Frost in the freezer, 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 holds little 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.
A sensor on the coil ends the heating once the ice is gone, so the cabinet is never heated for longer than it takes. On a Monogram that is the evaporator thermistor, which ends the defrost when it reads 38°F; the over-temperature disc beside it, which opens at 60°F and resets at 45°F, is only a safety cutoff. 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
Neither of these two defrosts on a fixed timer, and the intervals are not interchangeable. Each row is one series and one manual. Viking is left out: its VCRB service manual covers an all-refrigerator, which has no defrost system.
| 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; each defrost up to 35 minutes, under 16 on average by GE’s figure | No fault-code table in GE’s guide; the control cannot tell a broken heater from an open over-temperature disc |
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 and water rather than about the defrost heater.
- 01
Is anything stopping the door closing?
A package pushed to the back of a drawer, a bin that does not sit flat on 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, with the unit switched off. What desert dust does to a condenser.
- 05
Is the drain tube frozen?
Sub-Zero names a frozen freezer drain tube among the causes of frost build-up. It is the path the meltwater takes after every defrost, and if it is frozen, Sub-Zero’s instruction is to clean it.
- 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 relative humidity among the causes of excess moisture, frost and condensation in a refrigerator, and its freezer answer notes that condensation after a door opening shows more quickly in warm climates. Phoenix supplies the humidity on a schedule: the monsoon runs from June 15 to September 30. For those months the air a door opening lets in is carrying far more water than the same door in February, and much 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 can appear 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 aluminum 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.
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. Put towels at the base for the meltwater, and add no heat: no hair dryer, heat gun or steamer. GE’s current Monogram manuals, written for models that hold flammable isobutane, allow no electrical device for defrosting.
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. What we service on freezers.
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, each defrost running up to thirty-five minutes and, by GE’s figure, under sixteen on average. Sub-Zero runs the refrigerator off-cycle and the freezer on the first adaptive cycle after twelve hours of compressor run.
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 several different intervals across its own answers; these three bear on frost. 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 aluminum 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 can appear because a sensor failed weeks earlier.
Sources
- Sub-Zero, ice or frost build-up in the freezer: the causes it names, among them a frozen drain tube, the gasket inspection, the 0°F setting, the three-month condenser interval, the ice bin, condensation showing more quickly in warm climates, and the twenty-four hour wait.
- Sub-Zero, refrigerator has excessive moisture, frost, or condensation: high ambient temperatures or high relative humidity among the causes.
- 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, yearly maintenance: cleaning the condenser every three to six months, described as the only general maintenance Sub-Zero recommends. Its ice maker answer gives a third interval for the same job, six to twelve months.
- GE Consumer Home Services technical service guide 31-9091 (2002) for Monogram side-by-side refrigerators: at most sixty hours between defrosts with no door openings, each defrost up to thirty-five minutes and under sixteen on average, ended by the evaporator thermistor at 38°F, the over-temperature disc at 60°F and 45°F as the safety cutoff, and a control that cannot tell a broken heater from an open disc.
- Viking Preferred Service manual SMR-0022 for the built-in all-refrigerator VCRB5301 / VCRB5361, May 2012: an all-refrigerator that has no defrost system, which is why Viking is not in the comparison.
- National Weather Service Phoenix, monsoon awareness: the Phoenix monsoon season, June 15 to September 30.
Written from these sources and reviewed on (checked again ) 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
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- Refrigerator parts, 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.
- A door that will not close, and what it lets inSub-Zero’s order: food and shelves, the gasket, the hinge, then level. Designer hinges do not adjust. Who may do what, and what an open door costs.
Ice keeps coming back?
Tell us the make, where the ice is forming and what the display says. We answer every day, 7am–6pm.