HVAC Coil Corrosion: Should You Be Concerned?

Coil corrosion is an expensive problem in the HVAC industry, leading to coil replacement or entire system replacement. Corrosion results in failure, and is responsible for about 40% of equipment failures in industrial applications, according to CED Engineering.

As the authors of an article in Reliable Plant explain, coil corrosion comes in the form of either pitting or formicary deterioration. Corrosion may occur as quickly as a few weeks after an installation or it may take up to four years to present itself.

Pitting corrosion is most often caused by exposure to fluoride or chloride. Fluoride is present in municipal water supplies, while chloride is found in a variety of products including snow melt, detergents, cleaners, carpeting and fabrics. Pitting occurs when chloride or fluoride ions are transported to the metal via condensate. The ions attack the metal, forming pits that form pinholes, causing the coils to leak refrigerant.

Formicary corrosion is typically caused by exposure to acetic or formic acids. These acids are present in a host of household products including cleaning solvents, insulation, adhesives, paints, plywood and many other materials. This type of corrosion is not always immediately visible and sometimes presents itself as black or blue-gray deposits. Formicary corrosion creates tunnels within the tubing that result in pinholes forming in the coils, again often leading to a refrigerant leak.

Facilities located in more corrosive environments including near saltwater or in industrial sites are particularly prone to experiencing coil corrosion. Other environments that may contribute to a higher amount of corrosive materials being expelled include areas around pools, laundry facilities, water treatment plants, sewers and high traffic areas. In such highly corrosive areas, coils have been known to fail in less than one year, according to CED Engineering.

A potential cause of coil corrosion is Chinese drywall, also known as odorous wallboard. Chinese drywall is also hazardous to your health. According to an article in G3 Environmental & Industrial Hygiene, an environmental and industrial hygiene service company, the drywall was imported from China from 2004 to 2007 and installed in both residential and commercial buildings. The drywall emits sulfur compounds, which corrode metal, including air conditioning coils.

The Centers for Disease Control and Prevention confirms that exposure to Chinese drywall can cause certain health problems such as irritated eyes and skin, respiratory problems and headaches. Exposure to this type of drywall can also exacerbate asthma symptoms.

You can identify the drywall by looking at the back. It should contain a label with the words: “Made in China,” “China,” or “Knauf.” Some Chinese drywall is not marked. In that case, metal corrosion including in plumbing, electrical, and natural gas lines, combined with a rotten egg odor may signify a Chinese drywall installation

To combat the problem of coil corrosion, coil manufacturers are applying coatings prior to production. In addition, there are companies offering aftermarket coatings. In a brochure, Aeris Technologies Ltd., a manufacturer of aftermarket coil-protection coatings, explains that the consequences of coil corrosion can include reduced efficiency, unattractive surface deterioration and equipment failure.

Additionally, reduced heat rejection may occur, resulting in an increasing compressor temperature and lower cooling capacity, which in turn increases the power usage. Lower cooling capacity means the compressor doesn’t cycle as intended, which means increased power consumption. As systems work harder they become more stressed, experience more breakdowns and have higher maintenance costs. A lower cooling capacity may also mean occupants aren’t very comfortable, resulting in loss of business or productivity. Preventing coil corrosion is much more effective and cheaper than replacing coils or the entire system.

There are basically four types of coil coating materials: polyurethanes, epoxies, fluoropolymers and silanes, according to Reliable Plant. Choose carefully as the wrong coating, especially with aftermarket products, can reduce heat transfer and result in more expensive operating costs. An aftermarket application may affect the manufacturer’s specifications.

Thinner coatings have better heat transfer while thicker coatings restrict heat transfer. Hydrophobicity, or how effectively water drains from the coil, can affect heat transfer. Water buildup may also cause mold or mildew growth. The advantages of the four types of coatings differ as to how they resist scratching and corrosion; their weights/thicknesses; hydrophobicity; and heat transfer abilities.

Polyurethane is inexpensive, flexible and thin, but it’s not as long-lasting as other coating options. Epoxy is cheap, but it’s usually a thicker coating and can’t be applied in the field; the coils must usually be shipped to a factory for professional application. Fluoropolymers are highly resistant to acids and solvents, and are inexpensive, but the sprays generally adhere poorly and their effectiveness wanes quickly.

Silanes form a thin coating that affects heat transfer very little and they typically last longer than other coatings. But they’re more expensive and proper application is best done by a professional and usually off-site. Each coating type has varying toxicity levels. Technicians applying the coating should wear equipment as specified by OSHA and an appropriate breathing apparatus.

Whether coated or not, continuing maintenance of coils is the primary way to combat the effects of corrosion. Twice annual cleaning with an alkaline coil cleaner or ph balanced coil cleaner will clear away any accumulated deposits, keeping coils safe from corrosion but also positively impacting the efficiency of your system.

Have you experienced coil corrosion? How did you combat the problem? Or did you choose a cooling system with coil coating protection included? And have you been happy with the product?

Next Steps:

12 comments


  • Really interesting and great blog post on coil corrosion and what may be leading to it! Wasn’t sure how to reach you, but wanted to send you a message.

    I work with Owens Corning and thought I’d send you information about our updated HVAC duct solutions site. As you know every little thing can hurt or help air quality and your HVAC system as a whole. Would love for you to share this with some of your fans.

    Thanks! Good luck and keep writing!

    Adam
    Owens Corning

    August 10, 2012
  • Great post. An issue not many people talk about but I am glad you gave some great advice on how to deal with this.

    August 13, 2012
  • Great information in this post guys.

    We come across coil corrosion quite often with our service. Continued maintenance is so important to keep your HVAC system clean, and we are seeing more cases as people try to save their money.

    October 21, 2014
  • Wonderful post, everything is very well explained. Thanks for the information.

    April 6, 2015
  • Too informative, this is a common issue. Most of us face HVAC coil corrosion and your post will be helpful in dealing with the problem. We deal with best HVAC from branded companies which use latest technologies which reduce corrosion problems.

    July 11, 2015
  • Very well explained. Really people don’t put much attention to it to save money and pay much more later. Thanks for sharing this information!

    July 23, 2015
  • Kenneth Smith

    I have a outside wood burning boiler. with a water to water 50 plate water to water heat exchanger going to a inside gas boiler. The outside wood burner will NOT heat properly. It has been setting for 3- years without being used. I think the 50 plate water to water heat ex-changer is plugged up. Do you have anything that I could put in the 200 gallon of water that would clean the deposits out of the heat ex-changer?

    December 21, 2015
  • Great post.
    Thanks!

    March 15, 2016
  • Tim

    ScaleBreak is a liquid descaler that can help remove mineral deposits if that’s what you believe to be clogging the exchanger. We also have a variety of mechanical solutions that can help remove if tubes are plugged. Best bet is to call us and we can talk through the options.

    April 18, 2016
  • This is a great post that clearly explains the concerns of coil corrosion for both the HVAC industry and consumers alike. It was very enjoyable to read as you really did your research well. For example, you explained there are different types of coil coating materials and how choosing the wrong material can result in more expenses.

    Thank you for your Informative post.

    May 5, 2016
  • Phil Brinlee

    My dogs have destroyed some of the coils/heat transfer media on my outdoor uni,t exposing the pipes. I can’t afford to replace the coil or entire condense unit as suggested by a local company. I don’t have any leaks in the pipes, but heat transfer efficiency must ne way down. Is there something (like steel wool) I can pack around the pipes to increase heat transfer efficiency and get another few years of life out of the coil? Thanks. I know that this is not the “right” way to fix this. I’m just looking for a good “McGiver”.

    May 27, 2016
  • Tim

    Something we’ve heard of many times. To be honest, you probably are leaking refrigerant , you just don’t know it…yet. If not, it’s just a matter of time. At this point, as long as the fan is going strong, and you are keeping the coils free of debris, the heat exchange process is going fine. Stuffing it with anything will actually limit the air flow, which is the worst thing.

    July 11, 2016

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