The evaporator coil sits inside the box and absorbs heat from the air as cold refrigerant passes through it. Signs of trouble include heavy ice, weak airflow, water dripping, a warm box or oil stains. Icing usually points to defrost, airflow or refrigerant problems rather than the coil itself.
The evaporator is where cooling actually happens. It is also where many walk-in problems show up first, because ice, moisture and food dust all gather there. Learning to read the coil helps you describe a problem accurately.
What it does
Cold, low-pressure refrigerant flows through the coil tubes, and fans blow warm box air across the fins. Heat moves into the refrigerant, which boils and returns as vapor to the compressor. In a freezer, moisture in the air freezes onto the coil and must be melted by the defrost cycle. A clean, lightly frosted coil with strong airflow is normal.
Signs it is failing
- Thick ice covering the fins or the whole housing.
- Ice only on part of the coil, often the bottom or the inlet.
- Weak airflow from the fans.
- Water dripping or ice forming at the drain pan.
- Oil stains at fittings or on the coil.
- A box that will not reach temperature.
Common causes of failure
| Cause | What happens |
|---|---|
| Defrost failure | Ice builds up cycle after cycle |
| Blocked airflow | Stock stacked against the coil starves it of air |
| Low refrigerant | Coil freezes in patches from a leak |
| Dirty fins | Dust and grease reduce heat transfer |
| Door left open or bad gasket | Humid air adds moisture and frost |
What you can safely check
- Look, do not chip. Note where the ice is and how thick it is.
- Check the fans. Are they turning? Is the airflow strong?
- Check the drain. Is water reaching the pan and draining away?
- Clear stock. Move boxes away from the coil housing.
Never chip ice with a knife or screwdriver. Copper tubes are thin, and a puncture releases refrigerant. Turn power off before any cleaning, and keep water away from electrical parts.
What a technician does
- Checks defrost operation, heaters and termination.
- Inspects fans, motors and airflow.
- Measures superheat, pressures and temperature drop across the coil.
- Looks for leaks with a detector.
- Cleans the coil and drain pan.
Keeping it healthy
- Keep stock clear of the coil housing.
- Keep the drain pan and line clear.
- Check that fans turn freely and defrost runs on schedule.
- Wipe up spills and keep the door closing fully.
A quick look at the coil during a weekly walk-through takes a minute and tells you a lot. Note the ice pattern in your log so trends are visible.
Repair or replace
A coil that is dirty or iced is usually a symptom, so fix the cause first. A leaking coil may need repair or replacement, which involves recovering refrigerant and is a job for a technician. See cleaning the evaporator coil.
Related reading: the parts and components guide, freezing up and defrost repair and ice on the floor or ceiling.
Questions about this part
Is frost on the coil normal?
A light frost in a freezer is normal. Thick ice that blocks the fins is not.
Can I defrost it with hot water?
That is risky. See the page on defrosting with a heat gun or hot water.
Why is only part of the coil iced?
Usually low refrigerant, a feed problem or uneven airflow. A technician should check.