Waeco Fridge Cooling Fact or Fiction

Hi all,

I was checking out a new CT for a friend of mine earlier today, and during the conversation with the salesperson the topic got onto portable refridgeration, he put forward the idea that as the contents of the fridge get less, if you fill the empty space with a towel , this will help in power usage, as the fridge works on trying to keep the volume cool, and rather than trying to cool an empty space, it cools the towel instead.

What do you think Fact or Fiction.


Regards

Barry H
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Reply By: Member - Outback Gazz - Friday, Feb 10, 2012 at 16:58

Friday, Feb 10, 2012 at 16:58
G'day Barry

Not sure about the towel thing but mine certainly runs less and keeps my "liquid refreshments" colder when it's full rather than closer to empty
( which often is the case when I go bush )

Happy Travelling

Gazz
AnswerID: 477433

Reply By: Member - Barry H (WA) - Friday, Feb 10, 2012 at 17:08

Friday, Feb 10, 2012 at 17:08
Outback Gazz,

Mine does the same, so maybe there is some merit in filling the empty space with a towel, might have to give it a go next time I am camped up.

Regards

Barry H
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Follow Up By: Member - Outback Gazz - Friday, Feb 10, 2012 at 17:37

Friday, Feb 10, 2012 at 17:37
Barry

Depends where I am and where I'm going but I often put one or two 1.25 litre bottles of frozen water in my fridge - in my opinion this helps the fridge stay cold when in hot conditions and doubles up as drinking water when they eventually thaw out ! They have in some cases stayed frozen for up to a week !

Ps - Not sure if I want to go camping with you - when my "liquid refreshments" start running out I fill the new empty space in the fridge with "more liquid refreshments" ha ha !

All the best

Gazz

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Reply By: Allan B (Member, SunCoast) - Friday, Feb 10, 2012 at 17:09

Friday, Feb 10, 2012 at 17:09
Utter nonsense Barry,

Some people have great imagination!

Once the fridge has removed heat from the contents you place in it, then the only work then done is to remove heat entering through the insulation. The only effect of placing a towel, or anything else, in the fridge is to cool the towel down.

Similarly, placing water bottles in spare space within the fridge does nothing for the fridge efficiency. It adds load until the water is chilled then nothing......except you now have some cold water. Subsequently the water mass may affect the fridge running duty cycle but the operating amphours will be the same.

Cheers
Allan

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Follow Up By: Member - Barry H (WA) - Friday, Feb 10, 2012 at 17:14

Friday, Feb 10, 2012 at 17:14
Allan,

Thanks for that, as I said Fact or Fiction I wasn't sure which one it was.

Regards

Barry H
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Follow Up By: Patrol22 - Friday, Feb 10, 2012 at 17:32

Friday, Feb 10, 2012 at 17:32
I noticed someone putting the filled water bottles in to fill the empty space once and had a bit of a chuckle. I use old wine cask bladders (blown up like a balloon) as space fillers in my fridge....not to save energy though but rather just to stop stuff getting too shaken up on the corrugations :-)
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Follow Up By: Battery Value Pty Ltd - Friday, Feb 10, 2012 at 18:07

Friday, Feb 10, 2012 at 18:07
Hi Allan,

"utter nonsense"....NOT

As has been confirmed by others in this thread, the fridge runs more efficiently when there's something in it.

And yes, there's an explanation for this:

Heat gets conducted through radiation, conduction and convection, remember?

The one which is obvious is the conduction through the wall insulation.
And from there, it's convection.
Without convection (air circulation) heat conduction is reduced, because the still air (if it only were) on both sides would cause an additional layer of insulation.
And that's precisely what's being achieved on the cold side by stuffing towels into the empty space. Filling the fridge right up with goodies has a similar effect of course, so a full fridge will always run more efficiently than an empty one.

cheers, Peter
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Follow Up By: Member - Barry H (WA) - Friday, Feb 10, 2012 at 18:54

Friday, Feb 10, 2012 at 18:54
Peter,

Thanks for that.

I guess opinion will be divided on this one, as for me I think it has some merit, but I could be persuaded otherwise, I guess I will just have to try it on my next trip.

Regards

Barry H
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Follow Up By: Allan B (Member, SunCoast) - Friday, Feb 10, 2012 at 18:58

Friday, Feb 10, 2012 at 18:58
Oh Peter, nothing has been "confirmed by others" apart from their beliefs.

The original post referred to "power usage". The power used in a fridge is a function only of 1) the initial removal of heat from items loaded into the fridge and 2) the removal of heat from that entering through the insulation.

Any circulation of air, either cold or warm within the fridge has an effect on the temperature distribution within the fridge only. It has nothing to do with the removal of heat and thus power use. Nor does it affect heat conduction through the insulation. Without your "convection" heat from the contents or entering via the floor or lid where there is no evaporator surface will not reach the evaporator on the sides on be removed.

In point of fact, it is undesirable to restrict air movement within the chamber unless you are trying to achieve temperature gradients within the contents. Many modern domestic refrigerators use fan circulation to obtain more even temperature throughout the fridge.

Cheers
Allan

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Follow Up By: Off-track - Friday, Feb 10, 2012 at 22:18

Friday, Feb 10, 2012 at 22:18
And nothing has been confirmed by you Allan except that you are quick to point out that others are wrong, when in fact it is you that has a great imagination.

Have a read of olcoolones reply which provides an accurate practical application of why a fridge works more efficiently when full.
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Follow Up By: Battery Value Pty Ltd - Friday, Feb 10, 2012 at 22:53

Friday, Feb 10, 2012 at 22:53
You're welcome Barry,

yes, try it and see what the results look like.

And this is to test Allan B's nerves :)

....Any circulation of air, either cold or warm within the fridge has an effect on the temperature distribution within the fridge only. It has nothing to do with the removal of heat and thus power use. Nor does it affect heat conduction through the insulation.....

Allan, maybe you can re-think your statements above.
Let me explain one more time why you must consider convection on both sides of the fridge walls.

The rate of heat conduction through the insulation is determined by the temperature gradient between the outer and inner surface of the walls.
If you allow this gradient to be steeper, the rate of conduction will increase (the colder the wall surface gets on the inside, and the warmer the outer surface, the harder the compressor is made to work by the thermostatic control unit).
I think that's agreeable?

Because the heat source (condenser), and the heat sink (evaporator) are on the opposite sides of this wall the direction of the heat flow (as long as there's means of facilitating this flow) is from the condenser to the evaporator (duhh...).
So what facilitates heat flow between condenser and evaporator?
Because still air conducts heat very poorly, but nevertheless expands and contracts with temperature, it readily starts moving vertically if it's in contact with a wall surface of unequal temperature.
So what you get is a rotating mass of air on the outside and on the inside (air moving down along the relatively cool outer surface, and up on the relatively warm inner surface. Of course, on the surfaces of the evaporator/condenser, the direction of airflow is reversed, which completes the inner/outer circles of air movement.
And bingo, now we've got convection on both sides of the wall.
Convection increases the temperature gradient between the two surfaces, warm air is fed from the condenser (and ambient) to the outer surface and cold air from the evaporator to the inner surface.

If you artificially stop or at least restrict the air movement on either one, or both sides of the wall, the temperature gradient inside the wall decreases, and there's less heat flowing through this wall.

And I betcha, a towel is far more efficient stopping convection, than conducting heat from the relatively warm inner surface towards the cold surface of the evaporator.
The end result is a slightly warmer inner wall surface, thus a reduced temp. gradient inside it, and lower rate of heat conduction through that wall.
And a lower rate of heat conduction means less work to be done by the compressor, because now the temp. gradient between condenser and evaporator can be kept up more effortlessly.

To prove this, all you have to do is, stuff towels between the content and all wall surfaces (no towels between evaporator and content of course), and see how much more efficient your fridge will suddenly work.

....In point of fact, it is undesirable to restrict air movement within the chamber unless you are trying to achieve temperature gradients within the contents. Many modern domestic refrigerators use fan circulation to obtain more even temperature throughout the fridge....

Not restricting it between evaporator and content, but restricting it along the walls will definitely help.
Convection forming between the walls and content has to be restricted as much as possible.
Sure, fan assisted cooling is a slightly different kettle of fish (not sure if this pertains to this case).

I still remember how inefficient a fridge works if the evaporator is all iced up, and things don't get really cold inside although the compressor is running all the time.
I guess the main purpose of the fan inside is to reduce the temp. gradient between evaporator and content.
The associated losses due to the increased heat transfer rate through the walls are more than compensated for by the efficiency gains of a fan assisted refrigeration cycle (stuff is getting colder more quickly).

Of course it would be ideal to retain the low wall-temp-gradient, while the fan moves air between content and evaporator, but you can't have it both ways unfortunately, and some of this airflow will always increase the wall-temp-gradient to some extent (unless there's a towel shielding the walls from the cold draft).

cheers, Peter
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Follow Up By: Muntoo - Saturday, Feb 11, 2012 at 00:51

Saturday, Feb 11, 2012 at 00:51
Have to agree with Battery Value here,

It is proven to work, it aint a myth.

I thought everyone knew this, obviously not.

Same goes for your freezer/fridge at home.
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Follow Up By: Bush Wanderer - Saturday, Feb 11, 2012 at 01:22

Saturday, Feb 11, 2012 at 01:22
Ditto. It's easy enough to prove that it ain't a myth.
See how long a fridge runs empty, as opposed to full.
Fridges are far more efficient when full....towels chew up the space easier than bottles which can only help.
Seeing lots of snappy replies on this forum all too much these days.
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Follow Up By: Allan B (Member, SunCoast) - Saturday, Feb 11, 2012 at 01:28

Saturday, Feb 11, 2012 at 01:28
As I have already expressed, it is really very simple physics. The energy input (read amp hours) is purely a function of heat ingress through the insulation.
All the waffle in the world will not change this.

Peter, stick to the batteries. I respect your wisdom there.

And I'm off for the weekend, so no more from me.

Cheers
Allan

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Follow Up By: Battery Value Pty Ltd - Saturday, Feb 11, 2012 at 08:41

Saturday, Feb 11, 2012 at 08:41
And I really thought at some stage, yep that fellow has an engineering background :(

Anyway, enjoy your WE.

cheers, Peter
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Follow Up By: Bushranger1 - Saturday, Feb 11, 2012 at 09:07

Saturday, Feb 11, 2012 at 09:07
G'day Peter,
I have an engineering background & have been involved in the Metrology (precision measurement) side of it for 30 odd years. What you are saying is correct. The bigger the temperature differential the faster the heat moves towards the cold object.
This is a very important fact in calculating temperature compensation when doing dimensional measurement.

Also not many people realise that you cannot MAKE something cold you can only keep removing heat to cause the object to become colder. Heat moves away from the cold object & the rate increases with a greater temp difference between the 2 bodies.

Well I hope I have not confused too many people!

Cheers
Stu
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Follow Up By: olcoolone - Saturday, Feb 11, 2012 at 09:12

Saturday, Feb 11, 2012 at 09:12
I'm with Peter on this one....... Thermodynamics comes to mind!
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Follow Up By: Battery Value Pty Ltd - Saturday, Feb 11, 2012 at 09:33

Saturday, Feb 11, 2012 at 09:33
Thanks Stu and olcoolone,

actually Stu, you almost succeeded in confusing me when you wrote:

...Heat moves away from the cold object & the rate increases with a greater temp difference between the 2 bodies....

Heat's actually moving away from warm objects towards cold objects (from the heat source to the heat sink) - small lapse, no great deal :)

cheers, Peter

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Follow Up By: Bushranger1 - Saturday, Feb 11, 2012 at 10:08

Saturday, Feb 11, 2012 at 10:08
Hi Peter,
I think I confused my self with my explanation! I used a couple of contradictary words. Oh well we get the idea I think!

Cheers
Stu
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Follow Up By: Battery Value Pty Ltd - Saturday, Feb 11, 2012 at 10:38

Saturday, Feb 11, 2012 at 10:38
Sure thing Stu, it's all clear.

cheers, Peter
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Follow Up By: Allan B (Member, SunCoast) - Tuesday, Feb 14, 2012 at 10:28

Tuesday, Feb 14, 2012 at 10:28
To Peter @ Battery Value,

I take a risk of the Moderator barking about a "peeing contest" but I would like to comment on a post you made just after I signed off before the weekend.

You directed to me ........."And I really thought at some stage, yep that fellow has an engineering background :( "

The implication is that I do not have an engineering competence and clearly is an attempt to belittle me and thus extinguish my argument. It is "playing the man" rather than "playing the game" and is a ploy often used when your argument is weak.

I can assure you that I certainly do have extensive engineering and scientific qualifications and experience, some of it embracing the measurement and control of temperature and thermodynamics. I chose not to critically analyse your dissertation to avoid this becoming an extended and irresolvible argument and paid you the compliment of being expert with batteries. It would be nice to think that you were also prepared to respect my expertise.

Cheers
Allan

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Follow Up By: Bush Wanderer - Tuesday, Feb 14, 2012 at 10:52

Tuesday, Feb 14, 2012 at 10:52
Allan,

The exploroz forum too often becomes a slinging match on a number of post these days.
It seems to be the norm.
Shame really.
Everyone is entitled to their own opinion and experience, which can disagree with others....so others slam you down, then it all starts.
If members feel that the need to post comments that are negative or insulting they should not even both to post.

Brett
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Follow Up By: Allan B (Member, SunCoast) - Tuesday, Feb 14, 2012 at 11:06

Tuesday, Feb 14, 2012 at 11:06
Thank you Brett,

I felt it necessary to defend my expertise and qualification.

Our whole society seems to be going this way. Just look at the competitive TV Chef shows where belittling has become the norm in the name of entertainment.


Cheers
Allan

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Reply By: Member - Phil G (SA) - Friday, Feb 10, 2012 at 18:10

Friday, Feb 10, 2012 at 18:10
Its an interesting topic.

The only way I figure you can improve things is by reducing the loss of cold air and the ingress of hot air when you open the fridge. If the towel was to do this, then well and good.

But I store all our meat and veg in sealed systema plastic boxes in the fridge - not only does it keep most of the stuff in the fridge clean and orderly, but there is less "free air" to escape when you open it. When I open the fridge and have a stack of plastic containers looking at me, so I figure not much air escapes.

As for the beer, the gin, the 1.25L tonic bottles and the sav blanc bottles.....well we tolerate those losses but the glass and aluminium containers go into cut milk cartons to protect them from the corrugations, and maybe that helps too?
AnswerID: 477439

Follow Up By: Crackles - Friday, Feb 10, 2012 at 20:48

Friday, Feb 10, 2012 at 20:48
You're on the money Phil. The purpose of the towel (wine bladder/empty tupperware) is to fill the void so when you do open the fridge it minimises the loss of cold air. Less loss means less to cool down which in turn saves power.
Cheers Craig.......
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Follow Up By: supa D - Saturday, Feb 11, 2012 at 19:51

Saturday, Feb 11, 2012 at 19:51
When camping I always carry empty bladders from wine casks or drinking water packs, and use these to fill any empty void in the fridge or freezer. As you empty the fridge contents, inflate the badder or add another one. I also use the same system in the upright freezer at home because it is an older model that doesn't have individual drawers and inner compartments to keep the cold air from falling out with each opening of the door. The bladders ensure that the air that is in the fridge and pre-cooled before opening it, is the same air that is in the fridge when you close it. Saves a lot on power, and they are very easy to carry. Several will fold and fit in the Engel carry case.
Cheers
Glen
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Follow Up By: Member - Phil G (SA) - Saturday, Feb 11, 2012 at 21:59

Saturday, Feb 11, 2012 at 21:59
Gday Craig,
Yeah its not rocket science! Don't think I'd ever use a towel - whenever you pull it out to find the stuff in the bottom of the fridge, you'd lose a lot of the cold.

BTW, I keep eyeing off the FJCruisers - looks like a toy I need! But I can't depart with the HDJ79 - I reckon my wife would look better in the FJ than the Tiguan though :-)

Glen, I used to do the same but I started taking bottles instead of casks (happens as you get older and screwtops are great :-))
But I used cask bladders to stop the contents from rocking around when we'd freed up some space but I now know that it kept things cooler as well ! With the plastic boxes we just leave the empty ones in the fridge, so no need for bladders.

Cheers
Phil
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Follow Up By: Crackles - Monday, Feb 13, 2012 at 19:45

Monday, Feb 13, 2012 at 19:45
"I keep eyeing off the FJCruisers - looks like a toy I need!"
That's funny. Mine is just filling the gap until the kids are gone so I can get a Landcruiser ute with a Tailgate camper :-)
Cheers Craig...........
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Reply By: olcoolone - Friday, Feb 10, 2012 at 18:18

Friday, Feb 10, 2012 at 18:18
Yes and no..... if you have less in a fridge it will use more energy to keep cool if the fridge is opened or other items are added.... you have two cans of beer and each time you remove a cold one and restock it with a hot one.

If you have more in a fridge and it has cooled to a set point it will use less energy if you add an item or open the door as the heat transfer is over a greater area.... if you have 12 beers in the fridge and every time you open it you remove a cold one and add a hot one...... the differential in temp over the given area is less.

Think of it as a bucket of water with the bucket holding 2lts and the water is at 10 degC.... you add 1lt of water with a water temp of 30 degC... to cool that 1lt of water to a equal set point will take more energy then if you had a bucket with 5lts of water at 10 DegC to cool that 1lt.

The 5lt bucket of water would have a lower set point.... in other words it would make the 1lt of water colder as it can pull more heat out before becoming saturated.

In a fridge the set point is controlled via the thermostat so the more heat it has to remove to more energy it will use.

The more you have in a fridge the quicker something will cool.

Like all things in life nothing is for free and energy used to remove heat is no different.

The more ventilation you have around the condenser the more efficient it will run using less energy.





AnswerID: 477441

Follow Up By: aboutfivebucks (Pilbara) - Friday, Feb 10, 2012 at 19:20

Friday, Feb 10, 2012 at 19:20
Love your science project.

I'm going to swap the 5l bucket of water for a 5l Keg of Heineken.
Swap the 2l of water for a 2l cask of wine.
Reduce the temps to well under 10 deg

Big weekend coming - all in the name of science!!

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Reply By: Steve M1 (NSW) - Friday, Feb 10, 2012 at 18:24

Friday, Feb 10, 2012 at 18:24
I think some are confusing putting a warm towel/bottle of water with putting in, say, a pre-chilled or frozen bottle of water. With the former, the fridge has to work hard to cool it - with the latter it doesn't because it is already at the desired temp.
AnswerID: 477442

Reply By: CSeaJay - Friday, Feb 10, 2012 at 20:58

Friday, Feb 10, 2012 at 20:58
I also found the fridge running cooler with the towel. But I think it has to do with the towel stopping the cool air escaping when you open it up. Plus the kinetic enrgy stored in the cool towel not escaping like air when it is open.

Certainly works, maybe not for the reason stated in the original post but it works
AnswerID: 477454

Follow Up By: oldtrack123 - Friday, Feb 10, 2012 at 21:42

Friday, Feb 10, 2012 at 21:42
Hi CSeaJay



You have it right
Since the towel has low thermal mass ,it takes very little energy to cool
But it minimizes the amount of warm air that rushes in[&has to be cooled] whenever the door is opened
Any thing which limits the warm /cold air exchange will do the same thing provided it does have low thermal mass
Sealed empty bottles are better,they retain the cold air of that space they occupy
Plastic milk bottles are ideal
Unless you have a use for the cold water you have wasted energy by cooling it
Now the cans of beer does have a use ,so that is ok
Remember we are talking about energy loss
Energy is only LOST in 2ways
[a] Via insulation losses
[b]through loss of cold air being replaced with warm air
WE are not talking about temperate stability .precooling ,etc
Once the contents are down to set temp,If the door is not opened, the ENERGY USED used will be the same to maintain the set temp no matter if the fridge is full or empty
as [a] above is the only loss & it is constant for any set temp.

Peter
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Follow Up By: pop2jocem - Saturday, Feb 11, 2012 at 00:09

Saturday, Feb 11, 2012 at 00:09
CSeaJay and Peter,

Yep IMHO you guys have nailed it. Regardless of the 5 page essays on whatever, the amount of energy the fridge uses is governed by the amount of heat being added to the contents either through the insulation, by opening the lid, regardless of whether any item is removed and replaced by a warmer item and of course adding warmer items to the contents. A towel or anything that minimises the amount of warmer air entering the fridge will help keep energy use to a minimum. Dont open the lid and I think you will find the energy consumed will be the same regardless of the contents once the contents have reached whatever temperature setting was dialed in.Internal fans help a fridge keep an even temperature distribution.

Cheers
Pop
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Reply By: Member - John R (cQld) - Friday, Feb 10, 2012 at 22:12

Friday, Feb 10, 2012 at 22:12
This from Ergon (our electricity provider) energy-saving tips:

"Every time you open the fridge you let cold air escape.
Keeping your fridge full will use less electricity to
re-cool it after you close the door."

This would really apply only to upright fridges though, not the usual waeco or engel type camping fridge.

Cheers, John
AnswerID: 477461

Reply By: River Swaggie - Friday, Feb 10, 2012 at 23:48

Friday, Feb 10, 2012 at 23:48
"As has been confirmed by others in this thread, the fridge runs more efficiently when there's something in it.:


Its fact mate,Although with food it is,Well it certainly is with my Evakool RF60 fridge....
AnswerID: 477465

Reply By: Member - Brenton H (SA) - Saturday, Feb 11, 2012 at 15:29

Saturday, Feb 11, 2012 at 15:29
Well I thought it was simple.

When you're in the Simpson desert and you open the fridge the hot north wind blows all your cold air out..it sure as hell doesn't blow out your water bottles!
AnswerID: 477494

Follow Up By: Member - Phil G (SA) - Saturday, Feb 11, 2012 at 21:47

Saturday, Feb 11, 2012 at 21:47
Gday Brenton,
Hehehe...when I go to the Simpson, its the middle of winter. When I open the fridge in the morning, the cold south wind adds freezing cold air to the fridge.....contents which were at 2 degrees might drop below zero :-)

Cheers
Phil
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Reply By: mudbro2 - Saturday, Feb 11, 2012 at 15:36

Saturday, Feb 11, 2012 at 15:36
I use a towel in my fridge to stop the contents moving around when travelling (rattleing) and just restock it before driving for the day (aslong as we`re moving everyday) so i can turn the fridge up to cool the added contents. Oh and my brain hurts from reading this thread. Phil.
AnswerID: 477497

Reply By: Allan B (Member, SunCoast) - Monday, Feb 13, 2012 at 01:50

Monday, Feb 13, 2012 at 01:50
NOT ONE RESULT was turned up in my Google search for placing a towel in a fridge to reduce its energy consumption.

You might expect that if it were an effective practice then there would be at least one or two references to it on the internet. But no, only on ExplorOz forum!!!!!!

I'll leave it to you to draw conclusions to this.

Cheers
Allan

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AnswerID: 477610

Follow Up By: Off-track - Monday, Feb 13, 2012 at 18:53

Monday, Feb 13, 2012 at 18:53
Oh too funny!

How did you go regarding filling empty spaces in a fridge to increase efficiency? With the overwhelming information can you admit you were wrong yet?

...and yep, I think is fair to say we have all drawn some conclusions out of this...
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