Evaporator coil
The coil that absorbs heat from indoor air, causing the refrigerant inside it to boil from liquid to vapour. Because it runs below room temperature, it also condenses moisture out of the air, which is why every evaporator has a drain pan beneath it.
Also called indoor coil
What does the evaporator coil do?
The evaporator coil is where refrigerant boils and absorbs heat from indoor air. Because it runs below room temperature it also condenses water out of that air, which is what the drain pan collects.
Evaporator coil: how it works
Cold low-pressure liquid enters the coil and boils as room air passes over the fins, and the heat that boiling absorbs is the heat removed from the room. Because the surface sits below the dew point of the air, water condenses on it and runs into the pan, which is why a working system in humid weather produces a steady trickle at the drain.
Where you meet the term
- In a delta T measurement, because the temperature drop across this coil is what the reading describes.
- In an icing complaint, because this is the surface the ice forms on and the one that has to be thawed before anything is measured.
- In a musty smell complaint, where the permanently damp coil and pan are the surfaces being cleaned.
How to check it
- Check the filter before the coil. Air reaches the coil through the filter. A loaded filter starves the coil, and that is visible and reversible without opening anything. Owner check
- Look for ice or a water trail. Frost on the coil face or water dripping from the front of an indoor head both point at the coil rather than at the outdoor unit. Owner check
- Measure air temperature in and out. A thermometer at the return and at the supply gives the split across the coil, which is compared against the range the manual publishes for that model. Technician
Evaporator coil: what it is not
- Frost on this coil is a symptom, not the fault. It forms when airflow across it drops or the charge is low, and it will re-form after any thaw unless that cause is dealt with.
- A frozen coil is not fixed by running the system harder. Cooling has to stop so the ice can clear before anything can be measured.
Evaporator coil questions
Why does my evaporator coil freeze?
The coil surface drops below freezing when it cannot absorb the heat it was designed to, which happens when airflow across it falls or when the refrigerant charge is low. Both push the boiling temperature down.
Why does the indoor unit smell when it starts?
The coil and pan stay damp, and biological growth on those surfaces releases odour when air first moves across them. Cleaning the coil itself is a technician job on most indoor units.
Should the evaporator coil be wet in cooling?
Yes. Condensation on the coil is expected in cooling and is how the system dehumidifies. What matters is that the water reaches the drain rather than the floor.
Where this definition comes from
Written from manufacturer support article material held in the HVAC Bench evidence record. Where a document limits a definition to certain models, that limit is repeated here rather than generalised across a brand.
- Why Does My Mini-Split Smell?Trane ductless odor, moisture, drain, refrigerant, electrical, and professional cleaning guidance.
- Why Is My Mini Split Leaking Water?Trane ductless condensate, drain, filter, frozen coil, drain pan, and installation leak guidance.