Fridge and freezer temperatures

[CassidyNZ]

Having just “commissioned” my new freezer unit in an existing enclosure, I’ve added electronic controllers to manage the compressors in both the fridge and the freezer (identical compressor units) and it got me wondering where I should set the on/off temperatures.

I’ve always run the fridge at around +4C with the manual thermostat. Now I’ve set the compressor off at +3C and back on at +6C and I’m OK with that. My beers, wine and soft drinks seem to lose a lot of flavour when they’re really cold and food has always survived well between these temps so I recon that’s still good.

But Google appears to indicate the freezer should run at -20C (zero F). I’m thinking this is too cold and could see the compressor running for longer-than-I-want periods. I’ve set mine to turn the compressor on at -5C and off at -10C. The compressor runs about 40% of the time.

I don’t consider our freezer to be long-term cold storage and my perhaps skewed thinking is that for weeks rather than years, frozen is frozen. Minus 5 or colder should be OK.

What is consensus amongst those who live aboard with freezers?

Oh, the compressor units are Isotherm 12v units.

[tkeithlu]

US Food and Drug Admistration standards are 40 F for refrigerating foods and 0 F for freezing them. Restaurants that don't adhere to that get in trouble. Also, no re-freezing. Those are tough anti-food poisoning standards - we adhere to a basic 48 hour rule on anything refrigerated. Part of the problem is exposing the food briefly to a higher temperature and then putting it back. Apparently, that gives nasty organisms a chance to multiply, even though the food was keep cold nearly all the time. Can you be sure that your -5 C will not creep across to +1 when you open the lid/door?

[billknny]

In most cases with marine refrigerators, the temperature sensor is actually mounted right ON the evaporator plate. That is always the coldest spot in the entire box.

Your freezer settings, especially, will not keep things cold enough. But this is the internet... I am sure you can find someone to tell you it's ok to keep your food warmer than the experts suggest. It sounds like that is what you are looking for.

[a64pilot]

Frozen food should be kept at negative Fahrenheit numbers for long term storage. But that’s tough to do. Our Engel stays at -5 F and that’s about as cold as it will get.
Actually you can use Ice cream as a gauge, if it will keep ice cream hard it’s cold enough, unless your wanting to store for long term.
Our big spillover which is 14 cu ft and is not for long term storage I keep the freezer at +10F and the fridge just above freezing, about 34f. My divider isn’t good enough to keep a larger difference in temp, if I drive the freezer colder things begin to freeze in the fridge.

[john61ct]

Originally Posted by a64pilot

My divider isn’t good enough to keep a larger difference in temp, if I drive the freezer colder things begin to freeze in the fridge.

Aren't there ways to design the box to prevent that?

I would also like a movable divider, as the freezer contents get used up start to keep drinks cold (usually a luxury)

[CassidyNZ]

Originally Posted by tkeithlu

Can you be sure that your -5 C will not creep across to +1 when you open the lid/door?

Yep, I think I can. Deep cavity into which I’ve placed 15 litres of water in bottles, now frozen solid. Halfway down the shelf on which contents live. I worked inside repositioning stuff (sensors, shelves), top door was open for maybe 7 or 8 minutes. Temp went up from -7C to -3C and recovery was surprisingly quick. It would be unusual to spend seven minutes just taking out or putting in.

I guess the biggest issue would be putting, say, 5kgs of freshly caught tuna into the freezer but I’m not sure the temp rise would be more than 4 or 5 degrees. That’ll have to tested on our next trip out to sea.

[CassidyNZ]

Originally Posted by billknny

In most cases with marine refrigerators, the temperature sensor is actually mounted right ON the evaporator plate. That is always the coldest spot in the entire box.

Your freezer settings, especially, will not keep things cold enough. But this is the internet... I am sure you can find someone to tell you it's ok to keep your food warmer than the experts suggest. It sounds like that is what you are looking for.

No, that’s not what I’m looking for.

I want to know what you (and others) actually do, not what is recommended by the FDA and perhaps some gauge as to how that affects cycle times on the compressor. Experts expose themselves not only to criticism but also litigation when they get it wrong so over-compensation by them is not unheard of.

My temp sensors are mounted in/on expanded metal screens about 10mm from the evaporator surface as recommended by the maker. They sense air temp, not metal temp.

[Neptune's Gear]

Icecream needs min -15c. Most freezers are about -18c. Mine turns off at -21 and on again and -16c. If it is a eutectic type freezer, the fluid in the tank should be fully frozen at switch off, and liquid at switch on. This change of state will ensure maximum hold over periods between runs. There is a huge amount more energy stored in the change of state than in the temp change of either a liquid or a solid...

[a64pilot]

Originally Posted by john61ct

Aren't there ways to design the box to prevent that?

I would also like a movable divider, as the freezer contents get used up start to keep drinks cold (usually a luxury)

Of course, mine needs to be thicker, more R value and maybe taller.
However maybe a smaller fan would do it too, the hole is still there even with the fan off. I started out with just a hole no fan, but if you want to keep precise temps that takes a thermostatically controlled fan. Now that I think about it, I bet a flap that the fan would blow open but it closes when the fan is off would do the trick.
But what I have works well enough and as the Wx gets hotter, the issue is less, and you can’t really hold close to zero with my 30 yr old insulation and a 14 cu ft box with a single bd80 anyway without a 100% duty cycle.
So honestly the best answer is to rip the whole thing out and build a new box, and the pay back isn’t worth that to me, way too much work.
Want I do is pack the Engle solid with steaks vacuum packed to make the package as small as possible, well steaks and hamburger meat.
The chicken goes in the regular freezer with odds and ends stuff that gets frozen, we even keep rum in there because when you make frozen Daiquiri’s it melts less ice if you start with below freezing rum.

Then we eat our way to the bottom, I didn’t find decent meat in the Bahamas, but I didn’t go to the big towns either. Meat was very low quality.

[john61ct]

Drinks tip

freeze the non alcoholic ingredients in a ziploc to save space use that for your ice

[CassidyNZ]

Originally Posted by Neptune's Gear

Icecream needs min -15c. Most freezers are about -18c. Mine turns off at -21 and on again and -16c. If it is a eutectic type freezer, the fluid in the tank should be fully frozen at switch off, and liquid at switch on. This change of state will ensure maximum hold over periods between runs. There is a huge amount more energy stored in the change of state than in the temp change of either a liquid or a solid...

Thanks for that Matt.

Mine is not eutectic, has an evaporator so it’s all about the cold stored in the box and it’s contents.

Is it reasonable to assume that switching on at -16C and off at -21C provides more residual cold in the box/contents than my thinking (original post) and so cycle times don’t necessarily get longer just because it’s colder? Perhaps I’m being too pessimistic about the compressor running longer.

I should get the freezer full of frozen product at those temps and test.

[a64pilot]

Originally Posted by CassidyNZ

Thanks for that Matt.



Mine is not eutectic, has an evaporator so it’s all about the cold stored in the box and it’s contents.



Is it reasonable to assume that switching on at -16C and off at -21C provides more residual cold in the box/contents than my thinking (original post) and so cycle times don’t necessarily get longer just because it’s colder? Perhaps I’m being too pessimistic about the compressor running longer.



I should get the freezer full of frozen product at those temps and test.

No, most absolutely the colder you keep it, the higher the duty cycle will be and the more power you will consume, and it can be quite a dramatic increase, but not always, your box’s R value and evaporator size play into that big time.
One very big reason why is that the lower the compressor has to take the refrigerant temp. The lower it’s coefficient of performance is, when you see COP when discussing compressors that’s what COP means.

[a64pilot]

Originally Posted by john61ct

Drinks tip

freeze the non alcoholic ingredients in a ziploc to save space use that for your ice

Yes freezing the mixer would work too. I need to try that.

[Neptune's Gear]

Originally Posted by CassidyNZ

Thanks for that Matt.



Mine is not eutectic, has an evaporator so it’s all about the cold stored in the box and it’s contents.



Is it reasonable to assume that switching on at -16C and off at -21C provides more residual cold in the box/contents than my thinking (original post) and so cycle times don’t necessarily get longer just because it’s colder? Perhaps I’m being too pessimistic about the compressor running longer.



I should get the freezer full of frozen product at those temps and test.

Ok, in that case it’s not doing a change of state - you don’t want the stuff in the freezer to be thawing out. So as warm as you can but as cold as food safety requires will be most efficient. Most sources say -18c (0 f), and, dependent on the efficiency of the cabinet, and the thermal mass inside it won’t be off for long....

[jtsailjt]

Originally Posted by billknny

In most cases with marine refrigerators, the temperature sensor is actually mounted right ON the evaporator plate. That is always the coldest spot in the entire box.



Your freezer settings, especially, will not keep things cold enough. But this is the internet... I am sure you can find someone to tell you it's ok to keep your food warmer than the experts suggest. It sounds like that is what you are looking for.

Why the cynicism? He asked a reasonable question.

Mine (Seafrost DC units) has a temp sensor on the plate and another one away from the plate so I can see both the plate temp as well as how cold the air and food is and I think that’s not unusual.

When I’m on the boat so I can run the generator or engine as needed, and the box is being opened from time to time, I lower the freezer temps. But when I’m leaving the boat for several days and will be depending on the solar panels, I raise the freezer temps (not plate temps they’re about 10F colder) to about 20F- 25F in order to save energy in case it’s cloudy. I know that while I’m gone, the box will not be opened so I don’t need as big a margin. Also, any meat that’s being left aboard is deep in the freezer, where it stays colder than up where the temp sensor is. If I don’t remember to raise the freezer temp when I’ll be away, I’ve noticed that the battery bank is a little run down when I return. I don’t have numbers but there’s a significant difference in the amount of energy needed to keep a box at 10F vs 25F.