Quick answer
An evaporator coil is the indoor coil of a central air conditioner or heat pump where cold refrigerant boils and absorbs heat from the air blown across it, cooling and drying your home's air.
It sits on top of the furnace or inside the air handler, over a drain pan that collects the water it pulls out of the air.
Quick facts
The key facts about Evaporator Coil, with sources:
- Location
- Above the furnace, or inside the air handler
- Shapes
- A-coil, slab, N-coil, horizontal
- Does two jobs
- Removes heat (sensible) and moisture (latent) 4
- Common metering
- TXV, EEV or fixed orifice at the coil inlet 2
- Cleaning
- By a contractor as needed; ENERGY STAR lists it in a pre-season check-up 1
- Biggest enemy
- Dirt that gets past a poor or missing filter 1
- Water
- Drains to a pan and condensate line 1
Key takeaways
- The evaporator coil is where your house actually gets cooled.
- It removes both heat and humidity; the water goes down the condensate drain.
- Dirty coils make the system run longer and shorten its life 1.
- It must match the outdoor unit in size and refrigerant type.
- Ice on the coil is a symptom of low airflow or low charge, not a fix-it-yourself job.
What the evaporator coil does
Liquid refrigerant arrives at the coil under pressure. A metering device drops that pressure suddenly, and the refrigerant starts to boil at a low temperature. Boiling soaks up heat. So when the blower pushes warm room air through the coil's fins, the air leaves cooler.
The coil is usually colder than the dew point of the air. That means water vapor condenses on the fins, like drops on a cold glass.
This is the "latent" part of cooling: the system removes moisture as well as heat.
HVAC School notes that when humidity is higher, more of the coil's work goes to removing moisture, and the air temperature drop gets smaller 4.
The water drips into a pan and leaves through the condensate drain line. On a heat pump the same coil runs hot in winter and acts as the condenser. The name sticks, but the job reverses.
Refrigerant through the coil
- Warm liquid refrigerant from outdoors → expansion valve or orifice → pressure drops
- Cold, boiling refrigerant → coil tubes → absorbs heat from room air
- Fully boiled to vapor (with a few degrees of superheat) → suction line → compressor
- Moisture on fins → drain pan → drain line → outside or to a drain
What a healthy coil looks like in operation
- Air enters at room temperature and humidity.
- It passes through a clean filter, then through the coil fins.
- It leaves noticeably cooler. The temperature drop is often in the mid-teens to low 20s °F, but varies with humidity and blower speed 4.
- Water runs steadily from the drain outlet on a humid day.
- The suction line at the outdoor unit feels cool and may sweat, but should not ice.
If the drop is far outside the usual range, a tech checks airflow and charge before blaming the coil. See HVAC delta T.
Types of evaporator coils
| Type | Shape | Best fit |
|---|---|---|
| A-coil | Two slabs leaning together like the letter A | Upflow and downflow furnaces |
| N-coil or Z-coil | Three or more slabs for more surface in a short cabinet | Tight closets, high-capacity units |
| Slab coil | One flat panel | Horizontal air handlers, packaged units |
| Cased coil | Any of the above in a metal box | Added on top of a furnace |
| Uncased coil | Coil without its own box | Fits inside an existing plenum |
| Microchannel | Flat aluminum tubes instead of round copper | Some newer systems; repair options differ |
Example: how much water a coil removes
Example only, using the airflow targets in a published building-science reference and a common moisture-load equation. Inputs are illustrative, not a measurement.
| Step | Math | Result |
|---|---|---|
| Airflow, 3-ton system, humid climate | 3 × about 400 cfm per ton 3 | About 1,200 cfm |
| Moisture removed (grains per lb of air) | Say 70 grains in, 55 grains out | 15 grains |
| Latent heat removed | 0.68 × 1,200 cfm × 15 grains 3 | About 12,240 BTU/h |
| Share of a 36,000 BTU/h system | 12,240 ÷ 36,000 | About 34% of capacity goes to drying |
If the same system ran at 500 cfm per ton, the coil would be warmer and would remove less water 3. That is why humid-climate setups lean toward lower airflow. Our latent heat page explains the equation.
Where you will see the term
- On a replacement quote, as the indoor coil model matched to a new outdoor unit.
- On the AHRI certificate, which lists the coil model as part of the rated combination.
- On service invoices: "evaporator coil cleaned," "coil leak found," or "TXV replaced."
- In warranty terms, where coils often have their own parts coverage.
What a good coil does for you, and its weak points
Benefits
- Cools and dries air in one pass.
- A larger matched coil can raise efficiency and moisture removal.
- No moving parts of its own.
Weak points
- Hidden inside a cabinet, so dirt and leaks go unnoticed.
- Thin tubes can develop small leaks over time.
- Freezes when airflow or charge is wrong.
- Wet surfaces can grow mold if drains clog.
What the coil depends on
The coil is only as good as the air and refrigerant fed to it. DOE's review of 44 field studies found low airflow (under 350 cfm per ton) in 50% to 93% of systems tested, and charge outside the maker's range in many more 2. Both faults make a good coil underperform.
A coil also cannot fix humidity in a home that is oversized for its load. Short runs do not give the coil time to get cold and drain. See short cycling.
Sources: [2]
Cost drivers and lifespan
HyreHVAC publishes prices only with enough dated quotes. What drives the cost of coil work: coil size and type, whether it is cased or uncased, refrigerant compatibility with the outdoor unit, access (attic and crawlspace jobs take longer), refrigerant recovery and recharge, and whether the drain pan also needs replacing.
Leaking coils often fall under a parts warranty, but labor and refrigerant are usually extra. A coil leak on an older system is a common moment to weigh replacement. Use the repair-or-replace calculator.
How a coil is replaced
Licensed HVAC work, usually with a mechanical permit.
- A certified technician recovers the refrigerant 8.
- Disconnect the drain, then cut or unbraze the refrigerant lines.
- Remove the old coil and set the new one, sealed to the furnace or air handler.
- Braze in the new coil, often with a nitrogen purge to keep scale out.
- Pressure test, pull a deep vacuum and recharge to the maker's spec.
- Reconnect and test the drain and any float switch.
- Check airflow and temperature drop at startup.
Keeping the coil clean
- You: change the air filter on schedule. A filter is the coil's main protection.
- You: make sure the filter fits snugly with no gaps around the edge.
- You: watch the drain outlet in summer for steady flow.
- Tech: inspect and clean the coil during the spring check-up 1.
- Tech: check and clear the condensate drain 1.
Sources: [1]
Warning signs and when to call a pro
- Ice on the coil or suction line. Switch to fan only or off, and call. See frozen evaporator coil.
- Musty smell when the system starts.
- Water around the furnace or air handler.
- The system runs long and the house stays humid.
- A hissing sound or oily stain near the coil cabinet.
Rules that apply
Refrigerant matching. EPA's Technology Transitions rule sets a 700 GWP limit for new residential AC and heat pump systems, with no installation deadline where all specified components were made before January 1, 2025 5.
EPA says the restrictions apply to new systems, not installed ones 6. In practice a new outdoor unit using R-454B or R-32 needs a coil built for that refrigerant. Read our refrigerant transition research.
Service. Section 608 bans venting refrigerant during coil repair or replacement 8.
Drain pans. Where a coil sits above finished space, the mechanical code your area adopted usually calls for a secondary drain or overflow protection. Adoption varies.
Evaporator coil vs related parts
| Part | What it does | Where |
|---|---|---|
| Evaporator coil | Absorbs heat and moisture from indoor air | Indoors |
| Condenser coil | Releases that heat outdoors | Outdoor unit |
| Air handler | Cabinet that holds the coil and blower | Indoors |
| Heat exchanger | Transfers furnace flame heat to air | Inside the furnace |
Common misconceptions
- Myth A frozen coil means the AC is working extra hard.
- Reality It usually means too little airflow or too little refrigerant.
- Myth Coil cleaner sprays from the store make coils maintenance-free.
- Reality The indoor coil is hard to reach without opening the cabinet. A tech should clean it.
- Myth Any coil fits any outdoor unit.
- Reality Size, refrigerant and metering device must match.
- Myth Running the fan on "On" dries the house.
- Reality Continuous fan can blow water from the wet coil back into the air between cycles.
Climate and the coil
In humid states the coil's moisture work matters as much as its cooling. Lower airflow per ton and longer runs help it pull more water 3. In dry Southwest climates, setups can run higher airflow. Attic coils in hot states need working drain safeties.
When the coil matters to you
It matters if you replace your outdoor unit, if you find water near the furnace, or if the house feels clammy. When replacing equipment, ask the installer to confirm the coil is matched and listed together on an AHRI certificate.
Next step: if you suspect a leak, read about refrigerant leak repair. For humidity problems, see dehumidifier installation.
Related guides
Questions about Evaporator Coil
Where is the evaporator coil located?
It sits on top of a furnace in a metal box (a cased coil) or inside the air handler of a heat pump system. You usually cannot see it without removing a panel. The copper refrigerant lines and the plastic drain line both connect to it.
How do I know if my evaporator coil is bad?
Signs include weak cooling, ice on the coil or lines, water around the furnace, a hissing sound or an oily stain near the cabinet. These can also come from airflow or drain problems, so a technician should check pressures and look for a leak before replacing anything.
Can an evaporator coil be repaired instead of replaced?
Some small leaks at accessible joints can be repaired, but leaks inside the coil usually mean replacement. Microchannel coils are especially hard to repair. A technician will weigh the coil's age, warranty status and refrigerant type.
How often should the evaporator coil be cleaned?
ENERGY STAR lists coil cleaning as part of an annual pre-season check-up, done as needed. With a good filter changed on time, many coils stay clean for years. Pets, smoke, renovation dust or a missing filter make them dirty faster.
Why does my evaporator coil freeze?
The most common causes are low airflow from a dirty filter or blocked returns, and low refrigerant from a leak. Running cooling on cool nights can also cause it. Turn cooling off, run the fan to thaw it, then call a technician if it happens again.
Do I need a new coil if I replace my outdoor AC unit?
Usually yes. New outdoor units use refrigerants such as R-454B or R-32, and coils are rated and warrantied as part of matched pairs. An old coil may not be compatible, and keeping it can lower efficiency and void the warranty.
Why does my AC smell musty when it starts?
Moisture on the coil and in the drain pan can grow mold or bacteria if the drain is slow or the pan holds water. A technician can clean the coil and pan and clear the drain. EPA advises keeping indoor humidity below 60 percent, ideally 30 to 50 percent 7.
What is the difference between a cased and uncased coil?
A cased coil comes in its own metal cabinet that sits on top of the furnace. An uncased coil is bare and slides into an existing plenum. Cased coils are easier to service and seal. The choice depends on how much space and access the installer has.
Sources
- ENERGY STAR, Maintenance Checklist (heating and cooling), retrieved .
- U.S. DOE Building Technologies Office, Residential HVAC Installation Practices: A Review of Research Findings (June 2018), retrieved .
- Green Building Advisor, Converting Heating and Cooling Loads to Air Flow Needs, retrieved .
- HVAC School, What Should the Air Delta T Be? (Air Temperature Split), retrieved .
- U.S. EPA, Technology Transitions HFC Restrictions by Sector, retrieved .
- U.S. EPA, Frequent Questions on the Phasedown of Hydrofluorocarbons, retrieved .
- U.S. EPA, A Brief Guide to Mold, Moisture and Your Home, retrieved .
- U.S. EPA, Stationary Refrigeration and Air Conditioning (Section 608), retrieved .
Expert review
Written by the HyreHVAC Research team. Not yet reviewed by an outside expert. We say so rather than imply a review that has not happened; see our editorial policy.
How the numbers were checked: Coil function and the latent-heat equation follow the cited building-science sources. Fault rates are DOE's 2018 field-study review. The water-removal example uses illustrative grain values, labelled as such, and the published 0.68 latent constant.
Suggest a correction. We fix errors and say what changed.