Adjust your Freezer Thermostat

In almost every list of “what you can do to reduce your environmental impact” you will find the item “lower your thermostat setting”. This of course refers to the thermostat that controls the temperature of your home. However, there are other thermostats in your home that rarely make it onto such lists but have an equally significant impact. The remainder of this article deals with adjusting fridge, freezer, and deep freeze thermostats.

Measuring the current temperature

Using a digital indoor/outdoor thermometer (with remote temperature probe) it is a fairly simple matter to determine the temperature inside your fridge or freezer. Place the probe inside, next to something with a relatively high thermal mass (like a jug of water, or a frozen roast). Place the thermometer display outside where it can easily be read without opening the fridge/freezer door. Close the door and wait several hours for the temperature to stabilize before taking the reading.

I determined that my appliances were set as follows:

Fridge +1.8°C
Freezer -22.5°C
Deep Freeze -21.2°C

Permanent temperature monitoring

In the interest of being more aware of what’s going on inside my appliances I decided to install permanent digital temperature displays. The most economical temperature display I could find with an external temperature probe was an aquarium thermometer. The specific model I purchased is manufactured by a company called Coralife. I purchased 4 Coralife digital thermometers for $10 CAD ea from Mail Order Pet Supplies in Ontario, Canada. There were Cheaper sources in the US, but I live in Canada and I wanted to avoid any customs issues or higher shipping fees. My total including shipping and taxes was $49.72 and the thermometers arrived within a week.

To keep my fridge and freezers well sealed where the temperature probe wire entered, I installed a thin strip of foam tape over the wire. My fridge door had a rough surface that the included suction cup would not stick to, so I glued fridge magnet material to the back of the display. Todo: post pictures of thermometer installation.

Recommended temperatures

I found recommended fridge and freezer temperatures on several different websites, all of which agreed on roughly the same numbers. The recommended fridge temperature is from 2 to 5°C (34 to 40°F). A fridge does not stop bacterial growth, so the temperature chosen only affects the rate of bacterial growth. The lower the temperature, the longer it will take for your food to spoil. If you don’t typically leave food in your fridge very long, and aren’t particularly fond of cold drinks, you may safely raise the temperature above the recommended value.

The recommended freezer temperature is -18°C (0°F). All of the sites I found claimed that this is the temperature at which bacterial growth stops. Most sites noted that freezing doesn’t actually kill bacteria but just stops its growth/reproduction. I found it to be an odd coincidence that bacteria would stop growing at exactly 0°F. More on that later.

Adjusting the temperature

The hardest part of adjusting the thermostat settings is finding the adjustment dial. On my deep freeze, I found it on the outside, near the floor on the bottom left side. In my freezer, it was on the inside at the very back. In my fridge, it was at the very front, at the top.

Changing the thermostat settings is simply a matter of making an adjustment to the dial, monitoring the temperature with a thermometer until it stabilizes (possibly a day or more for a good deep freeze), and repeating the adjustment until the desired temperature is reached.

Consequences of raising the temperature

Given such an overwhelming agreement from different sources about the recommended freezer temperature, I was ready to take it as a gospel that keeping my freezer temperature below -18°C was the only way to safely store food for extended periods. But I’m not so easily convinced, so I decided to find out exactly what bacteria existed that could survive and reproduce down to -18°C. Not too much to my surprise, I couldn’t find any.

Image:Bacterial_growth_rate_vs_temperature.png

Bacteria are classified into one of five groups based on the temperatures at which they thrive. These groups are psychrophiles, psychrotrophs, mesophiles, thermophiles and hyperthermophiles. The table at right shows the growth rate vs temperature for each group.

It seems that -10°C is about the lowest temperature at which bacteria experience significant growth. So why is it that the recommended freezer temperature is -18°C? I couldn’t find this information anywhere, but some ideas I have are:

  1. 0°F just seemed like a nice round number to use that was well below the temperature required to halt bacterial growth.
  2. It might provide quicker recovery from defrost cycles. Most modern freezers are “frost free” which means they periodically heat up the “cooling coils” to prevent frost from forming on them. By keeping the contents of the freezer at -18°C it’s less likely the temperature of the contents will increase above -10°C during the defrost cycle.
  3. It might provide a margin of safety should the power go out, or should the performance of the appliance degrade. In this case you would have more time to notice the problem before the temperature increased too much.
  4. It might account for varying temperature throughout the freezer if you just happened to measure the temperature at the coldest spot. When measuring the temperature of my appliances, I tried different locations and took my final reading from the warmest spot.

My freezer has a defrost cycle, but my deep freeze does not. A better way to increase tolerance of defrost cycles and provide a margin of safety for power outages is to keep a larger thermal mass in the freezer (for example, some frozen containers of water). Contrary to intuition, having a greater volume of stuff in your freezer does not cause the freezer to consume more power (except during initial cooling). In general, it would appear that the consequences of raising the freezer temperature from the recommended -18°C to -10°C (at the warmest spot) are low.

Benefits of raising the temperature

Using a Kill-A-Watt Meter I measured the energy consumption of my appliances over a period of several days at different temperature settings. Dividing the energy consumed, by the time to consume that energy gives the average power usage of the appliance. My findings were:

Appliance T Before Power Before T After Power After
Deep Freeze -21.2°C 210 Watts -9.9°C 98 Watts
Fridge/Freezer +1.8/-22.5°C ??? Watts (to do) +4.2/-14.5°C ??? Watts (to do)

Where I live, electricity costs about $0.07 per kWh or $61 per year per 100 Watts.

Conclusions

  1. You can raise the temperature of your refrigerator above the recommended maximum of 5°C. The only consequences are that your food will be warmer and it will spoil faster. If you’re not particularly attached to cold drinks and don’t tend to leave food in the fridge very long, consider raising the temperature.
  2. If your freezer does not have an automatic defrost cycle, you can probably safely raise its temperature as high as -10°C (in spite of the recommended -18°C) without fear of your food spoiling any faster.
  3. If your freezer is “frost free” (ie it has a defrost cycle), you can still probably raise the freezer temperature as high as -10°C, but you may wish to place some additional thermal mass (ex frozen jugs of water) in the freezer. This will prevent the defrost cycle from raising the temperature of your freezer’s contents significantly.

Afterthoughts

When I wrote this article in July of 2007 I adjusted both my deep freeze and my freezer to around -10°C, and my fridge to 4°C. As of July, 2008 the only noticeable difference I’ve experienced is that my ice cream is no longer rock solid. It’s by no means over-soft either. It’s now just right.

One interesting thing to note is that if you heat with electricity, then an electric fridge/freezer located in a heated space effectively costs nothing to operate during the heating season. All the electricity that your appliances consume ends up as heat. Therefore, during the heating season, there is nothing do be gained by reducing the power consumption of your appliances unless they are located in spaces you don’t want to heat. My deep freeze is located in an unheated garage. To achieve better energy savings, I plan to move it inside so that the ~100 Watts of power it consumes to keep my food cold will also keep my house warm. The heating season here is about 8 months and I don’t have air conditioning. For those that do have air conditioning, it may make more sense to place heat producing appliances in spaces that aren’t air conditioned, depending on the relative lengths of your heating and cooling seasons.

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There are 14 comments on “Adjust your Freezer Thermostat”

  1. Jason Melnychuk said ... 2009-03-20 at 3:55 pm

    Thanks for the info, I found it useful to make the decision that I can safely raise the temperature of my fridge. In the after thoughts, the idea that in the heating season, the fridge is actually helping to heat the space may be true, but it is using electricity to do this instead of natural gas. Besides being cheaper to heat with natural gas, if you like most north americans is using electricity from coal fired power generation, that electricity represents about 3 times the carbon dioxide emissions then if you were heating with natural gas, on a joule for joule comparison.

    Also, the deep freeze located in the garage will consume less energy during the heating season. If you’re making the argument to save money, it makes more sense to consume less electricity by leaving it in the garage and use a little more natural gas to heat your home.

    A great book that you may find helpful is Carbonbusting for fun and profit, written by a local (to me) Edmontonian. I hope this is useful information to you.

    Cheers,

    Jason

  2. Anand Dhuru said ... 2009-05-19 at 9:12 am

    Great article! I feel vindicated; for years, I’d been suspecting that a badly set fridge was far more responsible for inflated bills than all the incandescent lamps that could be replaced with CFLs. I made up a simple Excel spreadsheet listing all the loads in my house, and drew a pie chart, which again pretty much confirmed my beliefs.

    The actual temperature-figures you have mentioned would certainly help me in setting the fridge to a more practical level, and significantly reduce consumption.

    Regards,

    Anand

  3. Rob Steves said ... 2009-05-19 at 3:11 pm

    Hi Jason,
    While I agree the savings would not be as great for someone who heats with natural gas (or other combustible fuel), to move their freezer inside, I suspect the savings would still be positive. Let me illustrate why.

    Where I live gas costs about $0.05/kWh and electricity costs about $0.07/kWh. Assume a gas heating efficiency of 90% (optimistic since most furnaces are less than 90% efficient). Suppose you have a freezer that consumes 100W if located inside during the heating season. Moving it outside will therefore cost you 100/0.9 = 111W of additional natural gas consumption. Given the relative cost of gas vs electricity, you would need to save at least 79W of electricity to make it worthwhile. So unless moving the freezer outside reduces its electrical consumption to 21W or less (a 79% reduction), you are better off keeping the freezer inside. I think that kind of reduction is very unlikely.

    To conclude, generally speaking, no matter what method you use to heat your home, you are likely to save money by having your freezer (or any other electrical appliances) operate inside your home rather than outside during your heating season. However, the reverse is true during the cooling season if you use air conditioning.

  4. Rowland Spear said ... 2009-07-13 at 11:18 am

    Very informative article! And that you are still around is proof of the safety! :-)

  5. Rob said ... 2009-09-04 at 7:56 pm

    Moving your freezer outside will increase the effeciency at which it runs. If the temperature outside is very low you can get better than 100% efficency (when the temperature is -20C) the freezer will consume 0 watts, in fact if the food cools to below 20deg C the freezer will also consume 0 watts as the food warms up to 20C on a warmer day. Another thing to consider, is the temperature in all of the cases mentioned is more like a sine wave which has a high and low value of a couple degrees up and down and your thermometer give you the average reading. So, if it is important to keep your food below a certain temperature you might want to consider that. Also the temperature in the middle of the fridge might be cooler than near the walls. Every time you open the door fills the fridge with warm air (an empty fidge can take half a day to actually cool down the warm air), but if you have mass inside the fridge the recovery time is faster. So its not so simple after all.

  6. Rob said ... 2009-09-08 at 12:39 am

    Hi Rob,
    If you are heating your home with electricity, then you effectively consume 0 additional Watts of power by running a freezer inside. Any energy it consumes is released into the home in the form of heat which simply offsets the energy your furnace or heaters would otherwise consume. You can’t get any more efficient than 0W consumed, so unless moving the freezer outside will cause it to generate power instead of consuming it (hint: it won’t), it’s more efficient to keep it inside.

    If you heat with something other than electricity, the comparison is not as simple. In the example I gave in an earlier comment, I showed that for heating with a 90% efficient natural gas furnace and assuming prices similar to where I live, the freezer would need to see a 79% reduction in electrical consumption for outside operation to cost less than inside operation. I said above that this kind of reduction is unlikely, but it is possible depending where you live. Assuming your freezer thermostat is set for -20C and assuming energy consumption roughly linear with temperature difference between inside and outside the freezer, you could see a 79% reduction in energy consumption if the average outside temperature during your heating season is below -12C. Where I live, the average outside temperature during my heating season is closer to 0C so I’d be better off keeping my deep freeze inside, even if I heated with natural gas.

    In so much as a thermometer sees the average temperature inside a fridge, so does your food. Most food has a much greater thermal mass than a thermometer, so the temperature of the food will fluctuate even less than that of the thermometer. Air temperature in the fridge will fluctuate most (it must, or the thermostat would not trigger the compressor to turn on and off) but the temperature of the food stays relatively constant (the same as the temperature indicated by the thermometer). Foods of low thermal mass (loosely packed, leafy plants for example) will experience the greatest temperature fluctuation.

    An empty fridge should not take half a day to cool down warm air that is allowed to enter it. The specific heat capacity of air is 1297 J/(m^3 degC). A large fridge might contain 1 m^3 of air. Assume the entire volume of air is replaced with room air at 22C and the air must be cooled by 20 degrees to 2C. The amount of energy to be removed is therefore 1297*20 J (Joules). A Watt is a J/s (Joule per second). Assume the fridge consumes 100W and has a COP (coefficient of performance) of 1 (very conservative). Therefore 100 J are removed each second and it will only take a little over 4 minutes to cool down the warm air. It is a different story if the entire fridge starts at room temperature. Then parts of the fridge must be cooled down in addition to the air so it will take more time, but probably still much less than half a day.

  7. Chandrakant said ... 2011-04-24 at 12:14 am

    Can I use my fridge as a Aircondtionar. Is it economical?

  8. Rob said ... 2011-05-02 at 9:01 pm

    Hi Chandrakant,
    You could use your refrigerator as an air conditioner but to do so you would have to put it in a doorway or cut a hole in a wall so the coils on the back of the fridge are outside. A fridge is a heat pump. It moves heat from inside it and released that heat through the coils on the outside. If the fridge is inside your home and you simply leave the door open, it will heat your home rather than cool it.

  9. Charles Barnard said ... 2011-06-13 at 2:00 am

    “One interesting thing to note is that if you heat with electricity, then an electric fridge/freezer located in a heated space effectively costs nothing to operate during the heating season.”

    Not so! Oh wise one. You are paying to heat the inside as heat seeps in and then paying to cool it down again.

    The lower maximum of -18C is probably safety factor to some extent–neither thermostats nor thermometers are noted for their accuracy–thermometers which are readable to .1 degree often have an uncertainty of up to 1 degree. And the actual reading may be far from that–take a look at all of th thermometers at a store display and you’ll see how inaccurate they are…lab thermometers are often only good to +-.5 to 1 degree.

    Bacteria are not the only reason to refrigerate!

    Food loses quality for chemical and physical reasons too.

    For example, ice cream needs to be kept below -20F (-29C) if it is to retain it’s polycrystalline structure (which affects mouth feel.) The colder you keep things the slower any chemical reactions will happen, and they can alter the flavor or nutrient values.

    Better insulation combined with a chest style opening (which can be obtained by converting a chest freezer to a refrigerator–which may be as easy as adjusting the thermostat.)

    Even a relatively inefficient chest freezer will run much more efficiently as a refrigerator than the best cabinet door refrigerators.

    Best for all purposes are the under cabinet drawer units, which combine the convenience of the front opening with the efficiency of the chest.

    But drawer units are rare and expensive. Cheap chest freezers abound. If you can afford the space, a larger chest refrigerator can be adapted to have things like pressure strut lifts to lift the top foods above the edge of the chest to work on the bottom foods. If you can find a cheap used unit, you can play with such things until you decide what you like best–you could even modify a chest to have drawers, though it would take some fair handyman skills to do, it’s not really complex,but like most things, it will take some time.

    Thermostats often have two adjustments, one is the knob you can see, the other is a screw which adjusts the contact spacing.

    When working with a conversion, find out what the highest temperature the freezer will hold at–if it 34F or above, you really shouldn’t need to change anything. Just measure the temperature at the bottom to see the difference–you should be able to store warmer holding freezer items in the bottom of the chest.

    Ideally you have multiple compartments and use small fans to move cooler air in as needed–allowing individual temperature chambers. These thermostats are much cheaper because the load for a fan is quite low compared with a compressor.

  10. Rob said ... 2011-06-18 at 9:44 am

    Hi Charles,

    You said “Not so! Oh wise one. You are paying to heat the inside as heat seeps in and then paying to cool it down again.”

    It is so. This is the law of conservation of energy. It’s basic highschool physics and I am frankly surprised at the number of people who are disputing it. ALL the electricity you put into a fridge is effectively released as heat into your home. The fact you are pumping heat around at the same time is completely irrelevant. The net result MUST be heat generation equivalent to the electrical consumption. If your fridge draws 100W of electrical power on average, then on average it outputs 100W of heat. Where do you suppose the energy goes otherwise? As far as your house is concerned, a fridge (or any electrical appliance) is just an electric heater. If you heat your home with electricity, it makes little difference to your cost whether you do it with heat from a refrigerator, light bulbs, toaster, or any other electrical appliance.

  11. dominic said ... 2011-12-12 at 8:28 am

    @Rob: I totally dont understand the confusion with the law of conservation of mass and energy either. Its very simple. You use the electricity to run the refrigerator to transfer latent heat inside it (food) to outside and the electricity consumed also eventually transform into heat transmitted to the outside of the appliance (from electric energy to kinetic energy to friction to heat and static to heat) becoming a more than efficient 2-in-1 heater (because youre also transmitting heat from one source to another using latent heat).

    enough babble. i bet somebody’s confused. in any sense, i put my computer on to download non-stop over the course of winter to function normally while heating my house to a nice comfy temperature without ever feeling guilty about being green. On that note, people ACTUALLY using heaters are the ones contributing to this greenhouse problem we’re facing.

  12. Harry said ... 2012-02-11 at 1:14 pm

    The suggestion of putting jugs of water in to fill the space in a half empty freezer is not as efficient as filling the gap with something you actually want to freeze. To freeze the water will take a considerable amount of energy which will be completely wasted unless you re-use the frozen water. ie. ice cubes in a large whiskey in the summer.
    The more efficient method would be to use loaves of bread or items with low mass as, if you have a cabinet freezer, the main object is to prevent the loss of cold air when the door is opened. This is less of an issue with a chest freezer as the cold air will stay in the freezer except where disturbed by moving items. If you do not have, or do not want to store, bread then just empty, but closed, cardboard boxes will also do the job of keeping the spare air-space filled with cold air so it won’t need cooling again.

    There are an awful lot of variables that have not been taken into account regarding the inside/outside theories.
    1. When the trial measurements were taken what was the ambient air temperature.
    2. What efficiency figures are you working with, using the freezer as a heater and the alternative boiler/air-con unit? I would wager a modern boiler is massively more efficient.
    3. The opening of the door will cause cold air to enter the room (or garage) and this will need to be warmed again if the freezer is inside. Basic balance sheet stuff and every event is subject to the inefficiencies.
    4. The reverse of item 3. What already used house energy is wasted to the outside world by opening the house/garage door to get items from the outside freezer. The idea of no loss of energy is flawed as you are not in a sealed chamber surrounded by a vacuum.

    Using the same ‘proof’ as this discussion could suggest that leaving the light bulb on, the TV on (not even in standby) and other electrical items as suggested such as toasters on would be saving energy. Wrong but a nice thought.

    I would suggest setting your freezer to the right temperature to be able serve ice-cream easily.
    Instead of taking 1 beer out of the fridge take 2. Invite a friend to drink the other but but make sure you do not leave the house or fridge door open for long when they arrive, if you have the heating on.
    Just be sensible with your/our energy. Use as little as possible but make life comfortable for yourself without feeling guilty.

  13. Rob said ... 2012-02-12 at 11:32 am

    Hi Harry,

    A freezer is a heat pump. If you are heating your home (especially if you are heating with electricity), and your freezer is inside your home, it doesn’t matter what you use to take up extra space. Freezing it does not “consume” energy… it converts the energy into heat and releases it into your home. There is no “wasted” energy if you desire to heat your home anyway.

    As silly as it sounds in practice, during your heating season, it would be a very efficient process to freeze water daily, inside your home, dumping the previous day’s ice outside. Such a process would effectively be more than 100% efficient since the energy transfered into your home would be the electrical energy “consumed” PLUS the heat of fusion of the water (about 0.1 kWh per kg). I suggest this only to illustrate a point. I don’t expect anyone to do it.

    Far from being “completely wasted” as you suggest, energy put towards freezing water in this manner has actually been used more efficiently than if you had “consumed” the same energy in an electric heater, or a gas boiler, which by definition can only achieve a maximum of 100% efficiency.

    My argument is not that leaving a light bulb, TV or toaster on would “save” energy. My argument is that, if you are heating your home with electricity anyway, leaving these appliances on will not “waste” any additional energy. These appliances heat your home just as efficiently as any electric baseboard heater. But moving any of these appliances outside clearly does “waste” the energy since it no longer heats your home. This is just as true for a freezer as it is for a light bulb, but it isn’t quite as obvious because the freezer may consume less power outside.

    Suppose you require 2000 watts to heat your home. Suppose your freezer consumes 100 watts (and thus releases 100W of heat) on average if you keep it inside. Suppose its consumption drops to X watts if you move it outside. So with the freezer inside, you need to produce an additional 1900 watts to heat your home for a total of 1900+100 = 2000 watts consumed. With your freezer outside, you need to produce all 2000 watts to heat your home, plus the X watts to run your freezer. Total consumption is 2000+X watts. I hope you can see that there is NO WAY that moving your freezer outside can reduce your total energy consumption, unless X is negative (ie your freezer starts to generate electricity instead of consume it).

    About relative efficiencies of heating with electrical appliances a gas boiler: ALL electrical appliances are at least 100% efficient heaters by definition. 100W of electrical input power always results in at least 100W of heat output into your home (more in the case of a heat pump that removes heat from a source outside the home and releases the heat inside). Any combustion based heating method (such as a gas boiler) is less than 100% efficient by definition since there will invariably be some heat left in the exhaust. That is a moot point though, since it is not “efficiency”, but “cost” that most people are concerned about.

    Going back to my above example, if you are heating with gas, it *might* be less expensive to have your freezer outside. It depends which is cheaper:
    Scenario 1 – freezer outside: 2000 watts (gas) + X watts (electric)
    Scenario 2 – freezer inside: 1900 watts (gas) + 100 watts (electric)
    Where I live prices are such that it’s very unlikely Scenario 1 would be cheaper. See my comment from 2009-05-19 for a more detailed example illustrating why.

    You suggested that I didn’t account for the heat energy required to “rewarm” cold air that is expelled from a fridge or freezer each time you open it. However, that heat is exactly balanced by the heat energy you extracted from that air by having it in the fridge or freezer in the first place. There is no need to include this in the calculation as it is a net zero sum.

    I agree with your argument about energy loss from opening doors to access a freezer located outside. It is an additional factor I did not consider, but it only pushes the argument further in favor of keeping the freezer inside.

    All my arguments above assume operation during the heating season. If you have a “cooling season” during which you use an air conditioner, then there IS an advantage to having your freezer outside at this time of year. If you don’t want to move your freezer back and forth each year, you’d have to consider which makes the most sense for a permanent location based on the relative lengths of your heating and cooling seasons.

  14. Mike54UK said ... 2013-02-19 at 2:19 pm

    This page has been a most interesting read and I am very happy to have accidentally come upon this site whilst trying to find some specifications on a particular freezer thermostat so that I could safely replace it with a cheaper one.
    Although my brain is beginning to overheat from struggling to keep up with all the arguments I would like to add a few points which occurred to me as I read.
    1. When you open the fridge or freezer door the cold air that drops out has to be heated back up by one or more of your appliances and you can’t easily choose which one. Or perhaps by heat lost from your body so that you have to eat more to compensate and depending on how and where your food is grown ..etc. etc.
    2. As I think was briefly mentioned before, you might get 100w at your home but a lot more than 100w worth of fuel had to be burnt to produce, maintain availability of and transmit that 100w of power to your home. Even if you’re using an alternative energy source, that probably remains true since it took a lot of energy to manufacture and install your generating equipment.
    3. Another factor in favour of turning down refrigeration stats is the reduced wear and tear on the appliances so they live longer. (Which has to be balanced against your having to go shopping or to your allotment more often if the food doesn’t keep fresh for as long. And in order to do so opening the door of the house and letting in the cold air etc. etc. Also against a very slim possibility that your own lifespan may be reduced should you get your temperature settings too far out. But then that might actually benefit the long term stability of the Earth’s climate.)

    I have bookmarked the site and will soon return to find out what else you get up to here.

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