A dehumidifier heats the room it is drying

The theory of dehumidification is well covered elsewhere. This is about the version you meet in practice.

Water leaves air by touching something colder

Air holds a limited amount of water vapour, and that limit rises steeply as the air gets warmer. Cooling air below the temperature at which it is saturated forces the excess to condense out onto whatever surface did the cooling. That temperature is the dew point, and it depends on how much water the air actually contains rather than on how it feels.

Every condensation-based dehumidifier works by creating a surface below that dew point and passing air across it. The same physics puts water on a cold window and on the outside of a chilled glass in summer.

The refrigerant type is a fridge that does not close

A compressor circulates refrigerant to make one heat exchanger cold and another hot, exactly as a refrigerator does. Room air is drawn across the cold coil, where moisture condenses and drips into a tank or a drain hose.

In practice, the now dry and chilled air then passes over the hot coil, which reheats it before it returns to the room. Because both coils are in the same room, the cooling and heating cancel and the compressor's own energy remains as warmth. Performance falls in cold rooms, since the coil must run below freezing and frost then blocks the airflow.

The desiccant type dries chemically

A desiccant machine passes air through a slowly rotating wheel coated with a material that adsorbs water vapour directly. A separate small stream of heated air passes through another part of the wheel, driving the collected moisture back out. That moist stream is then cooled so the water condenses and drains, and the wheel returns to the intake dry again.

The short version: because it does not rely on chilling a coil, it keeps working in cold spaces where a compressor type frosts up. It uses a heater to regenerate the wheel, so it consumes more electricity and warms the room considerably more.

Where all the energy goes

A dehumidifier does not remove heat from a room, because it has nowhere outside the room to send it. Essentially all the electricity it consumes ends up as warmth, plus the heat released when vapour condenses into liquid.

The room therefore gets warmer and drier, which feels more comfortable even though the temperature has risen. In summer that added heat is unwelcome, which is why an air conditioner, which does exhaust heat outside, is a different machine.

An air conditioner dehumidifies as a side effect, since its cold coil sits below the dew point for exactly the same reason.

Relative humidity is a ratio, not an amount

A relative humidity reading compares the water present with the maximum that air at that temperature could hold. Cooling air without adding any water raises the reading, which is why an unheated room reports high humidity in winter. Two rooms at very different temperatures can hold identical amounts of water and display completely different figures.

At the protocol level, this is why heating a cold damp room reduces the reading immediately even though nothing has been removed. Condensation problems track the dew point rather than the ratio, so the useful question is which surfaces sit below it.

This is the general case; a specific device may behave differently by design.

Where the water in a house actually comes from

Occupants, cooking, washing and drying laundry indoors all release substantial quantities of water vapour into the air. Condensation appears first on the coldest surfaces, which are typically window frames, external corners and poorly insulated walls. Those cold spots exist where insulation is bridged by a structural element, and no amount of drying changes their temperature.

At the protocol level, ventilation removes moist air and replaces it with colder outdoor air that holds less water, which is why extractor fans matter. A dehumidifier treats the symptom effectively, and persistent damp usually reflects ventilation or insulation rather than the air itself.

The takeaway

It dries the air by chilling it, and every joule it spends stays in the room.

Understanding the failure mode tells you more than the feature list does.