Here’s what’s actually happening:
Formicary (ant-nest) corrosion is the biggest culprit. Microscopic tunnels form inside the aluminum tubing, eventually creating pinhole leaks that let refrigerant escape. It’s driven by organic acids — mainly acetic and formic acid — that off-gas from ordinary household sources: adhesives, particleboard and new cabinetry, latex paints, cleaning products, air fresheners, silicone sealants, even cooking. Modern tightly-sealed, energy-efficient homes trap these VOCs, and the coil is constantly wet with condensate, so the acids concentrate right where the metal is most vulnerable. The corrosion is invisible from the outside until the coil suddenly won’t hold a charge.
Contributing factors:
• The copper-to-aluminum switch. Around the early-to-mid 2010s many manufacturers moved to all-aluminum coils to cut costs and dodge copper price swings and theft. Aluminum is more susceptible to formicary attack in the wrong environment, and the transition came with a learning curve.
• Thinner tubing walls. Cost pressure pushed wall thickness down, leaving less material before a corrosion tunnel breaks through.
• Higher operating pressures. R-410A runs at meaningfully higher pressure than the old R-22, stressing thinner-walled coils. The newer A2L refrigerants continue that trend.
• Galvanic corrosion at copper-aluminum joints, where dissimilar metals plus moisture set up a small electrochemical cell.
• Condensate chemistry — anything that makes the moisture more acidic or salty (pet urine, coastal salt air, certain cleaning agents) accelerates the whole process.
A few practical takeaways: coils rarely fail from one big cause, it’s the environment plus the material. Improving ventilation, avoiding storing chemicals near the air handler, and choosing coils with protective coatings (e-coat) or copper where available all help.