Adjust your Freezer Thermostat2008-07-06 by Rob
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:
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.
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 5Â°C (34 to 40Â°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 -18Â°C (0Â°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 0Â°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 -18Â°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 -18Â°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 -10Â°C is about the lowest temperature at which bacteria experience significant growth. So why is it that the recommended freezer temperature is -18Â°C? I couldn’t find this information anywhere, but some ideas I have are:
- 0Â°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 -18Â°C it’s less likely the temperature of the contents will increase above -10Â°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 -18Â°C to -10Â°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||-21.2Â°C||210 Watts||-9.9Â°C||98 Watts|
|Fridge/Freezer||+1.8/-22.5Â°C||??? Watts (to do)||+4.2/-14.5Â°C||??? Watts (to do)|
Where I live, electricity costs about $0.07 per kWh or $61 per year per 100 Watts.
- You can raise the temperature of your refrigerator above the recommended maximum of 5Â°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 -10Â°C (in spite of the recommended -18Â°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 -10Â°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.
When I wrote this article in July of 2007 I adjusted both my deep freeze and my freezer to around -10Â°C, and my fridge to 4Â°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.