute or 14,400,000 every 24 hours of operation.

While it is possible to reduce or eliminate damage during filter installation with the implementation of training and good operational procedures, it is more difficult to deal with the problems presented by viable organic contamination. With the exception of high end cleanroom fabrication labs, many facilities using HEPA filters are not staffed with technicians who have the knowledge necessary to maintain the environment that HEPA filtration is designed to provide.

Air handlers equipped with HEPA filtration usually have both pre-filters and secondary filters upstream to protect the HEPA filters. This minimizes the load and improves the life of the more expensive HEPA filters. With this design larger particles entering the air handler are therefore removed before they reach the HEPA filter.

This filtration scenario works well until viable organic matter starts the growth process within the HVAC system. Conditioning coils, particularly the cooling coils, are ideal for culturing microorganisms. The constant temperatures, moisture and an abundant food source equate to laboratory conditions for growing and sustaining a multi-species microbial population4. Eventually these organisms will travel downstream and become entrapped by the HEPA filter.

Filters treated with an antibacterial preservative typically show less tendency to develop microbial growth5. Under ideal conditions for microbial growth the treatment will, at the best, delay the process.

It is one thing to stop a small inorganic dust particle but a completely different thing to stop a small living organism. The situation becomes even more cumbersome if moisture is finding its way to the filter. This establishes conditions on the filter media that are similarly ideal to the ones on the coil. Again, studies show that when filters are loaded with microbial growth and moisture, it is very likely that the same organisms can be found on the supply side of the filters7.

The picture shows a typical final filter located downstream from the cooling coil at a hospital. The organic growth on the upstream side of the filter is clearly visible as white and dark areas (see arrows). Condensation water coming off the coil virtually saturated the filter. Besides creating an ideal environment for organism growth, the water also increased the delta pressure across the filter adding 1″ (W.G.). It is quite clear that this filter has lost much of its protective properties and instead assumed a roll as an incubator of contamination. Unfortunately this situation is not rare and can be found in many air handlers varied environmental settings.

Severe contamination of the cooling coil and drain pans are the root cause of this condition. The contamination causes water and organisms to come off the coil surface and travel down to the final filter. Fouled and clogged drain pans act as a secondary reservoir for microbial growth.

Contaminated air handlers not only yield reduced filtration efficiency, they also may increase indoor air pollution. Studies of office buildings suggest that once filters are colonized with fungi, they produce Volatile Organic Compounds (VOCs) that are offgassed, adding to indoor air quality problems6,7, especially for building occupants that are immune compromised or suffer from allergies.

Building owners who install VIGILAIR® Air Handler Protection Systems experience clean coils and drain pans. Coils are returned to their ‘as designed’ efficiency and drain pans work as intended instead of exacerbating the problem. Filters remain dry and free from viable organisms. Microorganisms captured by the dry filter will find it difficult to survive and reproduce.

In summation, the HEPA filter is the highest efficiency filter available for HVAC systems. Like all filters there exists a determinable inefficiency that belies the myth of the HEPA as an ‘absolute’ solution to airborne contamination removal. The presence of microbial matter within HVAC systems raises the bar for contamination control of conditioned environments.

VIGILAIR® is a proven, highly effective system that provides an uncontaminated air handler environment. High efficiency Ultraviolet Germicidal Irradiation (UVGI) ensures that cooling coils are completely free from organic growth. VIGILAIR® UVGI compatible filters allow UVGI exposure of the filter surfaces which ensures inactivation of any organisms trapped on the filter surface.


1. Michele R. Evans, David K. Henderson, Infection Control in the Healthcare industry in the 21st Century, Hospital Engineering & Facilities Management 2005, Issue 2 pp. 58-62

2. Colin Perllman, Are Hospitals Getting Left Behind?, Cleanroom Technology, October 17, 2005

3. Andrew Streifel, Control Factors in Hospital Building Maintenance and Operations, Hospital Engineering & Facilities Management 2005, Issue 1 pp. 55- 58

4. R.B. Simmons, D.L. Price, J.A. Noble, S.A. Crow, D.G. Ahearn, Fungal Colonization of Air Filters from Hospitals, AIHA Journal (58) December 1997

5. D.L. Price, R.S. Simmons, S.A. Crow, D.G. Ahearn, Mold Colonization during Use of Preservative-Treated and Untreated Air Filters, Including HEPA Filters from Hospitals and Common Locations over an 8-year Period(1996-2003), Journal of Industrial Microbiology Vol. 32: 319-321

6. M. Möritz, H. Peters, B. Nipko, H. Rüden, Capability of Air Filters to Retain Airborne Bacteria and Molds in Heating, Ventilating and Air-conditionng (HVAC) Systems, Int. J. Hyg. Environ. Health 203, 401-409 (2001)

7. D.G. Ahearn, S.A. Crow, R.B. Simmons, D.L. Price, S.K. Mishra, D.I. Pierson, Fungal Colonization of Air Filters and Insulation in a Multi-Story Office Building: Production of Volatile Organics, Current Microbiology, Vol. 35 (1997)