How does a compressor dehumidifier work?

The short version

Warm air can hold more water vapour than cold air. Cool a parcel of air enough and it can no longer hold all its water, so some condenses into liquid. That temperature is called the dew point. A compressor dehumidifier creates a very cold surface inside the machine, pulls room air across it, collects the water that condenses and sends the air back out drier.

It is the same thing that happens when a cold can of drink from the fridge gets wet on the outside in a warm kitchen. The dehumidifier just does it continuously, with a fan, a tank and a way of keeping the surface cold. Our dew point explainer covers the science with a worked example.

The refrigerant cycle step by step

Inside the machine a sealed loop of refrigerant moves heat from one coil to another. The steps below follow the refrigerant and the air.

1. 1

   The compressor squeezes refrigerant gas, which makes it hot.

2. 2

   The hot gas flows to the condenser coil, where it gives up heat and becomes liquid.

3. 3

   The liquid passes through a narrow restriction and its pressure drops, which makes it very cold.

4. 4

   The cold refrigerant flows through the evaporator coil, which becomes much colder than the room.

5. 5

   The fan draws room air over the cold evaporator. Water condenses on the fins and drips into the tank.

6. 6

   The now cooler, drier air passes over the warm condenser coil and leaves the machine slightly warmer than it went in.

7. 7

   The refrigerant, warmed again by the room air, returns to the compressor and the cycle repeats.

What each part does

Knowing the main parts helps when reading a manual or describing a fault to a repairer.

Why the air comes out slightly warmer

A compressor dehumidifier does not cool the room, even though it has a cold coil. The heat removed from the air at the evaporator is put back at the condenser, and the electricity used by the compressor and fan also ends up as heat. On top of that, turning water vapour into liquid releases heat. The result is that the air leaving the machine is usually a little warmer than the room, by a small number of degrees.

In winter this is a small bonus. In a warm summer room you may notice it, which is one reason dehumidifiers are not a substitute for air conditioning.

Why output falls in cold rooms

The amount of water collected depends on how far below its dew point the air is cooled. In a warm, humid room the dew point is well above the coil temperature, so lots of water condenses. As the room gets colder, the dew point drops towards the coil temperature and less water condenses. Below about 15 °C output falls noticeably, and the coil can drop below freezing and frost over, so the machine has to stop and defrost.

That is why compressor models suit heated homes, and desiccant models, which use a zeolite wheel instead of a cold coil, suit garages, cellars and cold rooms. See what is a desiccant dehumidifier and our guide to the best dehumidifiers for UK homes.

See the best dehumidifiers for UK homes →

Why you might need one

A dehumidifier removes water from the air that would otherwise condense on windows and cold walls, or slow down drying washing. It does not fix leaks, rising damp or penetrating damp, which need building repairs. Our article on whether dehumidifiers are worth it covers the signs that a home would benefit, and what size dehumidifier do I need helps with sizing.

When choosing a compressor model, compare litres a day at the same test condition, the stated watts, the tank size and the noise figure. The best 20 litre dehumidifiers guide compares the popular family size.

Frequently asked questions

Does a dehumidifier cool the room? No. A compressor dehumidifier usually makes the room very slightly warmer, because the heat removed at the cold coil is put back at the warm coil, plus the heat from the motor.

Why does my dehumidifier hum? The hum is the compressor running. It stops when the room reaches the target humidity and the compressor switches off, though the fan may keep running.

Why does a dehumidifier need defrosting? In cool rooms its cold coil can fall below freezing, so water freezes on the fins. A defrost cycle stops the compressor to melt the ice.

Is a compressor dehumidifier the same as a refrigerant one? Yes. Both terms describe machines that use a refrigerant loop and compressor to create a cold coil.