In short
An evaporator coil is the indoor half of an air conditioner: a finned coil above the furnace where refrigerant boils at a low temperature, pulling heat and humidity out of the air. It must match the outdoor unit and its refrigerant. Ice on the coil, water at the furnace or an oily leak means it needs service, not just a refrigerant top-up.
The cooling you feel at the vents is made in a sheet-metal box above your furnace, not outside. What is an evaporator coil? It is a bank of copper or aluminum tubing threaded through thin aluminum fins, usually folded into an A shape, that sits in the supply plenum on top of the furnace or inside an air handler. The blower pushes house air through it every time the AC runs.
What an evaporator coil does in an Ontario summer
Liquid refrigerant passes through a metering device at the coil inlet, its pressure drops, and it boils inside the tubes at a temperature a few degrees above freezing. Boiling takes heat, and that heat comes out of the air crossing the fins. The same cold surface condenses water vapour, which runs down into a drain pan and out through the condensate line. In a humid Lake Ontario summer that water removal is half the job: a coil that is dirty, oversized or starved of refrigerant cools the air but leaves the house clammy.
The coil is one half of a matched pair. Its size, its metering device and its listed refrigerant have to agree with the outdoor unit, which is why a coil built for R-410A cannot simply be left in place under a new R-454B or R-32 condenser unless the manufacturer lists it for that refrigerant. On a replacement quote the coil should appear as new and matched, with its model number.
How an evaporator coil fails
- Refrigerant leaks: pinhole corrosion in the copper tubing lets the charge seep out over months. Signs are longer run times, ice on the coil or the copper line, and an oily film at the tube bends.
- Icing: low airflow (a plugged filter, a dirty coil face, closed vents) or a low refrigerant charge drops the coil below freezing and it grows into a block of ice. Our post on why an AC freezes up covers thawing it safely.
- Dirt: dust that gets past the filter mats the underside of the coil. The restriction is there in heating season too, so a furnace that overheats and trips its limit in January can trace back to the AC coil.
- Drain problems: a plugged pan or drain line overflows onto the furnace below and soaks the cabinet and control board.
- Metering device faults: a stuck expansion valve starves or floods the coil and mimics a low charge.
A leaking coil is not fixed by adding refrigerant. A top-up without a leak search puts the same charge back into the same hole, and handling refrigerant is restricted to technicians holding an ODP card. Ask for the leak location in writing before approving a recharge.
Repair or replace: what an evaporator coil job costs
- Diagnostic visit: $90–$150.
- Refrigerant leak repair and recharge: $300–$800+ where the leak is reachable and the coil is otherwise sound.
- Coil replacement: quoted in writing after we see the coil size, the plenum and the refrigerant, because the sheet-metal work differs with every installation.
Two words on the quote matter. A cased coil comes in its own insulated cabinet that bolts to the furnace; an uncased coil is the bare A-frame that slides into an existing or site-built plenum. Cased costs more in parts and less in sheet-metal labour.
Our rule of thumb: on a system past 12 years old with a leaking coil, price evaporator coil replacement against a full matched system before spending on the coil alone. Check the warranty first, since a registered coil may still have parts coverage. The answer on what an AC refrigerant recharge costs shows why repeat top-ups are the worst of the three options.
What to do if you see ice or water at the coil
Set the thermostat to off and the fan to on, check the filter, and let the ice melt fully before anyone tests the system; gauges on a frozen coil give false readings. Put a tray or towel under any drip at the furnace. Then book AC repair or message us through the contact page with the age of the system and the refrigerant type printed on the outdoor label.
Questions people ask about Evaporator coil
Can an evaporator coil be cleaned instead of replaced?
Yes, when dirt is the only problem. A technician reaches the coil through the plenum, cleans the upstream face and flushes the drain pan; heavily matted coils are pulled out and washed. Cleaning will not fix a refrigerant leak or corroded tubing. If airflow is weak in both heating and cooling and the filter is clean, a dirty coil is a likely cause.
How long does an evaporator coil last?
A coil that stays clean and drains properly often lasts as long as the outdoor unit. Corrosion leaks tend to appear later in its life, and sooner where the drain pan stays wet or the filter is neglected. A good filter changed on schedule and a clear condensate line do the most to extend it, and both are checked during a spring tune-up.
Does the evaporator coil have to be replaced with a new outdoor unit?
In nearly every case, yes. The efficiency rating is certified for a specific outdoor and indoor pairing, the manufacturer's warranty assumes a matched coil, and the newer refrigerants need a coil listed for them. Reusing an old coil to save money leaves you with an unrated combination and a used part that is the next thing to leak.