Why does my freezer get a little warmer every 14 hours?

Question

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:

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?

Answer 1

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.

Answer 2

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.

Answer 3

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.