Adjust your Freezer Thermostat
2008-07-06 by RobIn 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.
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:
- 0°F just seemed like a nice round number to use that was well below the temperature required to halt bacterial growth.
- 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.
- 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.
- 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
- 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.
- 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.
- 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.
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
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
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.
Very informative article! And that you are still around is proof of the safety!
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.
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.