What a defrost cycle is and why yours might be wrong
My evaporator keeps freezing over. Is the defrost set wrong?
Published 30/10/2026. Written by the Pristine Cooling team, Kalgoorlie.
Possibly, though a defrost setting is a less common answer than people expect. An evaporator freezing over is usually telling you about airflow, moisture or a door, rather than about the controller. Worth understanding what defrost is actually for before changing any numbers on it, because a setting adjusted to chase somebody else's fault makes the room worse.
Why a cool room needs defrost at all
The evaporator coil inside the room runs colder than the air around it. That is how it takes heat out.
In a room below a certain temperature it runs colder than freezing, so moisture in the air condenses onto the coil then freezes there. Every door opening brings in more moisture. So does warm product. So does a tired seal.
Left alone, that ice builds into a layer across the fins. It insulates the coil from the air, the same blanket problem a dirty condenser has, then eventually it blocks the airflow through the coil entirely. The room starts climbing while the plant runs flat out.
Defrost exists to take that ice off on a schedule, before it gets that far.
The three ways it gets done
Off cycle defrost. The refrigeration simply stops and the room air, which is above freezing, melts the ice off. Only works in rooms that sit above zero.
Electric defrost. Heating elements inside the coil melt it off. Common, simple, effective.
Hot gas defrost. The system sends hot refrigerant back through the evaporator to warm it from the inside. Faster, more efficient on bigger plant, more complex.
The three settings that can be wrong
How often it happens. Too few defrosts and ice accumulates faster than it is removed. Too many and you are heating the room you are paying to cool.
How long it runs. Too short leaves ice behind, which builds from a base that never fully clears. Too long dumps heat into the room after the ice has gone.
How it ends. This is the one people never think about. A defrost that ends on a timer runs the same length whatever the conditions. A defrost that ends on temperature stops when the coil is actually clear, which is better in almost every case.
What wrong actually looks like
Ice building up anyway, visit after visit. Not enough defrost. Or something putting far more moisture in the room than the settings assume.
Room temperature climbing at the same time every day. Defrost running too long or too often. Look at when it happens rather than at the ice.
Ice on the floor under the evaporator. That is a drain problem rather than a defrost problem, which is the next section.
Ice on the suction line outside the room. That is usually a charge or airflow fault rather than defrost at all.
The drain line is the quiet one
A defrost melts ice into water. That water has to leave the room.
The drain runs from under the evaporator, through the wall, into a trap. On a freezer room that line is heated, because water sitting in an unheated drain in a sub zero room does exactly what you would expect.
When that heating fails, the defrost works perfectly and the water freezes on its way out. It backs up, refreezes on the coil tray, builds into a block of ice under the evaporator, then drips down the wall. People look at the controller. The fault is in the drain.
Why it is often not the defrost
Before anybody changes a setting, the things worth ruling out are the ones that put extra moisture or extra load into the room.
Stock stacked hard against the evaporator, blocking the airflow it needs. A failed evaporator fan. A door seal that has given up, which is covered in when a cool room door seal is done. A door propped open during deliveries, every day, for twenty minutes.
Any of those will ice a coil up on a defrost schedule that was perfectly correct last year. Changing the schedule then hides the real fault for a while, until it gets worse.
The same pattern turns up on display cabinets, which is in why a display cabinet ices up.
The short version
Ice on the coil is a symptom. The defrost settings are one possible cause among several, usually not the first one worth changing.
If a room keeps icing up after somebody has already adjusted the defrost, that is a good sign the defrost was never the problem.
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