Evaporator Coil Leaks: The Expensive Failure That Starts Quietly
A slow coil leak looks like a system that is gradually getting weaker. By the time it is obvious, the decision is usually repair-or-replace.
An evaporator coil leak causes gradual loss of refrigerant, producing declining cooling performance, longer run times, ice on the refrigerant line, and rising electric bills. Coil leaks are often caused by formicary corrosion from indoor air contaminants. Replacing a coil is expensive enough that on systems past ten years, full replacement is usually the better decision.
How it presents
Coil leaks are rarely dramatic. Refrigerant escapes slowly through microscopic pinholes, so performance degrades over weeks or months rather than failing outright.
What homeowners notice, in roughly this order: the house takes longer to cool than it used to, the system runs more hours, the electric bill creeps up, some rooms stop keeping up on hot afternoons, and eventually ice appears on the copper suction line at the outdoor unit.
Because the decline is gradual, people adapt to it. We regularly find systems that have been losing capacity for two seasons where the homeowner simply assumed the house had always been like that.
Why coils develop pinholes
The main culprit is formicary corrosion — sometimes called ant-nest corrosion for the microscopic tunnel structures it produces in copper.
It is caused by volatile organic compounds in indoor air reacting with copper in the presence of moisture and oxygen. The sources are ordinary household items: cleaning products, adhesives, new carpet and paint, air fresheners, and building materials off-gassing. Over years, those compounds attack the copper from the outside in until a tunnel reaches the refrigerant passage.
This is why coil leaks often appear in homes where nothing obviously went wrong, and why they can affect systems well before the end of their expected life.
The signs, specifically
- Cooling performance declining gradually over a season or two
- Longer run times with the thermostat never quite satisfied on hot days
- Ice on the copper suction line at the outdoor unit, or on the coil itself
- A hissing or bubbling sound near the indoor unit
- Oily residue on or below the evaporator coil — refrigerant carries oil, and it leaves a trace where it escapes
- Refrigerant added previously, with the system low again within a year or two
That last one deserves emphasis. Refrigerant is not consumed. A system that needed refrigerant last year and needs it again has a leak, and topping it off without locating the leak is paying repeatedly for the same problem.
Repair or replace
Coil replacement is labor-intensive: recover the remaining refrigerant, remove the old coil from the air handler or furnace cabinet, install and braze the new one, pull a deep vacuum, and weigh in a full charge. On some installations the coil is difficult to access, which adds significantly.
| System age | Typical recommendation |
|---|---|
| Under 5 years | Replace the coil; it is likely under parts warranty |
| 5–8 years | Usually replace the coil, especially if efficiency is reasonable |
| 8–12 years | Genuine judgment call — depends on overall condition and efficiency |
| 12+ years | Replacement of the full system is almost always the better decision |
| Any age on R-22 | Replace the system |
Reducing the risk on a new coil
You cannot eliminate formicary corrosion, but you can reduce the exposure. Improve ventilation when using solvent-based products, painting, or installing new flooring. Reduce reliance on aerosol air fresheners and heavily fragranced cleaners near return grilles. Keep the return path clean and sealed so the system is not pulling air from a garage where chemicals are stored.
Some manufacturers offer coils with corrosion-resistant coatings or all-aluminum construction, which sidestep copper corrosion entirely. If you have already lost one coil to corrosion, that is worth asking about at replacement.
On leak sealants
Sealant products injected into the refrigerant circuit are marketed for exactly this problem. We do not use them, and most reputable contractors will not.
They can circulate and clog metering devices, filter driers, and in bad cases the compressor. They also contaminate recovery equipment, which is why many shops refuse to service a system that has had sealant added — meaning a temporary fix can make you harder to help later.
Frequently asked
How do I know if my evaporator coil is leaking?
The pattern is gradual: declining cooling over weeks or months, longer run times, rising bills, then ice on the refrigerant line. Oily residue near the coil is a strong indicator, since refrigerant carries oil that leaves a trace where it escapes.
Is it worth replacing an evaporator coil?
Under eight years old, usually yes — and it may still be under parts warranty. Past twelve years, replacing the full system is almost always the better decision, because coil replacement is labor-intensive and you are investing significantly in aging equipment.
What causes an evaporator coil to leak?
Most often formicary corrosion — volatile organic compounds from household products reacting with the copper over years, producing microscopic tunnels that eventually reach the refrigerant passage. Vibration, manufacturing defects, and physical damage account for the rest.