Replacing an evaporator coil: the indoor failure with the widest quote spread
Contents (9)
The outdoor unit gets the attention because it is the part you can see and hear. The part that actually fails first, on a great many American central systems, is the one nobody looks at: the evaporator coil sitting in a closet, an attic or on top of the furnace. It leaks slowly, the house stops cooling well over a period of weeks, and the homeowner is handed a quote that ranges — for what sounds like the same job — from about a thousand dollars to the price of a complete new system. That spread is not dishonesty. It is three genuinely different jobs wearing the same name.
Why the indoor coil is the half that leaks
The evaporator lives in the wettest, dirtiest air in the house. Every cooling hour it runs below the dew point, so its surface is permanently wet, and everything the return duct picks up lands on that wet surface. Three failure mechanisms follow from that environment.
The first is formicary corrosion — also called ant-nest corrosion, because a cross-section of the tube wall shows a network of tiny interconnecting tunnels. Volatile organic compounds released indoors by adhesives, cleaning products, solvents, new flooring and building materials dissolve in the condensate and form weak organic acids, which attack the copper from the outside in. It produces pinholes rather than a single obvious split, it is famously hard to locate, and once it starts in a tight, low-ventilation house it can perforate a tube in months rather than years. It is not brand-specific and it is not something a homeowner did wrong.
The second is mechanical: tubes rubbing where they pass through a support, a hairline fracture at a factory braze joint, or a return bend fatigued by vibration. The third is simple age combined with acidic condensate and a drain pan that has been standing full since the pan float stopped working.
The symptom is almost always the same and almost always misread: gradually weaker cooling, longer run times, sometimes ice on the coil or the suction line. Ice is a low-charge signature as much as a dirty-filter one, and our guide to an air conditioner that freezes up sets out how to tell those apart before anyone quotes a part.
Confirm the leak before you agree to a coil
A coil is an expensive answer to a question that has not always been asked properly. Before accepting one, insist that the diagnosis is a located leak rather than an inference from low pressure. That means one of: an electronic leak detector that finds the spot, dye circulated and then found under UV light, or — the definitive test — the system isolated and pressurised with dry nitrogen and held. A technician who cannot say where the refrigerant is going has established that the system is low, not that the coil is the reason.
This matters financially because a refrigerant top-off is sometimes offered instead, at a fraction of the price. It buys a season at best; the arithmetic of repeated top-offs against a real repair is worked through in our guide to refrigerant recharge costs. Venting refrigerant is also illegal, so a system knowingly left leaking is not a neutral choice.
What the job actually involves
A correct coil replacement is a half-day to a full-day job for one or two technicians: recover the remaining refrigerant into a machine, cut the coil out, fit the new one, adapt the sheet metal where the new cased coil does not match the old plenum opening, braze the line connections under flowing nitrogen so the inside of the tube does not oxidise, fit a new liquid-line filter drier, pull a deep vacuum to remove moisture and non-condensables, then weigh in the full factory charge plus the line-set adder. Skip the drier and the acid from the old failure stays in the system. Skip the nitrogen purge and the scale you create circulates into the metering device. Skip the vacuum and the new coil starts life with moisture in it.
Three things make one coil job cost twice another: access — an attic in August is slower and more expensive than a basement in October — whether the coil is cased or uncased and how much plenum fabrication the swap needs, and whether the metering device (a thermostatic or electronic expansion valve, or on older equipment a fixed-orifice piston) comes with the coil or has to be sourced and matched.
Indicative costs
These are market reference ranges for a residential two to four ton system, not quotes; regional labour rates and access move them substantially.
| Line item | Indicative range | What drives it |
|---|---|---|
| Diagnosis and located leak search | $100–$350 | Nitrogen test or dye adds time; often credited against the repair |
| Cased coil, part only | $500–$1,300 | Tonnage, brand, whether an OEM match or a universal coil |
| Labour, 4–8 hours | $500–$1,400 | Attic or crawlspace access, sheet-metal adaptation |
| Filter drier, nitrogen, brazing, deep vacuum | $100–$300 | Non-negotiable; a quote without them is a shortcut |
| Refrigerant, weighed in | $50–$180 per pound | R-410A pricing has risen through the phasedown |
| Out-of-warranty total | $1,500–$3,500 | Higher for five-ton, high-efficiency or awkward installs |
| Part under warranty, labour billed | $700–$1,800 | The part is free; nothing else is |
The warranty line that catches everyone
Evaporator coils are usually covered by the manufacturer’s parts warranty, and on a registered system that term is commonly around ten years. What the parts warranty covers is the coil. It does not cover the labour to fit it, the refrigerant, the drier, the freight in some cases, or the diagnosis. So the homeowner with a genuinely covered failure still writes a four-figure cheque, and does so with the distinct feeling of having been sold something. Whether your coverage is the five-year base term or the registered ten is decided by paperwork filed in the weeks after installation, which is the subject of our guide to HVAC warranty registration deadlines. Ask, before work starts, for the claim to be filed and shown on the invoice as a separate line: a covered part that is quietly billed at retail is the single most common overcharge on this job.
Can you still fit an R-410A coil in 2026?
Yes. This is the question that generates the most misinformation on a coil quote, and the rule is clear. The EPA’s Technology Transitions restrictions under the AIM Act set a global warming potential limit of 700 for new residential air conditioning and heat pump systems, with an installation compliance date of 1 January 2025 — which is why new equipment now ships with R-454B or R-32. The same rule states that components used to repair existing systems are not subject to those restrictions. A replacement R-410A evaporator coil for the system already in your attic is a repair component, and fitting one is legal.
What you cannot do is mix eras. An R-454B coil will not work with an R-410A condenser: the pressures, the oil and the safety classification differ, and the pairing has no certified rating. Any contractor telling you that a leaking coil forces a whole-system replacement because « R-410A is banned » is overstating the rule. What is true is that R-410A costs more per pound each year, that OEM coils for older platforms get scarcer, and that both facts belong in the decision rather than in the diagnosis.
Coil, or the whole system?
The honest test is age and pairing, not the size of the invoice. Under roughly eight years, with the compressor healthy and the part under warranty, replacing the coil alone is straightforward and sensible. Past twelve or thirteen, a $2,500 coil buys a few years on a machine whose other half is already near the end of its life, and the same money is a meaningful deposit on a matched system — the ranges are in our guide to central AC replacement costs. In between, the deciding factors are whether the compressor has ever been repaired, whether the ducts are sound, and whether the pairing is a certified combination at all. The mirror-image version of this decision, when the failure is outdoors, is set out in our guide to replacing only the outdoor unit — and the same principle applies here: a coil chosen without reference to the condenser it will serve is a coil chosen badly.
What a good coil quote shows
Ask for the leak location and how it was found; the new coil’s model number and confirmation that the pair appears in the AHRI certified-ratings directory; the metering device supplied; the drier, nitrogen purge and evacuation as stated line items; the refrigerant type, weight and price per pound; the warranty claim reference if the part is covered; and the labour warranty on the work itself. A quote that carries those seven lines can be compared with another quote. A single number cannot.
Frequently asked questions
How do I know if my evaporator coil is leaking?
The pattern is gradual: cooling that fades over weeks, longer run times, warmer supply air, sometimes ice on the coil or the copper suction line, and an outdoor unit running almost continuously without satisfying the thermostat. None of that proves a coil leak — a dirty filter, a failing capacitor or a blocked condenser produce overlapping symptoms. Confirmation is a located leak: an electronic detector finding the point, dye under UV light, or a nitrogen pressure test that holds or does not.
Can a leaking evaporator coil be repaired instead of replaced?
Occasionally, and only for a single accessible pinhole on a tube a technician can reach and braze. It is not usually the right call, because formicary corrosion is a condition of the whole tube surface rather than a single defect: repair one pinhole and the next one appears elsewhere within months. Ask whether the leak is one identifiable point or a corroded area, and treat a proposed spot repair on an older coil as a way to buy a season, not a fix.
Does the coil have to match my outdoor unit exactly?
It has to match on refrigerant, on capacity and on the metering device, and the resulting pair should appear as a certified combination in the AHRI directory. A universal coil of the right tonnage and refrigerant, correctly matched, is a legitimate option and is often cheaper and quicker to get than an OEM part. A coil selected only on tonnage, with no reference to the condenser, is how systems end up delivering less cooling than the label promises.
Is the labour covered if the part is under warranty?
Almost never by the manufacturer. Standard residential coverage is parts-only: the coil is supplied free, and diagnosis, labour, refrigerant, drier and sometimes freight are billed. Labour is covered only if the installing contractor gave a separate workmanship warranty that is still running, or if you bought an extended service agreement that includes labour. Check which of those you hold before the visit rather than after the invoice.
How long does an evaporator coil replacement take?
Typically four to eight hours on site for an accessible installation, spread across recovery, removal, sheet-metal work, brazing, evacuation and charging. Attic and crawlspace installations take longer. Beware of a two-hour version of this job: the corners that get cut to reach two hours are the nitrogen purge, the filter drier and the deep vacuum, and each of them shortens the life of the coil you just paid for.
Sources
Read next
- HVAC zoning: motorized dampers, a thermostat per floor, and the bypass duct that undoes it
- HVAC permits: what a legal system replacement involves
- Central AC replacement cost: an honest 2026 breakdown
- AC compressor replacement: the repair that is really a system decision
- Does a home warranty cover your air conditioner? Reading it before August