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Comment by bluGill | Hacker News Reader

I'm not sure how any of that is actionable. I already know how to make my fridge more efficient - but if I knew how to get my kids to close the door instead of leaving it open while the pour their milk (on the other side of the kitchen) I'd do that.

I get mail from my utility that I use more electric than average. However that doesn't give me anything actionable to do about it. I don't need another nag about things I can't do anything about.

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You’re only losing cold air, which has very little mass compared to what you’re actually storing in your fridge. The energy cost to re-chill the air in the unoccupied volume of your fridge can’t be that significant, I’d imagine.

It’s be an interesting experiment to put a temperature probe in something like a block of cheese, let it cool to the ambient fridge temperature, then open the fridge door for a few minutes and see if the temperature of the cheese even measurably changes over the next few hours or whatever amount of time.

I’m still on the fence about closing the door for a short task like pouring milk. My understanding from the physics of opening the door, you’re mostly already replacing all the air in the fridge with air from outside the fridge. So convection already did its job of robbing your fridge of cold. Closing the door to open it again a minute later just gives you a second convection inducing action for air that might have been trapped the first time.

I would love to see someone at the level of technology connections do a breakdown of the physics here, and where the likely crossover point point is for different kinds of fridges. My current (probably illformed) belief from the suction and imagined resulting air exchange of all the fridges I’ve lived with is that leaving the fridge open on the order of seconds is better than opening-closing-opening-closing in terms of cold loss.

"In theory but the benefit would be pretty negligible i think. Let's say for argument that most food is water and your fridge is 25% food and 75% air. The specific heat of air is 1 kJ/kgK, specific heat of water is 4.2 kJ/kgK. But also the density of air is 1.2kg/m3 and the density of water 1000kg/m3. Assume 1 m3 volume. So that would be 250kg food and 0.9 kg air. To change temp by 1C would take .9kJ for the air and 1050kJ for the food. So the air in a fridge holds a fraction of a percent of the total thermal mass of the contents. Whether or not that air changes temperature by a few degrees won't change things much at all. You would spend far more on balloons that you would save on energy."

* https://www.reddit.com/r/AskEngineers/comments/1618mku/would...

The air exchanges in seconds, yes (as evidenced by the post-door-closing suction), but the air doesn't stop flowing through the fridge as soon as the air changes once, and the air enters warm and leaves cold, so (by conservation) heat is being robbed from internal surfaces. So you're still losing more heat by leaving the door open longer.

Door open --> Door close --> Suction --> No further heat loss afterwards

Door open --> Continuous heat leak

its not

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