Why do HVAC coils fail?
Why HVAC Coils Fail: The Corrosion Problem and the Case for Protective Coatings
Evaporator and condenser coils are the thermal heart of any HVAC system, and they are also its most failure-prone component. Industry data collected by manufacturers and independent researchers over the past two decades points to the same conclusion: while mechanical damage accounts for some coil losses, the dominant long-term cause of premature coil failure is corrosion. Understanding how that corrosion develops — and how modern protective coatings interrupt it — has real financial consequences for building owners, facility managers, and equipment manufacturers alike.
The Two Root Causes of Coil Failure
Heat-transfer coils fail for essentially two reasons: mechanical damage and corrosion. Mechanical damage includes bent or crushed fins, physical impacts, vibration fatigue, and debris erosion. These failures are largely addressed through physical protection — coil guards, careful handling during installation, and routine cleaning — but there is little that can be engineered out of the metal itself to prevent them.
Corrosion is a different story. Unlike mechanical damage, corrosion is a chemical and electrochemical process that can be substantially slowed, and in some cases nearly eliminated, through material selection and surface treatment. That distinction matters because corrosion, not physical damage, is responsible for the majority of coils that fail years before their designed service life.
How Coil Corrosion Actually Develops
Coils fail through several distinct corrosion mechanisms, and the mechanism at work usually depends on the coil's environment and the volatile compounds it is exposed to.
Formicary corrosion. This is one of the most notorious and least visible failure modes in evaporator coils. Volatile organic compounds — acetic acid and formaldehyde released from adhesives, particleboard, cleaning products, and even fresh paint — react with moisture on the coil surface to form organic acids. These acids attack copper tubing through microscopic, ant-nest-shaped tunnels (hence "formicary," from the Latin for anthill), eventually perforating the tube wall and causing a refrigerant leak. A widely cited 2007 Carrier Corporation industry research report documented that this problem affects evaporator coils across essentially every manufacturer, not just one brand, and that some coils have failed within just a few years of new construction because indoor air was saturated with VOCs from cleaning and finishing work.
Galvanic corrosion. Most coils pair aluminum fins with copper tubing. Whenever two dissimilar metals are in electrical contact in the presence of an electrolyte — condensation, humid air, or salt-laden moisture — the less noble metal (aluminum) corrodes preferentially to protect the more noble one (copper). Over time this sacrificial process eats away at the fins, degrading heat transfer and structural integrity long before the copper tubes themselves fail.
Pitting and atmospheric corrosion in coastal or industrial air. Airborne chlorides from ocean spray, industrial pollutants, and de-icing salts accelerate oxidation on both aluminum and copper surfaces. In seacoast environments this can be dramatic: HVAC contractors serving coastal regions routinely classify units by exposure zone, with quarterly coil inspection recommended for equipment within roughly 1,000 feet of tidal water and semi-annual service as a minimum out to about 3,000 feet, because standard annual maintenance schedules are not sufficient to keep pace with salt-driven degradation.
General surface oxidation and grease contamination. In commercial kitchens, manufacturing plants, and other grease- or chemical-heavy environments, contaminants that settle on the coil surface can trap moisture against the metal and accelerate localized corrosion, independent of the broader galvanic or formicary mechanisms.
The Financial and Operational Consequences
Corrosion rarely announces itself as a sudden catastrophic failure. It shows up first as reduced efficiency: a corroding coil loses heat-transfer surface area, which raises compressor head pressure and lowers cooling capacity. The system compensates by running longer and harder, driving up energy consumption before a leak or perforation ever occurs. As the equipment works harder under stress, mechanical wear accelerates, maintenance calls increase, and occupant comfort — and by extension productivity or tenant satisfaction — declines. Eventually the coil perforates, refrigerant escapes, and the unit either needs the coil replaced or, if corrosion has also compromised the cabinet or mounting structure, the entire unit replaced.
Replacement is expensive by any measure. Coil and unit replacements in corrosion-prone coastal markets are commonly cited in the range of several thousand dollars per unit once labor, refrigerant, and downtime are factored in — costs that recur every few years in the harshest environments if nothing changes about how the coil is protected.
How Protective Coatings Interrupt the Corrosion Process
Corrosion-resistant coatings work by physically isolating the metal surface from the moisture, acids, and salts that drive these reactions, while still allowing heat to pass through the coil efficiently. Several coating families are used across the industry:
Phenolic/epoxy dip or bake coatings (often referred to generically by the trade name "Heresite," even when applied by other manufacturers) form a thin, chemically resistant barrier over fins and tubing.
E-coat (electrocoat) systems, such as Modine's ElectroFin, apply a uniform epoxy layer via an electrodeposition process.
Silane-based coatings cure to a barrier under 10 microns thick, are highly hydrophobic (helping condensate shed rather than pool), and are typically field- or factory-applied by trained technicians because surface preparation is critical to their performance.
Field-applied epoxy or polyurethane sprays allow existing, uncoated equipment to be retrofitted with corrosion protection without full coil replacement, provided the tubing has not already been perforated.
Tin coating of copper hairpins, offered by some manufacturers as a factory option, sidesteps formicary corrosion specifically, since tin is not vulnerable to the acid attack that damages bare copper.
Coating performance depends heavily on proper surface preparation. Applying a coating over existing oxidation, grease, or contamination effectively seals the corrosion in rather than stopping it, so a thorough cleaning cycle before application is a non-negotiable step regardless of which coating chemistry is used.
What the Data Shows
Manufacturer and third-party data on coating performance is consistent on a few key points:
High performancing HVAC corrosion coatings are engineered to extend coil service life four to five times relative to an uncoated coil, while affecting thermal efficiency by less than 1%. This a critical figure because any coating that meaningfully insulates the coil defeats its own purpose by reducing heat transfer.
Independent laboratory work on coated aluminum alloys in coastal exposure has shown that coatings can maintain corrosion resistance for extended periods, though even well-applied systems eventually lose protective capability after long-term exposure (on the order of two decades in some marine test programs), underscoring that coatings extend service life significantly rather than making it indefinite.
Financial Savings and Other Benefits
The economic case for coatings rests on a straightforward comparison: coating cost versus avoided replacement cost, amortized over the additional years of service life the coating provides.
Avoided replacement cost. Field examples from coastal markets describe coatings that add a modest upfront cost preventing a full unit replacement — commonly cited in the thousands of dollars — just a few years down the line. Multiplied across a commercial property with dozens of rooftop units, or a portfolio of buildings, avoided replacement cost is typically the single largest line item in the coating ROI calculation.
Extended service life. Field-applied epoxy recoatings on existing equipment are commonly described as adding several years of protection to a coil, deferring replacement capital expenditure rather than eliminating it — still a meaningful cash-flow benefit for building owners planning capital budgets.
Energy savings. Because corrosion degrades heat-transfer surface area over time, a protected coil that resists pitting and fin loss maintains closer-to-design efficiency for longer, avoiding the gradual rise in energy consumption that accompanies a degrading, uncoated coil.
Reduced maintenance and downtime. Coils that resist formicary tunneling and galvanic fin loss are less likely to develop refrigerant leaks that trigger emergency service calls, unscheduled downtime, and refrigerant replacement costs (a factor that has grown more significant as refrigerant prices have risen under phase-down regulations).
Sustainability and certification value. Coating suppliers note that extending coil life and reducing early equipment replacement can support LEED and other green-building certification goals by reducing material waste and the embodied carbon associated with manufacturing replacement units.
Practical Limits
Coatings are not a universal fix. Once a coil's refrigerant circuit has already been breached by corrosion, a coating cannot restore function — replacement becomes the only viable repair. Coatings are most cost-effective when applied proactively, either at the factory or in the field, before pitting or formicary tunneling has compromised tube walls. Poor application — inadequate cleaning, coating outside the manufacturer's recommended temperature window, or excessive coating thickness — can also reduce heat-transfer performance or fail to bond properly, so coating quality and installer competence matter as much as the choice of chemistry.
The Bottom Line
Coil failure is overwhelmingly a corrosion story, driven by indoor VOCs, galvanic pairing of dissimilar metals, and airborne salts or industrial contaminants. Protective coatings address the mechanism directly, with manufacturer data showing service-life extensions on the order of four to five times for well-engineered coating systems, minimal impact on thermal efficiency, and a return on investment that generally comes down to one comparison: a modest coating cost now versus a full coil or unit replacement — often running into the thousands of dollars per unit — a few years sooner than necessary. For any building in a coastal, industrial, or VOC-heavy environment, corrosion coatings are one of the highest-leverage investments available for extending HVAC equipment life and controlling long-term operating costs.