Many people have noticed that if we try to open a fridge door again only a few seconds after closing it, it suddenly opens much harder.
It sometimes feels as if some mechanical latch has engaged or as if the magnet in the door seal has become stronger.
In reality the main reason is simple:
after the door is closed, a slightly lower pressure than in the room is briefly created inside the fridge.
What happens when we open the door?
The inside of a fridge is considerably colder than the air in the room.
When we open the door, part of the cold air leaves the fridge and warmer air from the room takes its place.
That air is usually more humid as well than the air that was already in the fridge.
When we close the door again, a certain amount of that warmer air stays inside.
It then starts to cool down quickly.
As the air cools, the pressure falls
To understand the phenomenon it is enough to recall the basic relation between the pressure, the temperature and the volume of a gas.
If the amount of gas is approximately the same and the volume of the closed space is practically unchanged, a drop in temperature also brings a drop in pressure.
In other words:
warmer air enters the fridge → the door is closed → the air cools down → the pressure inside falls.
The outside atmospheric pressure then becomes somewhat higher than the pressure on the inner side of the door.
And it is exactly that difference that presses the door against the sealing gasket even more.
How can such a small pressure difference create such a large force?
At first sight it seems that a huge pressure difference would be needed for a noticeable „sticking“ of the door.
It is not so.
A fridge door has a relatively large area.
The force acting on it depends on:
the pressure difference × the area of the door.
That is why even a small difference of a few tens or hundreds of pascals, spread over the whole area of the door, can create a noticeable additional force.
At the handle it feels as if someone were pulling the door from the inside.
The role of water vapour
There is one more effect that can add to the fall in pressure.
The warm air that enters the fridge contains water vapour.
When it cools, part of that water vapour can condense on the cold surfaces.
As water vapour turns into liquid water, the number of molecules in the gas phase decreases, which further contributes to the fall of pressure in the closed space.
That is why the effect can be more pronounced when the air in the room is warm and humid.
Does the magnet in the door seal lock the fridge?
No.
The magnet in the sealing gasket has an important role, but its task is above all to keep the door pressed neatly against the cabinet and to make the seal sit properly.
It provides good closing and reduces the ingress of warm air.
The sharply increased force needed to open the door again right after closing, however, is most often not the result of a change in the magnetic force.
The magnet is practically the same before and after closing.
What has changed is the air pressure inside the fridge.
Why can the door be opened easily again after a short time?
A fridge is not a perfectly hermetically sealed vessel.
Over time the pressure gradually equalises through very small leaks in the system, the seal, the condensate drain and construction openings.
When the pressure inside the fridge comes close to atmospheric, the additional force pressing the door disappears.
What is left is mainly the force of the magnetic seal and the door opens normally again.
That is why the effect is strongest immediately after closing and then gradually disappears.
Is that a sign of a fault?
In most cases it is not.
On the contrary, a moderately harder opening right after closing can be a sign that the door and the sealing gasket close well.
If the door can be opened without any resistance at any moment, that in itself does not mean there is a fault, but on an older appliance it can be useful to check the condition of the sealing gasket.
On the other hand, if the door is constantly extremely hard to open, if the seal sticks, deforms or if unusual sounds are heard, the cause can be mechanical and it is worth checking the state of the hinges, the seal and the door itself.
Why is it good not to open a fridge several times in a row?
Every opening of the door brings in a new amount of warm and humid air.
The fridge then has to cool that air down, and part of the moisture has to be condensed and drained away.
Frequent and long opening of the door therefore increases the thermal load on the cooling system and lengthens the running time of the compressor.
The brief „locking“ of the door after closing is only the visible consequence of the same process:
warm air has entered the fridge and the system is now trying to bring it back to the temperature of the interior.
The essence of the phenomenon
The process can be reduced to a few steps:
we open the door ↓ warmer air enters the fridge ↓ we close the door ↓ the air cools down quickly ↓ part of the water vapour condenses ↓ the pressure inside the fridge falls ↓ the atmospheric pressure outside presses the door against the seal even more ↓ for a short time the door opens with more effort ↓ the pressures gradually equalise and the door opens normally again
So a fridge door is not temporarily blocked mechanically.
For a few dozen seconds it is held shut by the atmosphere itself.