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Old 08-27-17, 01:37 AM   #1
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Default Chest freezer insulation

I recently bought a used chest freezer. It is larger than we expected (we wanted 110 liters, turned out to be 168 l), so I'll add extra insulation to reduce the extra load. According to the label, the compressor needs 70W, and the unit should use 0.52 kWh/day. Here is the plan to reduce that:
  • The freezer will be in the basement, where the temperature is lower than in the house - now (late summer) it is 16-17C, in the winter it drops to 9-11C.
  • I will remove the 15W light which turns on when the lid is open.
  • There is about 3cm of space between the condenser coils and the rear wall. I want to increase this by 10-15mm, and insulate the rear wall with 5mm of styrofoam with a reflective coating, to keep the heat off (when testing, the coils heated from 16 to 26C in just 2 minutes).
  • The freezer walls are already 85mm thick, but I would stick an extra 3-5cm of styrofoam on the front and sides.
  • The bottom stands 4cm above the floor - should I raise it more, or add insulation?
  • What about the lid and the seal? Are these worth the effort to insulate?
Possibly in the future we may change our tariff so that electricity is cheaper at night, then the freezer could be put on a timer to only work the compressor during cheap hours.

I also had a plan to add a second thermostat for refrigerator temperatures, with a switch allowing me to choose between fridge and freezer mode. This would be for example during holidays, when we have lots of food and barely enough space in our primary fridge. But I'll have to see if this is at all needed.

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Old 08-31-17, 11:33 AM   #2
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Most chest freezers in the US have their coils buried in the sides of the box itself - not a bank of coils with a fan forcing air over them, but the entire outside surface of the freezer radiating heat naturally.

This is a terrible design of course, since it also puts those warm coils right against the very insulation that is supposed to keep heat out of the cold compartment. Dumb.

If your freezer's coils are in a separate compartment, then I would say add all the insulation you can stand and/or afford. This includes adding insulation underneath, so long as air flow to the coils is unimpeded.
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Old 09-12-17, 09:33 AM   #3
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Not sure about the insulation question. You can put some newspaper in there as a heat/cold sink. So that when you open the door you won't loose extra air/energy with your extra unused space.
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Old 09-12-17, 11:28 AM   #4
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Newspapers don't have much heat capacity, but filling empty space with plastic jug of water is what I do. I use large and small jug/bottles, depending on the empty space, and use the bottles of ice to keep things cold while traveling.
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Old 09-12-17, 11:53 AM   #5
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Originally Posted by Piwoslaw View Post
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wow, great set of measures there, Piwoslaw.

It should lower the projected consumption to nearly half. Be sure to brag about the results!
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Old 09-12-17, 09:58 PM   #6
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Not too 'negative nancy' your ideas, but there isn't much you can do to improve on the basic chest freezer. However tinkering can be fun, but any minimal gains will be negated by time/cost of any mods.
[*]I will remove the 15W light which turns on when the lid is open.
> It's a momentary use so little to save. As long as you don't take extra time to find things [longer lid is open, more cooling you lose] or use a different light like an overhead one you wouldn't normally use.
Opening the lid once or twice a day for no more than 10secs will use 15Watts the entire year; it would take 67 years to use 1KwH that costs 6-20 pennies
[*]There is about 3cm of space between the condenser coils and the rear wall. I want to increase this by 10-15mm, and insulate the rear wall with 5mm of styrofoam with a reflective coating, to keep the heat off (when testing, the coils heated from 16 to 26C in just 2 minutes).
> The coils already have a insulated air gap to the rear, another 10-15mm won't markedly change the resistance value. Adding more insulation won't make much difference as far as the coils go; and there isn't really any such thing as reflective coating as far as non-solar heat goes. Manufacturers would already do these things if they were effective.
You could try to enclose the coils in a separate enclosure that vented outside.
[*]The freezer walls are already 85mm thick, but I would stick an extra 3-5cm of styrofoam on the front and sides.
> There is a point of diminishing returns on extra insulation, and of course the end product can be a nightmare.
[*]What about the lid and the seal? Are these worth the effort to insulate?
> Making sure the seal is effective is worthwhile, especially on an old unit.
You can use thick firm magnetic seals, and put weight on the lid to keep it firmly shut.


[]Possibly in the future we may change our tariff so that electricity is cheaper at night, then the freezer could be put on a timer to only work the compressor during cheap hours.
> This can work, but you must be careful not to open the freezer often, especially during the day. The issue is temperature swings, and the humidity/frost. You want the food to stay not only good, but not freezer burnt or encourage frost build up. Freezer items are often valuable, like meats and such.

[]I also had a plan to add a second thermostat for refrigerator temperatures...
> I've seen models with this feature. Generally the compressor designs are optimized for the type of use. It won't be efficient, but it will be useful.


>>Other thoughts:
If it has a auto defrost mode, turn it off. The trade off is timing a manual defrost that doesn't take too much time/energy. Also adding insulation internally will short circuit this auto defrost mode, and make manually defrosting very difficult.

Can you dump the heat from the coils elsewhere? Separate enclosure, a hot water tank preheater, run to the bottom of a sump pump well?

Can you get water on the coils to cool them quicker, similar to how many window A/C units do. Of course, it's going to evaporate into the basement, which may be undesirable. The gains here could be trivial.

Spending money on newer units with more efficient compressor designs save energy; the issue is whether the added cost of purchase saves money long term. The new designs also save by keeping frozen food fresher longer.

Good Luck
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Old 09-13-17, 01:03 AM   #7
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Quote:
Originally Posted by ctgottapee View Post
...[*]There is about 3cm of space between the condenser coils and the rear wall. I want to increase this by 10-15mm, and insulate the rear wall with 5mm of styrofoam with a reflective coating, to keep the heat off (when testing, the coils heated from 16 to 26C in just 2 minutes).
> The coils already have a insulated air gap to the rear, another 10-15mm won't markedly change the resistance value. Adding more insulation won't make much difference as far as the coils go; and there isn't really any such thing as reflective coating as far as non-solar heat goes. Manufacturers would already do these things if they were effective.
You could try to enclose the coils in a separate enclosure that vented outside.
...
Coils also radiate heat (the solar heat you mention) and tin foil reflects 90%+ of this heat. So it serves some purpose.

Of course, the 10-15 mm movement of coils is mostly useless
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Old 09-13-17, 01:53 AM   #8
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Thank you all for your feedback

I am aware that the freezer design is already pretty efficient, so I am not expecting 30 or 50% improvement. But even if I get 5-10%, it will still count. My goal would be get the freezer to turn on only 1-2 times per day, to reduce wear on the compressor. If I can do that, then I can easily time it run only at night.

To relate to some of the things which came up:
  • No autodefrost, only manual
  • Also, I will defrost when the outdoor temps are very low (mid-winter), so that the food does not heat up too much during the few hours when it is out of the freezer.
  • This freezer will be only for extras (we will still use the smaller freezer compartment in the kitchen fridge on a daily basis). Hopefully it won't be opened for days at a time.
  • I would not want to vent the heat from the coils to the outdoors. Our basement is cool and adding some heat will only reduce the heat loss of the upstairs floor. Anyhow, it is mostly heat which came from the basement anyway, so moving the heat outdoors would be like installing A/C in the basement. Pumping the heat to a hot water tank would be good for summer, but a bit too complicated for the expected savings.
  • I am not worried about the 15W light when the lid is open, I am worried about it sticking in 'ON' when the lid is closed, pumping almost 15W of heat into the freezer continuously.
  • I may freeze a few liters of water just for thermal mass, but I could also look into some material which changes phases at freezer temps. Any ideas?
  • Insulation will be added only to the outside, not inside.
  • There is a drain plug at the bottom, I could use that opening to see how well the lid seal holds (pressure test). I will also use my IR thermometer to check whether I am gaining heat through that drain.
  • To increase efficiency I could wire a small fan to run with the compressor, blowing over the coils to increase heat transfer (and blow it away from the freezer's back wall), but since it will be on only 1-2 times per day, probably not worth the extra watts the fan would use.
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Old 09-13-17, 09:37 AM   #9
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[*]I may freeze a few liters of water just for thermal mass, but I could also look into some material which changes phases at freezer temps. Any ideas?
Add something like salt or propylene glycol to the water to lower its freezing point.
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Old 09-13-17, 02:31 PM   #10
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Add something like salt or propylene glycol to the water to lower its freezing point.
Sea water freezes at around -2C or -4C, iirc, and I don't think it would go much lower, with more salt in it.

Glycol solution may be an option, but I would first need to find what the temperature range will be in the freezer, then get the glycol/water proportions for a freezing temp in that range.

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