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
(2) Formicary 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.