Ice where it should not be.
A freezer is built to grow frost. The evaporator collects it, a heater melts it off on a schedule, the meltwater runs to a drain, and the cycle starts again. So when ice builds where it should not and keeps growing, the refrigeration is usually working perfectly and one of two other things is true: the defrost cycle has quietly stopped, or humid air is getting in faster than any cycle was designed to clear. Where that meltwater goes when the drain is blocked.
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What a defrost cycle actually is.
Cold air cannot hold moisture, so the moisture in a freezer leaves the air and settles on the coldest surface in it: the evaporator. That is not a flaw in the design; it is the design. Left alone the coil would pack solid within weeks, so the control interrupts the cooling, runs a heater against the coil until the ice has cleared, sends the water to a drain, and goes back to making cold.
An over-temperature disc on the coil cuts the heater once the ice is gone, so the cabinet is never heated for longer than it takes. On a Monogram that disc opens at 60°F and resets at 45°F. The whole interruption is minutes, and it happens while nobody is watching.
Which means the useful question is never “why is there frost”. It is: why is this frost still here.
What Each Maker Publishes
None of these three defrosts on a fixed timer, and the intervals are not interchangeable. Each row is one series and one manual.
| Defrost | What reports a fault | |
|---|---|---|
| Sub-Zero Built-In 600-2 | Refrigerator off-cycle. Freezer adaptive: the first cycle after 12 hours of compressor run | Codes 20–24 name under-heat, overheat and missing voltage feedback |
| GE Monogram ZIS side-by-side | Adaptive: at most 60 hours between defrosts with no door openings, about 35 minutes each | Over-temperature disc opens at 60°F, resets at 45°F |
| Viking VCRB built-in | Ships in adaptive defrost; can be set to conventional | An open or shorted thermistor leaves the section calling for cooling, with an error shown |
Sixty hours with no door openings is the Monogram ceiling, not a target: every time the door opens, the control has more reason to defrost sooner. The three built-ins compared in full.
Sub-Zero’s Own List
Six things Sub-Zero asks an owner to look at before anyone is called, and most of them are about air rather than about the defrost system.
- 01
Is anything stopping the door closing?
A package pushed to the back of a drawer, a bin proud of its runners, a door left ajar for an afternoon. Sub-Zero puts this first, and it is first for a reason.
- 02
Is the gasket torn, split or dry-rotted?
Sub-Zero asks for the door gaskets to be inspected for tears, rips or dry rot, and replaced if damaged. A gasket that no longer pulls tight is an open door that looks shut.
- 03
Is the freezer set to 0°F?
That is the setting Sub-Zero recommends on an electronic control panel.
- 04
Has the condenser been cleaned in the last three months?
Three months is what Sub-Zero asks for on this fault. Its yearly maintenance answer says three to six, and its ice maker answer allows six to twelve. In a valley that spends the summer full of dust, follow the strictest one. What desert dust does to a condenser.
- 05
Is the toe kick blocked?
Air comes in low and leaves high. Anything pushed against the grille turns the whole cabinet into a machine working uphill.
- 06
Is the ice bin frosted over?
Sub-Zero says the bin frosts as the freezer runs through its defrost cycles, and asks for it to be emptied, rinsed and dried once or twice a month to clear the ice particles. Ordinary housekeeping, and it is on the maker’s own list.
Then wait a day. Sub-Zero asks for twenty-four hours for the unit to return to normal temperatures and for the frost to clear after any of those is corrected. Judging it sooner only produces a second opinion of the same afternoon.
Where the humid air gets in.
Sub-Zero names high humidity among the causes of frost build-up, and Phoenix supplies it on a schedule: the monsoon runs from 15 June to 30 September. For those months the air a door opening lets in is carrying far more water than the same door in February, and every gram of it ends up on the coil.
That is why a gasket matters more here than in a milder climate. A seal that would be merely imperfect somewhere else is, in a Phoenix August, a continuous supply of moisture to the one surface in the machine that is trying to stay clear of it. The coil then ices faster than any defrost cycle was designed to handle, and the symptom looks like a defrost failure when the defrost system is working exactly as built.
The tell is where the ice is. Frost spread evenly across a back wall behind the panel points at the defrost system. Ice concentrated around a door, a drawer front or a hinge points at the seal.
The three parts that stop it.
When the checks are clean and the ice keeps coming back, a defrost system has three things in it that fail, and none of them is inside the sealed refrigerant circuit. The heater itself, which stops warming the coil. The over-temperature disc, which either cuts the heater too early or does not cut it at all. And the sensor the control reads to decide when a cycle is needed and when it is finished. What each of those does, and how it fails.
There is a fourth part that is not part of the defrost system but ends up in the same complaint: the evaporator fan. Once ice reaches the blade the compartment goes warm even though the refrigeration is fine, and an owner describes a machine that has stopped cooling rather than one that has stopped defrosting. When one side is warm and the other is not.
On a Sub-Zero Built-In 600-2 the control says which it suspects. Codes 20 through 24 separate defrost under-heat, defrost overheat and a missing voltage feedback at defrost start. They name what was detected rather than what failed, and a defrost code often appears because a sensor failed weeks earlier. The full code list.
The one thing not to do.
Never chip ice off an evaporator. It is thin aluminium and copper carrying refrigerant under pressure, and one puncture turns a defrost repair into a sealed-system repair: regulated work, a recovered and reweighed charge, and a bill several times the one you started with. A screwdriver has ended more built-ins than any defrost heater ever has.
Sub-Zero publishes a manual defrost procedure for its own machines, and that is the route if a cabinet has to be cleared before a visit. Otherwise the safe version is the slow one: empty it, switch it off, leave the doors open and let the room do the work.
The visit is $119, waived in full when the repair goes ahead. On a defrost fault the parts are ordinary and the afternoon is short, which is the better half of this particular piece of bad news.
Questions
Is frost in the freezer a fault?
Not by itself. A freezer is designed to grow frost on its evaporator and then melt it off on a schedule: a heater warms the coil, the meltwater runs to a drain, and the cycle begins again. Frost becomes a fault when it builds where it should not and keeps growing, which usually means the defrost cycle has stopped rather than the refrigeration failing.
How often is a freezer supposed to defrost?
It depends on the series and it is not a fixed timer. GE’s guide for the Monogram ZIS allows at most sixty hours between defrosts with no door openings, about thirty-five minutes each. Sub-Zero runs the refrigerator off-cycle and the freezer on the first adaptive cycle after twelve hours of compressor run. A Viking ships in adaptive defrost and can be set to conventional.
Why does ice keep coming back after I clear it?
Because clearing ice treats the symptom. If the heater, the over-temperature disc or the sensor has failed, the sequence stops again as soon as the machine is closed up, and the coil packs until the evaporator fan can no longer move air past it. The other cause is air: a gasket that no longer pulls tight lets humid air in continuously, and no defrost cycle was designed to clear that much.
How often should the condenser be cleaned?
Sub-Zero publishes three different intervals across its own answers. The frost build-up answer asks for the condenser to have been cleaned within the last three months. The yearly maintenance answer says every three to six months, and calls condenser cleaning the only general maintenance it recommends. The ice maker answer allows six to twelve months. In a valley that spends the summer full of dust, the strictest figure is the one worth following.
Can I chip the ice off myself?
No. An evaporator is thin aluminium and copper carrying refrigerant under pressure, and a single puncture turns a defrost repair into a sealed-system repair, which is regulated work and a far larger bill. Sub-Zero publishes a manual defrost procedure for its own machines; that is the route, not a screwdriver.
What does a defrost code mean?
On the Sub-Zero Built-In 600-2 the defrost codes run from 20 to 24 and separate under-heat, overheat and a missing voltage feedback at defrost start. They name what the control detected, not which part failed, and a defrost code often appears because a sensor failed weeks earlier.
Sources
- Sub-Zero, ice or frost build-up in the freezer: the causes it names, the gasket inspection, the 0°F setting, the three-month condenser interval, the blocked toe kick, the ice bin, and the twenty-four hour wait.
- Sub-Zero Built-In (600-2) Series technical service manual: adaptive defrost on the first cycle after twelve hours of compressor run, off-cycle defrost on the refrigerator side, and codes 20 to 24.
- Sub-Zero published answer on yearly maintenance: cleaning the condenser every three to six months, described as the only general maintenance Sub-Zero recommends. That is the third interval the same maker publishes for the same job.
- GE Consumer Home Services training guide 31-9091 for Monogram side-by-side refrigerators: at most sixty hours between defrosts with no door openings, about thirty-five minutes each, and the over-temperature disc at 60°F and 45°F.
- Viking Preferred Service manual SMR-0022 for the built-in all-refrigerator VCRB5301 / VCRB5361, May 2012: adaptive defrost as shipped, and the NTC thermistor failure mode.
- National Weather Service: the Phoenix monsoon season, 15 June to 30 September.
Written from these sources and reviewed on by Aphelion Appliance Repair, Phoenix. Intervals and figures belong to the series named beside them and series differ, which is why the rating tag settles it. Nothing here is a manufacturer’s instruction, and work inside a sealed system is a technician’s job.
Read Next
- Warm on one side, perfect on the otherWhy a warm fresh-food side means a whole stopped system on a Sub-Zero.
- The part that failed, in plain wordsTwenty parts, what each does, and which are sealed-system work.
- After a power outage: keep it, or throw it out?The USDA four-hour rule, and the keep-or-discard chart.
Ice keeps coming back?
Tell us the make, where the ice is forming and what the display says. We call you back to set a time.