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After closing my refrigerator's door I noticed that it's much harder to reopen it immediately, as if there's an underpressure. Only after a minute or so it opens normally. How can this be explained?

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  • $\begingroup$ Google for "horror vacui" $\endgroup$ – Georg Oct 24 '11 at 9:31
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When you open the door the cold air flows out and is replaced by air at room temperature. When you close the door this air gets cooled through contact with the stuff inside the fridge and therefore the pressure decreases, according to ideal gas laws by some 10%. That's not very much, but the surface of the door translates this small pressure difference to a larger force.
Note that the effect is even stronger with deep freezers, where the difference in temperature (and hence pressure) is even bigger.

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Like Anna says there must be air leaking in if the door opens normally after some time. This air will be cooled as well, but the pressure will increase with more air being sucked in, until an equilibrium is reached: cold air at atmospheric pressure.

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    $\begingroup$ @downvoter - Please tell why you do when you downvote an answer. $\endgroup$ – stevenvh Oct 24 '11 at 10:11
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    $\begingroup$ I had this problem at home with a one-year-old Siemens fridge and at work also. The explanation above is 100% right, but as this problem only started after a year of use, I thought there must be an reason as to why that suddenly started. I found out that the little hole in the lower back of the fridge (inside the fridge, not behind it) which is responsible for draining water, was blocked by som black crap generated over time. I cleaned this hole and the problem disappeared and of course stopped having water piling in the bottom. Try it as it will solve the problem. $\endgroup$ – Ambran Oct 22 '18 at 9:31
  • $\begingroup$ I can confirm @Ambran observation - my fridge doors started to get stuck, cleaning the draining hole solved the problem $\endgroup$ – Rsf Feb 12 at 19:03
  • $\begingroup$ If the pressure decreases by 10% then the pressure difference between the inside and outside is 0.1 atm. If the area of the fridge door is 1 m$^2$, then the amount of force you'd need to open it is $0.1\,\mathrm{atm} \times 1\,\mathrm{m}^2 \approx 10^4\,\mathrm{N}$. That's equivalent to the force needed to lift a 1000 kg weight. So I'd say there's gotta be some mechanism for equalizing the pressure inside and outside the fridge... $\endgroup$ – Alex Feb 16 at 5:22
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What @stevenh said.

I always look for that resistance to make sure that the door has really closed when I overfill the refrigerator. If yours equalizes after a while air must be leaking in, look at the seals around the door if they have deteriorated. Mine is fairly resistant once closed well, both the freezer compartment and the normal one.

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There are magnets in most fridge seals.

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    $\begingroup$ That doesn't explain why the door opens more easily after a minute or so. $\endgroup$ – Federico Russo Sep 9 '13 at 10:32
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many modern refrigerators have a mechanism that pumps out air from the fridge after you close the door, to save money on cooling the region. This creates an immediate pressure drop. Of course it can't be maintained forever, but it still improves the energy efficiency.

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    $\begingroup$ Are you sure about this? I have never seen any refrigerator, old or new that has a vacuum pump or something similar. $\endgroup$ – Alexander Sep 2 '13 at 19:30
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    $\begingroup$ The manual that came with the fridge said so. $\endgroup$ – joseph f. johnson Feb 10 '14 at 20:22
  • $\begingroup$ If it were true, then the opposite effect would be observed, because immediately after closing it would be less of a vacuum than before opening it. $\endgroup$ – user21820 Feb 26 '15 at 15:02
  • $\begingroup$ @user21820 I think the pump down is done to make a firm seal and then the pump is turned off. I have read about these pumps as well but never met one though I know of lots of freezers with good enough seal that will cause a vacuum strong enough to resist re-opening for a couple of minutes due to cooling of the room air that was exchanged. $\endgroup$ – KalleMP Nov 11 '16 at 18:59
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    $\begingroup$ @KalleMP: As someone else asked 3 years ago, have you ever seen such a pump in the refrigerator? Reading about it from online forums doesn't count. Also, for my own refrigerator the resistance is strongest the first time; opening and closing a second time results in much less resistance. This can be easily explained because the insides warm up after the first opening, and so on the second time the air gets cooled less. $\endgroup$ – user21820 Nov 13 '16 at 15:56
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the door has magnetic lock inside the seal to close.check out by placing a piece of iron on the seal.but remove it while closing.when the door is closed, the seal around it compresses and closes all gaps from air entering or loosing.when we try to open it, actually the seal expands to original shape and creates no gap until an inch or so thereby a under pressure is generated which required more force to open.

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protected by Qmechanic Sep 8 '13 at 14:18

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