Indoor Coil Leaks

With A/C season officially here, I have been asked to address an issue that has been around our industry for a long time now and that is indoor coil leaks also known as formicary corrosion.

Indoor coil corrosion failures are an issue in the HVAC industry today. Although the occurrence rate of these failures is low nationwide, some geographic areas have experienced higher incidence rates. For instance, some homes experience multiple failures while those around them have none. Failures are typically characterized by leaks that form in the fin pack area of the coil after one to four years of installation and use. This issue exists industry-wide. A competitive study has shown identical corrosion failure leaks in all coil brands investigated. (I have attached an industry study paper that Carrier put out that goes into great detail on this industry wide problem. There is a  lot of great information in this paper and some excellent pictures showing microscopic views of the various types of corrosion).

There are two main forms of pitting corrosion found in indoor coils: (1) general pitting; and (2) formicary corrosion, sometimes called “ant’s nest” corrosion.

(1) General pitting corrosion is caused by aggressive anion attack on the copper tube. An anion is a negatively charged chemical species. Due to this negative charge, anions aggressively search for positively charged species called cations. Copper is an abundant source of cations.Large pits resembling bite marks characterize the footprint of general pitting. These pits can often be observed with the human eye. Chlorides are the most common source of the aggressive anions known to cause general pitting corrosion.

Common household substances that may contain chlorides include:
• Aerosol sprays                                             • Carpeting
• Degreasing and detergent cleaners         • Dishwasher detergents
• Laundry bleach                                            • Fabric softeners
• Paint removers                                            • Tub and tile cleaners
• Vinyl fabrics                                                 • Vinyl flooring

(2Formicary corrosion, on the other hand, appears as multiple tiny pinhole leaks at the surface of the copper tube that are not visible to the human eye. Upon microscopic examination, the formicary corrosion pits show networks of interconnecting tunnels through the copper wall, hence the association with ants’ nests. The agents of attack involved in this corrosion mechanism are organic acids.

There are three conditions required for formicary corrosion to occur:
• The presence of oxygen
• The presence of a chemically corrosive agent (organic acid)
• The presence of moisture.

There is increasing evidence linking the primary cause of indoor coil leak failures to agents present in the household environment. Significant levels of corrosive agents known to cause these failures have been quantified in indoor condensate sampling during studies. (see list in attached paper) The trend toward decreased home ventilation rates likely contributes to the elevated levels of indoor contaminants. In addition, increased environmental awareness to identify and fix refrigerant leaks will continue to focus attention on these indoor coil failures as an industry issue.

So why didn’t it occur long ago and what is being done to address this issue? Both are very good questions.

So why didn’t this occur long ago? In an effort to increase efficiency of air conditioning systems, the heat transfer capabilities of the coils needed to be increased. To accomplish this, thinner walled copper tubing is now used and that tubing has rifling (grooves) inside the tubing which also decreases the wall thickness of the copper tubing. In much older systems, the copper tubing had thicker walls but the corrosion was still taking place — it just took a lot longer for it to occur and cause leaks like we now see  occurring in  the short time with present day coils.

When the coil leaks in the fin pack, the only solution is to replace the coil. But putting in another “copper tube” coil will eventually fail again depending on the “indoor environment”. To address this, manufacturers are using both “tinned” copper tubing in the coils or have gone to or will be going to all aluminum coils. Since aluminum and tinned coils do not have the anions available for the cations to attack they should resist the general pitting/formicary corrosion.The other issue that will help is providing more air changes to the occupied space.

Feel free to use this post to help explain it to your customers when you encounter the indoor coil leak.

Thank you to Carrier Corp for the information in the attached paper.



About yorkcentraltechtalk

I have been in the HVAC industry most of my life. I worked 25 years for contractors on anything from residential to large commercial boilers and power burners. For the past 23+ years I had been employed by York International UPG Division ( a division of Johnson Controls) as a Technical support/Service Manager but I am now retired. One of my goals has always been to "educate" dealers and contractors. The reason for starting this blog was to share some knowledge, thoughts, ideas, etc with anyone who takes the time to read it. The contents of this blog are my own opinions, thoughts, experiences and should not be construed as those of Johnson Controls York UPG in any way. I hope you find this a help. I always welcome comments and suggestions for postings and will do my best to address any thoughts, questions, or topics you may want to hear about. Thanks for taking the time to read my postings! Mike Bishop
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5 Responses to Indoor Coil Leaks

  1. Steve Hewitt says:

    I have heard that installing uv lights on the coils might help with the formicary corrosion. Do you know if there is any truth to this?

    • UV light will NOT prevent formicary corrosion. Since it is caused by CHEMICALS and not biologicals, UV lights have little or no effect of formicary corrosion. Likewise, they will not harm the coil either

  2. Glad to see your update on the ongoing issues with infant mortality in high efficiency evaporator coils. It’s telling that older homes often have their original coils after 20 years and experience failure in two years or less after the new system is installed. It is also interesting that Carrier, in their “research” paper, claimed no solution exists; however that conclusion was offered in an undated paper with references from year 2000 or before – implying perhaps that the problem was well known 15 or 20 years ago and the paper may be equally old. Clearly older manufacturing methods passed the test of time in these older homes; perhaps because of no rifling and thin copper as you mention. It also seems likely that aluminum coils are resolving the issue as long as they can hold up to the much higher R410 pressures. For the last 10 years I advised the customer to purchase the longest parts and labor warranty available from the hvac manufacturer, an investment that has often payed off for them and the installing company, unlike most other product’s extended warranties. The question now may become can American suppliers embrace quality and other ways to achieve hvac efficiency, such as inverter based motors and electronic expansion valves, before the market is lost to imports who have already achieved results? With the introduction of full sized indoor air handlers by the imports, the competition may be close to conclusion.

    • David Glenn says:

      Consider for a second the difference in construction standards between homes built when “older homes often have their original coils after 20 years”. It is without question that homes built 20 years ago were not built with the focus on energy conservation that are the hallmark of newer homes. Energy consumption is cut when the envelope of the space is tightened and air infiltration is cut. When volatile organic compounds (VOCs) build up in the home, this corrosion takes place — but, it isn’t just this corrosion that takes place, hence the newer building standards that call for the introduction of fresh outside air so that occupant health inside the home is not compromised. I enjoy reading this blog because normally it it isn’t a place to take pot shots against any manufacturer. Whether I sell or service York equipment, I can appreciate the fact that Yorkcentraltechtalk has been professional in respecting the industry. I respect his opinions much more than the opinion of someone who offers a reply with a definite bias against a manufacturer — and it would be difficult to suggest that your reply was unbiased when you call out the Carrier papers as “research” in quotation marks as if there is a legitimate reason to doubt the statements made within it…

      • David, thank you for your support and comments. I was not singling out Carrier but, rather using their research to show that this is an industry wide problem. If you look at the first page, it shows pictures of various manufacturers and names them too.
        Again, I thank you for sharing your thoughts and opinions.

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