Walk-In Cooler and Freezer Defrost System: How It Works and Fails

Defrost is the most common reason a walk-in freezer stops freezing. When it fails, ice builds on the evaporator until air can't pass through the coil. When it runs too long or too often, product warms and energy is wasted. Knowing how your system defrosts makes both problems easier to spot.

Why walk-ins need defrost

The evaporator coil runs colder than the box air. Moisture from the air, from door openings and from product condenses on the fins and, below 32°F, freezes. A thin layer is harmless. A thick layer insulates the coil and blocks airflow, so the box warms even though the compressor runs. Defrost melts that frost on a schedule and drains the water away.

Three types of defrost

Heatcraft's engineering manual pairs coolers held at 25 to 34°F with hot-gas or electric defrost, and Heatcraft controls default to air defrost for a 35°F cooler. A cooler held below the mid-30s may need positive (heated) defrost to keep its coil clear; your technician or the equipment manual can confirm what your system is set up for.

How defrost is started

• Mechanical time clocks start defrost at set times of day and hold it for a set maximum.
• Electronic controllers run a set number of defrosts per day. On Heatcraft's Beacon II, defaults are four a day for electric defrost and two a day for air defrost.
• Demand or adaptive defrost skips defrosts that aren't needed. Heatcraft's Smart Defrost programs eight defrost "opportunities" a day instead of a fixed number of defrosts.

Heatcraft's condensing unit manual says "typical settings are two to four defrost cycles per day for freezers." A box with heavy door traffic or humid product may need more; a lightly used box may need fewer.

How defrost is ended

Defrost should end as soon as the coil is clear. Two things end it:

1. Temperature termination. A sensor or a snap-disc switch (often called a klixon) on the coil ends defrost when the coil reaches a set temperature. Beacon II defaults are +60°F for electric defrost and +45°F for air defrost. Smart Defrost uses 55°F.
2. Fail-safe time. If termination doesn't happen, defrost ends after a maximum time. Beacon II defaults are 30 minutes electric and 40 minutes air; Smart Defrost uses 60 minutes. Heatcraft's manual recommends setting fail-safe at the normal defrost length plus 25%.

These are defaults for specific Heatcraft controls. Your unit may use different settings, so check its manual.

Drain-down and fan delay

After electric defrost, the coil is wet. If the fans started right away, they would blow water droplets into the box and onto product. So the system waits:

• A drain-down period lets water run off the coil into the pan. Beacon II uses 2 minutes.
• A refreeze or fan delay keeps fans off until the coil is cold again. Beacon II holds fans off until suction reaches 28°F or 3 minutes pass. Other systems use a fan delay switch on the coil.

The drain pan and drain line have to stay above freezing through all of this, which is why freezers have drain line heaters.

Signs of defrost problems

Controllers often flag defrost trouble. Heatcraft's intelliGen shows IN01 (too many defrosts) and IN05 (too few defrosts, time-terminated eight times in a row). HTPG's EcoNet shows T001 (defrost failure, max time expired) on freezers. Carel ir33 shows Ed1/Ed2 when defrost ended on timeout. On a Dixell XR60CX, a P2 evaporator probe failure makes defrost end on time instead of temperature. The controller alarm codes page has more.

Checks staff can do safely

1. Check the controller display for defrost, drain or delay status, and write down any codes.
2. Look at the coil from the floor right after a scheduled defrost. It should be clear. Frost that survives defrost points to a problem.
3. Log temperatures before, during and after defrost for a day or two. This shows whether the rise is normal and recovers.
4. Check under the evaporator for ice in the pan area or water on the floor.
5. Keep doors closed and check that gaskets seal; moisture from outside is what defrost has to remove.

Safety: Staff should not force a manual defrost by adjusting timers or controller settings unless the manufacturer's instructions and the operator's procedures allow it. Never chip or pry ice from the coil. See evaporator coil frozen for safe steps.

What a technician will check

A technician will test defrost heaters for resistance and current, check the termination sensor or klixon and the fan delay, verify timer or controller settings, confirm the drain pan and drain line heaters work, and look at why frost is forming as fast as it is. Heatcraft recommends checking evaporators for proper defrost "once a month or more often," which is a good reason to add a quick post-defrost look to your routine.

Defrost and energy use

Electric defrost heaters draw a lot of power while they're on, and every minute of defrost adds heat that the system then has to pull back out of the box. That makes defrost settings one of the less obvious drivers of a high electric bill. Demand-style controls aim to defrost only when the coil needs it. For Heatcraft's electronic controls, see the site's page on Heatcraft smart systems; walk-in cooler energy efficiency covers other ways to cut running costs. Any change to defrost frequency or length should be made by a technician who has looked at the coil after defrost.

Getting the settings right

Too little defrost ices the coil. Too much defrost wastes energy, warms product, and can create steam that refreezes on the ceiling. The right settings depend on the box temperature, how often doors open, and what's stored. If your system seems to need constant defrost tweaks, the underlying cause may be infiltration from doors and gaskets, a failed component, or a coil that doesn't match the application. See walk-in freezer not freezing and ice buildup in a walk-in freezer.