Evaporator Coil Corrosion (Formicary)

Definition

Evaporator coils are made of thin copper tubing in u-bends through aluminum fins. Refrigerant runs through the copper. When organic acids (formic acid, acetic acid) reach the copper surface — usually from VOCs in indoor air — they react with the copper and gradually etch microscopic tunnels through the tube wall. Under magnification the pitting pattern looks like ant tunnels, hence the name.

Common acid sources in a typical Austin home: cleaning products with vinegar, scented candles, aerosol sprays, certain construction adhesives, formaldehyde from new cabinetry, pet odor neutralizers, and some pesticides. Outdoor sources include nearby gas stations and industrial sites. Newer homes (post-2010) with tighter envelopes and synthetic-heavy interiors are more prone.

Detection is a leak search: nitrogen pressure test, electronic leak detector, or UV dye. Once formicary is confirmed, the failed coil is replaced. The new coil should be all-aluminum (microchannel) or epoxy-coated copper — both are highly resistant to formicary corrosion.

Why it matters in Austin

An AC that's losing refrigerant year after year, gets recharged, then loses it again next summer is almost certainly running on a coil with formicary corrosion. Recharging only buys a season — the coil needs replacement.

Most R-410A residential systems built between 2010 and 2018 used uncoated copper coils. Many of those are in year 8-14 of their life right now and beginning to leak. If your system is in that vintage and is losing charge, replacing the coil with a corrosion-resistant version is usually the right call. If the system is also at end-of-life on age and SEER2, replacing the whole unit (with R-454B and a microchannel coil) is the cleaner long-term answer.