We’ve all probably heard it before as children from our parents: “Close the door son. You’re letting all the cold air out”!
So, this is basically nonsense right? The real investment is getting items from room temp to cold. Which takes hours. Opening the door for 2 or 3 minutes tops, does nearly nothing.
I guess the question is: How much energy is junior wasting when he leaves to door open for 3 minutes while he contemplates what to eat?
Are there any other cons to leaving the door open?
The calculation would depend on the fridge and the relative difference between the temperature inside and outside.
This morning, I opened our fridge door while standing barefoot. The cold air falling out of the fridge was obvious.
A lot depends on how much and what kind of stuff is in there. Say the refrigerator was packed with jugs of water. They’ll retain a lot of the ‘coldness’ versus a few condiment bottles. Or versus the fridge packed with something light but bulky like, I dunno, cereal.
The OP’s point is that the air doesn’t count for much.
Say you have a 500 liter fridge. Air has a density of 1.29 g/l, and a heat capacity of about 0.7 J/g-K. So the overall heat capacity is about 450 J/K, or 9000 J if it’s 20 C below ambient.
That’s not much. A fridge can probably pump heat at around a kilowatt, so that’s 9 extra seconds of operation. Or, it’s like putting 100 grams of room-temperature water in the fridge.
Here are some ball park numbers :
1> Volume of a typical fridge = 20 ft3 or 0.6 m3 (trying to use round numbers
" When it comes to the interior, the refrigerator itself should provide you with 14 to 20 cubic feet of space. " What's the Standard Refrigerator Size? - THOR Kitchen.
2> Volume of a beer can = 12 oz or 355 ml or 0.0004 m3
3> For estimation purposes, lets assume the fridge is filled with beer cans. With packing efficiency and space - lets assume that 70% of the fridge volume is filled with beer cans. That gives : 0.6 x 0.7 / 0.0004 ~ 1000 cans of beer
4> The surface area of a 12 oz beer can is 67 in2 What Is the Surface Are of a Soda Can? So 1000 beer cans will have a surface area of 67 x 1000 = 67,000 in2 = 43 m2 (Not all surface area will be exposed but for our case - lets assume it is )
5> Lets assume the beer is cooled inside the fridge to 50 F (10 C) and the room temperature is 75 F (24 C)
6> Heat transfer coefficient in air is typically about 10 W/m2/K Convective Heat Transfer Coefficients Table Chart
7> So the heat lost over 3 mins (180 seconds) is :
10 x 43 x (24 - 10) x 180 = 1083600 Joules
8> Coefficient of performance of a refrigerator is about 3 to 4. Lets assume 4 https://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.438.9480&rep=rep1&type=pdf This means 1J of energy is consumed per 4 joules of energy removed.
9> So the electrical energy consumed over the 3 minute period is 1083600/4 = 270900 J = 0.08 kWh
10> 1 kWh in the US is about 10c. So 0.08kWh is about 1c. So you waste about 1c in electricity if you leave the fridge open for 3 mins.
11> Do not use my calculations to convince your parents or spouse. It doesn’t work.
We always got crap for opening and closing the door repeatedly. Example: you need to grab seven or eight things to fix your lunch. Scenario A: you open the door and keep it open while you dig out your ingredients and set them on the counter/table. This invariably got us What the hell are you doing leaving the door open? Scenario B: you open the door, take out two items and close the door, putting the items on the counter/table. Repeat until all items are fetched. This invariably got us What the hell are you doing opening the door so many times? Scenario C: the police find Dad stuffed in the basement freezer several weeks later.
Once upon a time, long long ago, people had iceboxes. They were boxes cooled with ice. Keeping an icebox and its ingredients cold was difficult. I think these habits started back then when my grandparents were young themselves. The science may not have supported it but at the end of the week and your ice box is warm you’d be wishing you didn’t open the door as often.
My fridge doesn’t let the cold air out when I open the door. Unfortunately, it does tend to let the warm air in.
So if you leave the fridge door open for too long, the interior pressure steadily increases until it explodes?
That’s okay for a fridge full of beer cans, but what about the same fridge with a few limp veggies, some cheese and a couple of cartons of milk?
In this case the energy loss will be significantly less than 270900J (0.08 kWh). The analysis was done for a worse case scenario.
Being 70% full of containers of liquid sounds like more of a close to best case scenario.
Worst case scenario would be having an empty fridge.
I dunno. Beer at 50 F is a pretty bad scenario but certainly not the worst.
I seem to remember some college professor saying that for every second you had the door open, it would take 7 seconds of the fridge running to return to the initial temp. I guess there are too many variables to have such a rule of thumb but t sounded reasonable at the time. That was a long time ago.
(bolding mine)
Using those numbers, for a 3 min (180 seconds ) of open time, the fridge will have to run for 21 mins.
Typical fridge is 250 watts or 0.25 kW, and 21 mins of running = 0.25*21/60 = 0.09 kWh which is close to the 0.08 kWh I estimated.
Its always great to have an independent validation.
It’s a cumulative thing.
If, all over the civilized world, people are leaving their refrigerator doors open for long periods while they stare slack-jawed into the interior, vast amounts of power are being wasted, refrigerator motors are taxed, and the toll of carbon emissions mounts. It’s a scary scenario.
However it’s not going to take very much energy to cool that air-only fridge back down either. I’ve seen these “full vs empty fridge” discussions before, and usually they end up at a stalemate because there’s not really all that much difference in energy usage because it’s less about thermal mass, air volume vs product volume, and door opening/closing than about the temperature of the food you’re putting into the fridge and the standby losses through the walls.
A typical home refrigerator/freezer that’s never opened will still spend about 1/3 of its time running. So your baseline energy usage regardless of any other factors is already pretty significant. Big jugs of water may have some impact on total energy consumption either because of the air displacement or their thermal mass or both, but the benefits are more temporal and less cumulative. They’ll keep the temperature from rising too much due to door opening, or the defrost cycle, it also takes more time to recover from those things.
A good example from HVACR Videos on YouTube is when he repairs a broken walk-in cooler or freezer at a restaurant. Normally a walk-in freezer would be about -10ºF and if the unit has been down all day it may be 40º when he finally gets it running again. The temperature will quickly drop, say to 20º and then it seems to stall. The unit is still pumping out as much cold air as it can, but the room temperature is barely changing. That’s because 20º is (in this example) the temperature of all the product being stored. It was easy to cool the air alone, but once the temperature of the product is reached, then it’s no longer helping the refrigeration system, it’s fighting against it.
So, in a situation like that, similar to opening your home refrigerator door, an empty walk-in freezer may have warmed up to nearly room temperature, so having a lot of product stored would definitely help it maintain temperatures. But an empty freezer would also cool down much faster, so you’re kind of paying for it all at some point, it’s just a matter of when.
Yes, it starts to beep incessantly like some irritating nag. 
Yep
Suppose the empty fridge gets filled completely with room temp air. So now we have to calculate the energy needed to cool down 20 ft3 (0.6 m3) of 70F air to 50 F.
That works out to be 9680 Joules (If you need the calcs let me know).
9680 Joules is significantly lower than the 1083600 Joules we calculated for the beer cans heat loss.
So I think the beer cans are the worst case.
My fridge will shut off the fan and I assume the compressor when the door is open. Not totally sure about the compressor, the thing is so damm quiet I can’t tell.
If the fridge does not have this feature then the warmer air will be circulated though the cooling coils, this does increase the load on the compressor taking more power to turn it and I’m not sure if that was also taking into account in the above calculations. During the time the door is open the compressor will work harder and consume more power. Since the cooling coils are designed for rapid heat exchange I suspect that this may be more significant than just cooling down the air that is in the box once the door closes as you already ran a bunch of warm air into the refrigerant that you cooled and spilled onto the floor during the 3 minutes.