Walk-In Freezer Ice Buildup: Where It Forms and Why

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Some frost in a walk-in freezer is normal. A block of ice on the evaporator coil, icicles on the ceiling, a frozen puddle on the floor or a frost-rimmed door frame is not. Each one points to a specific cause, usually warm, moist air getting in or a defrost system that is not finishing its job. This page shows where to look, what staff can check safely, and when the fix belongs to a refrigeration technician. It is part of our Commercial Refrigeration Help Center.

Where the ice is tells you why it is there

Why a freezer coil ices up

A freezer evaporator runs well below freezing, so moisture in the air freezes onto the fins. The defrost system melts that frost on a schedule: electric heaters (or hot gas) warm the coil, the water runs into a heated drain pan and out a heated drain line, and a termination control ends defrost when the coil is clear. Turbo Air's walk-in systems manual says that for most applications two to four defrost cycles per day should be adequate, and that ice usually starts at the bottom of the coil in the middle rows, where it can be hard to see (Turbo Air manual).

When any part of that chain fails, ice builds on itself. The same manual lists the causes of a coil not clearing of frost during defrost as heater problems, an insufficient defrost period and a defective timer or defrost thermostat, and warns that a partial defrost leaves water and slush that refreeze into ice on the next cycle, which then needs a manual defrost. Left alone, long-term ice formation will crush the refrigerant tubes in the coil and cause leaks (Turbo Air manual). That is why an iced freezer coil is worth a call even when the box still feels cold: it is heading toward a much bigger repair.

Ice in the drain pan and drain line

Defrost water has to leave the freezer before it refreezes. Turbo Air's troubleshooting chart lists four causes of ice accumulating in the drain pan: a defective heater, a plugged drain line, a defective drain line heater, and a defective timer or defrost thermostat (Turbo Air manual). Once the drain freezes, each defrost adds more water to the pan until it overflows onto the floor or back onto the coil. If you see water dripping from the unit cooler or a frozen puddle below it, our refrigeration water leaks page covers the drain side in detail.

Frost around the door

Freezer doors have heaters in the frame (and often in the pressure relief vent) to keep the gasket from freezing to the jamb. Kolpak's walk-in manual says frost or condensation around the door jamb or heated pressure relief vent indicates that the heater is inoperable or that excessive moisture is present, that the gasket should seal around the entire door perimeter, and that metal surfaces, magnetic gaskets and door sweep gaskets should be cleaned often with a mild detergent (Kolpak manual). A door that freezes shut or takes a hard pull to open is a sign the heater has failed.

Ice on the ceiling, walls and floor

This is usually air infiltration. Norlake warns that leaving entry doors open for more than five minutes can form a film of ice or moisture on the floor, ceiling and walls because warm, moist air condenses inside the walk-in, and suggests vinyl strip curtains to reduce it (Norlake walk-in manual). Federal walk-in standards require automatic door closers on most walk-in doors and strip doors or another way of limiting infiltration while the door is open (10 CFR 431.306). Deliveries are the usual culprit: the door is propped for a full pallet while the box behind it fills with humid air. Ice that forms at a panel seam or around a light fixture, away from the door, can also mean a gap in the panel joints that a technician should seal.

Overloading and warm product

A freezer is built to hold frozen product at temperature, not to freeze large loads quickly. Stacking warm or fresh product into the box, or crowding the unit cooler so air cannot circulate, drives up humidity and run time and leaves frost on everything. Keep a clear space in front of the evaporator fans and stagger large deliveries.

What staff can safely do

1. Check product first. California requires frozen food to be stored and displayed frozen unless it is being thawed by an approved method (HSC 114018). If product is soft, treat it as a refrigeration failure and follow our walk-in not cooling guide and your county's food safety guidance.
2. Close the door and watch the closer. The door should swing shut and seal on its own. Look for torn gasket sections, a bent door sweep or a door that sags.
3. Read the controller. Note the set point, the defrost schedule if it is shown, and any alarm codes. Do not change defrost settings unless a technician has shown you how.
4. Look at the coil and drain. With a flashlight, check whether the coil fins are open or iced, and whether ice is building in the pan or at the drain outlet.
5. Start a manual defrost only if you have been trained. Keep the door closed, protect product under the coil from dripping water, and check whether the coil clears. If it ices again within a day or two, the defrost system has a fault.
6. Clean up and mark floor ice and remove loose ice from the floor. Leave ice on the coil, lines and ceiling to the technician.

Stop and call when

• The evaporator coil is solid with ice, or the fans are striking ice.
• Ice returns after a defrost, or water is overflowing from the drain pan.
• The door is freezing shut, or there is heavy frost on the frame with the door closed.
• The box temperature is rising, or frozen product has started to soften.
• You see oil at a fitting, hear hissing, or smell refrigerant.

Defrost heaters, drain line heaters and door heaters are wired at line voltage inside a wet, frozen space, and the controls sit next to the refrigerant circuit. Staff should not open unit cooler panels, and only an EPA Section 608 certified technician may open a refrigerant circuit (40 CFR 82.161).

How we fix a freezer that keeps icing

We defrost the coil fully and safely first, then find out why it iced. That means checking each defrost heater and the drain pan heater for continuity and current draw, checking the drain line heater and clearing the line, and confirming the defrost timer or controller starts and ends each defrost as it should. We look at door heaters, gaskets, sweeps and closers, and at refrigerant readings, because a low charge or a failing expansion valve can also frost a coil unevenly. The repair is usually a heater, a termination control, a drain line heat trace or a door part rather than new equipment. Our walk-in cooler repair cost guide explains what drives the cost of these repairs, and our walk-in cooler and freezer repair service covers every job with a 1-year parts and labor warranty.

Keep the ice from coming back

• Hold deliveries at the door with a strip curtain in place, and close the door between loads.
• Wipe gaskets and check that the door seals on every shift.
• Keep the drain outlet and floor under the unit cooler clear and visible.
• Book quarterly preventive maintenance. Norlake recommends quarterly service by a certified technician that includes gasket and sweep checks, heated vent port inspection and evaporator and condenser coil cleaning (Norlake walk-in manual). Our commercial refrigeration maintenance plans and our maintenance schedule guide lay out who does what.

Book a walk-in freezer service call

Frequently asked questions

Written and reviewed by Jim McKinny, owner of Cool Aid Air Conditioning & Refrigeration — California C-20 and C-38 licensed, EPA Universal certified, second generation of a family business serving the Bay Area since 1966. Every job we complete is backed by our 1-year parts & labor warranty.