Walk-In Evaporator Coil Frozen: Read the Ice Pattern

A frozen evaporator coil is one of the most common reasons a walk-in stops cooling, and one of the easiest to spot. Air has to pass through the fins to be cooled; once ice fills the gaps, the fans spin, the condensing unit runs, and the box warms anyway. The good news is that the shape of the ice tells you a lot about why it formed.

Why a coil ices up at all

The evaporator runs colder than the box air. Heatcraft's engineering manual balances general-purpose coolers at a 10 to 12°F temperature difference (TD) between the box air and the coil, which keeps relative humidity around 80 to 85%. In a cooler held at 35°F, that puts the coil below freezing, so the moisture in the air freezes onto the fins as frost.

That's by design, and defrost removes it on a schedule. Coolers usually use off-cycle (air) defrost, where the compressor stops and the fans melt the frost with box air. Freezers use electric heaters or hot gas. Ice becomes a problem when more moisture arrives than defrost removes, or when defrost, airflow or the refrigerant supply stops working.

Read the ice pattern

With a flashlight, look at the coil face and fins without touching them.

The causes, in more detail

Defrost failure

If defrost doesn't happen, or doesn't finish, frost accumulates cycle after cycle until it becomes a solid block. In a freezer, the usual suspects are a failed heater element, a defrost termination sensor that ends defrost too early, or a timer or controller that isn't starting it. In a cooler, air defrost can fail if the timer or controller isn't stopping the compressor long enough, or if the setpoint was lowered far enough that the box air is too cold to melt the frost.

For reference, Heatcraft's Beacon II controller defaults to two air defrosts a day ending at 45°F on coolers, and four electric defrosts a day ending at 60°F on freezers, each with a fail-safe time. Some controllers tell you when defrost isn't working: Heatcraft intelliGen shows IN05 "Too Few Defrosts" after eight time-terminated defrosts in a row, and Russell's EcoNet shows T001 "Defrost Failure: Max Time Expired" on freezers. See defrost system.

Evaporator fan failure

Without airflow, the coil gets colder and frosts rapidly, and defrost can't keep up. A single failed fan on a multi-fan coil causes a patch of ice in front of it, which spreads. Fan blades can also pick up ice and become unbalanced and noisy. See evaporator fan motor.

Low refrigerant or a starving valve

When too little refrigerant reaches the coil, it boils off in the first rows, which get very cold and frost heavily, while the rest of the coil stays bare. Suction pressure drops, and the system runs longer and cools less. Low charge means there is a leak, which only an EPA Section 608 certified technician may repair. An expansion valve that is stuck, has lost its bulb charge or sits behind a clogged strainer produces the same pattern with a full charge.

Moisture from outside

Every door opening brings in humid air, and in summer that can be a lot of water. A door propped during deliveries, a gasket that doesn't seal, missing strip curtains or a door that doesn't self-close can overload defrost. Frost that builds most on the side of the coil nearest the door, with frost on the ceiling or walls near the door too, points this way. The ice buildup in the freezer page follows that moisture around the rest of the box.

Drain problems

If melt water can't leave the drain pan, it refreezes in the pan and climbs into the bottom of the coil. Freezer drain lines need heat, and Heatcraft lists ice in the drain pan as a sign of a defective heater, bad pitch, a plugged drain or a failed drain-line heater. See drain line heater.

Clearing the ice safely

Clearing ice is necessary, but it is not a repair. The ice will come back unless the cause is fixed, so plan on a technician visit either way.

Safety: Never chip, pry or scrape ice off an evaporator coil. The fins and tubes are thin; one slip with a knife, screwdriver or ice pick can puncture a refrigerant tube, turning a defrost problem into a leak repair and possibly releasing refrigerant into an enclosed box. Don't use heat guns, torches or hot water either.

What staff can do while waiting:

1. Move at-risk product out of the airflow below the coil and to another cold space if the box is warming. Follow food safety guidance for anything that has warmed.
2. Put towels or a catch tray under the evaporator for melt water, and keep the floor dry. OSHA requires floors to be kept, as far as feasible, dry and free of ice.
3. Keep the door closed as much as possible so no more moisture comes in.
4. Ask the technician by phone whether to start a manual defrost from the controller, if your controller offers one and a manager has been shown how. Many electronic controllers do; follow the manual.
5. Don't change settings such as defrost times or setpoints on your own.

A technician will typically melt the ice with a controlled defrost (heaters, hot gas, or with the compressor off on a cooler), confirm the drain is open, and then run the system and watch the next defrost cycle to find the cause.

Checks staff can do safely

1. Photograph the coil before anyone melts the ice. The pattern is evidence of the cause.
2. Count the fans and note any that are stopped, slow or noisy.
3. Look under the unit for ice in the drain pan, icicles or ice on the floor.
4. Check the door closes on its own and seals, and that strip curtains are intact.
5. Check the controller for defrost status or codes, and note the setpoint.
6. Ask staff whether the door has been propped, or whether the setpoint was recently lowered.

What a technician will check

• Defrost heaters, termination sensor, timer or controller settings, and fail-safe.
• Fan motors, blades and the fan delay after defrost.
• Superheat, pressures and charge, with a leak search if low.
• Expansion valve operation and the filter drier for restriction.
• Drain pan heater, drain line heater, pitch and trap.
• Box humidity sources: door, gaskets, closer, strip curtains.

Related symptoms

Water blowing out of the unit after defrost is covered on evaporator dripping or spitting water. For freezers specifically, see walk-in freezer not freezing. The evaporator coil component page explains how the unit cooler works, and the site's guide to Heatcraft evaporator options covers coil types.