A dehumidifier dries the air by making it cold. A fan pulls damp room air across a chilled coil, the moisture in it turns to droplets, and the water drains away while drier air is blown back out.
A dehumidifier takes water out of the air. Damp air holds a lot of invisible vapour, and that vapour is what makes a room feel clammy and encourages mould. The machine's trick is simple: cool the air down, and its vapour turns back into liquid water you can collect and pour away.
It uses the same refrigeration loop as a fridge or air conditioner — a refrigerant that absorbs heat on one side and releases it on the other — but here the goal is not cold air, it is dry air.
Outside, it is just a box with a grille and a tank. Hiding the housing shows the single part that does the work — the cold, finned cooling coil — and how air is guided across it from the inlet grille to the outlet.
Two ideas matter most: the coil only works because it is held below the air's dew point, and the fins exist to give the air as much cold surface to touch as possible.
The fan creates a steady flow of room air, drawing it in through the inlet grille. This air is warm and carries a lot of invisible water vapour, which is exactly what the machine is about to remove.
Cold refrigerant flows through the finned coil, so its surface is colder than the air's dew point — the temperature at which the air can no longer hold all its vapour. The thin fins spread that cold over a large area so more air touches it.
As air passes the cold surface, its vapour condenses into tiny droplets, the same way a cold drink fogs up. The droplets grow on the coil until they are heavy enough to fall.
Everything that condensed falls off the coil into the drip pan below. From there it runs into a collection tank, or through a hose to a drain, and the air is now free of that water.
The same fan pushes the dried air out through the outlet grille and back into the room. Because its moisture was left behind on the coil, the room slowly becomes drier.
From the air. When humid room air passes over the cold coil, its water vapour condenses into droplets, exactly like the beads of water that form on a cold glass.
A refrigerant inside the coil boils and absorbs heat, so the metal drops below the air's dew point. That cold surface is what squeezes water out of the air.
It cools the air slightly as it passes the cold coil, but the compressor and the warm side add heat back, so the net effect is often a small temperature rise.
The thin fins give the coil far more surface area, so more air touches a cold surface and more water condenses in the same amount of time.
Drops fall onto the drip pan and run into a collection tank that you empty by hand, or out through a hose if one is connected.
Its moisture has been removed on the coil, so the same fan pushes out air that is cooler and much drier than when it went in.