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I equipped my fridge-freezer (Bosch KGN 33X48/13, bought around 2013) with a self-made temperature monitoring system to warn me whenever the internal temperature of the freezer rises above a certain level.

Looking at the resulting curve I'd expect the temperature to more or less steadily swing around a target value.

What I rather see is this:

freezer temperature course

So the built-in thermostat switches on the cooling unit whenever the internal temperature rises above around 18 °F. However, quite consistently about every 14 hours the cooling unit appears to hold off a bit, allowing the temperature to rise up to 23 °F for a short time. After that the cycle continues normally again.

These short temperature spikes look so equably and intentional that I assume a purpose. Why would the freezer designers arrange for this small temperature increase every 14 hours?

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    some kind of anti-frost measure is my first guess, my second is that something needs to be recalibrated every so often. – ratchet freak Nov 8 '17 at 10:06
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    Incidentally, your freezer is set far to the warm side of recommendations from the USDA. "Check the temperature of your refrigerator and freezer with an appliance thermometer. The refrigerator should be at 40 °F or below and the freezer at 0 °F or below." And yes, it's normal automatic defrost cycling. – Ecnerwal Nov 8 '17 at 14:51
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    Super cool data by the way. +1 – GManNickG Nov 8 '17 at 19:36
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    Your temperature monitor might happen to be placed in an area close to where the cold air comes out or where the heat tape is, causing it not to accurately reflect the air temperature in the freezer. Also, as Ecnerwal said, your freezer is set way too warm. – David Schwartz Nov 8 '17 at 19:38
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    @JonH RRDtool surely. – jscott Nov 9 '17 at 1:30
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Every Freezer including yours has to have a defrost cycle designed in or the cooling coils become so encrusted with frost the recirculation air can no longer pass over them. Then everything goes downhill.

So the engineers design for and install an element between the coils, Control it with a timer and turn it on periodically to eliminate the frost build-up. They operate this element for the shortest possible time span to melt the frost build-up and at the same time affecting to overall box temp the least.

What you are seeing on the graph is that system working.

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    And here I always thought that the defrost function was to remove frost from the part of the freezer where the food is, not (only) in the cooling coils. Is it just coincidental that this also prevents frost in the box portion, without actually warming up the contents? – Michael Nov 8 '17 at 17:40
  • @Michael: The change in temperature comes with a change in relative humidity that likely causes tiny amounts of evaporation to take place from the surface of any frost/ice that has developed. – R.. Nov 8 '17 at 18:35
  • @Michael, the coils are the coldest part of the freezer, so frost build-up starts there. – Mark Nov 8 '17 at 22:26
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    "Every" is incorrect; some freezers (particularly chest freezers) do not have this cycle, and have to be periodically defrosted by hand (perhaps months or years depending on ambient humidity and the care you take with them). – Joe Nov 8 '17 at 22:26
  • Do any use a reverse cycle instead of a heating element? – DaveInCaz Feb 8 '18 at 9:57
102

It is defrosting.

Frost free refrigeration equipment goes through a defrost cycle every few hours. This normally involves turning off the cooling compressor and activating a heat tape that is attached to the evaporator coil for about 20 minutes. This melts any frost and ice that has accumulated on the evaporator coil and keeps it from accumulating.

It is completely normal to the operation of modern frost free equipment.

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    FWIW, it is normal, but does contribute to food spoilage, dehydration, development of smells, etc. Keeping items that won't be consumed quickly well-sealed and well-insulated (e.g. inside cardboard boxes they're sold in) rather than in the open and exposed directly to the air in the freezer will help mitigate these things. – R.. Nov 8 '17 at 18:33
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    I believe also keeping the freezer relatively full (just throw water bottles in there if needed) tends to keep the food cooler, due to increased thermal mass. i.e. the heat spreads itself thinner. – Scott Nov 9 '17 at 3:47
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    This is why you vacuum-seal anything you want to keep for long periods of time, and why some frozen foods (especially frozen meat) comes vacuum sealed from the factory. – user4302 Nov 9 '17 at 4:49
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    @ThreePhaseEel - that's why God invented hair dryers. :-) (Seriously - works like a charm, and no chiseling). – Bob Jarvis Nov 9 '17 at 10:14
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    @ratchetfreak in case you've got still frozen food you want to place back inside without defrosting – Pureferret Nov 9 '17 at 17:39
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The other answers are correct that you are seeing the normal defrost cycle of your freezer. I'd just like to add two additional observations:

1) There are commonly two kinds of defrost timers: mechanical and adaptive. Your fridge seems to have a mechanical defrost timer due to its regular 14hr cycle regardless of usage. That means it's often wasting energy defrosting when it doesn't have to.

Adaptive defrosters OTOH try to be more efficient and not run so frequently. They have a computer monitor things like how often the doors have been open since last defrost, how long the compressor has been on since last defrost, how long the heater was on during the last defrost, etc.

(Unfortunately, that's not how it necessarily works out in practice. For my GE fridge, at least, the circuit board has been through many revisions and still fails quite often...and it's darn expensive to replace, too, compared to a simple timer! It's a neat idea, but there's clearly more work to be done in this area. It doesn't take too many CPU - and food - replacements to outweight the minor savings of the defrost cycling running a little more often IMHO.)

2) It looks like you're directly measuring the air temperature in the freezer, so you're catching every little fluctuation. The actual food in your fridge isn't changing so rapidly, so more accurate temperature monitoring for things like food, vaccines, etc. usually involve a probe with a temperature buffer of some kind to better reflect the slower temperature changes that the items in the freezer are actually experiencing (e.g. a probe bottle filled with glycol). And while the claim that a fuller freezer is more energy efficient than an emptier one is still a bit controversial, from a temperature stability standpoint, a fuller freezer should have a more stable temperature, too.

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    I'd just add that if the freezer is sitting unopened for an extended period of time, I'd expect an adaptive defroster's resulting schedule to be just as regular as a mechanical defroster. – JakeRobb Nov 9 '17 at 16:47
  • Very good answers here. I'd like to add that around 80% of refrigerators in the garbage just have a broken defrost component. I would often take them out of the garbage and repair them for an average of 30 bucks and sell them. Sometime for over a thousand dollars. I work in rich neighbourhoods and those people just throw away everything as soon as it shows any sign of failure. – Joe Fala Feb 19 at 12:25

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