A dehumidifier takes water out of the air. It does not take water out of a wall that is fed by a pipe, the roof or the ground. So it helps in some homes and only burns electricity in others. This guide explains what it does, the two main types, what capacity to choose, where to put it and how to clean it.
Updated on 4 November 2026. General information, not a substitute for a technician’s assessment.
The essentials
- DECO PROteste (Portugal’s consumer body, 21 March 2026) says to use a dehumidifier as a complement to ventilation when the problem is condensation. If the unit and ventilation do not lower humidity, the cause is probably structural: infiltration, or a defect in the building or its envelope.
- The Swiss health authority (FOPH) writes that dehumidifiers must never be switched on before the mould is removed. See mould on walls.
- DECO PROteste gives up to 16 litres a day for rooms up to about 25 m² and above 16 litres a day for rooms above 20 m².
- The EU has not defined an energy label for dehumidifiers: compare watts and standby use.
- Cleaning the filter and tank is part of normal use.
What the unit does, and what it does not
It draws room air through, removes water vapour and returns drier air. The water ends up in a tank or leaves through a hose. It acts on the air only. It does not repair a roof, a leaking pipe or a wall touching wet soil.
| Situation | Does it help? | Basis |
|---|---|---|
| Condensation from vapour (showers, laundry, breathing) | Yes, as a complement to ventilation | DECO PROteste, 2026 |
| Rain infiltration, pipe leak | It does not fix the cause: repair the source | DECO PROteste, 2026 |
| Damp rising from the ground or coming from soil | It does not fix the cause; see rising damp and basements and buried walls | DECO PROteste, 2026 |
| Mould already present | Remove the mould first | FOPH, 2021 |
To work out which damp you have, see the types of damp. Airing and heating are in bedroom condensation.
Two types: compressor and desiccant
DECO PROteste does not explain the types in the pages we read. We used a page from Meaco (20 October 2025), a maker of both types. The figures are the manufacturer’s, not verified by us.
| Compressor | Desiccant (adsorption) | |
|---|---|---|
| How it works | Cools a surface and the vapour condenses | A wheel of absorbent material holds the vapour; an internal heater regenerates it |
| Cold room | Yields less below 10 ºC and spends more time defrosting | Steady output in the cold, per the maker |
| Stated power (20 ºC, 60 % RH) | 150 to 260 W, by size | about 330 W |
| Heat into the room | Outlet air about 2 ºC warmer | Outlet air about 10 to 12 ºC warmer |
The maker suggests a compressor for rooms above 10 ºC and a desiccant unit below that, and says compressors are usually cheaper to run. In a cold garage or store the type matters. The manual of an old compressor model (De’Longhi DH40) gives 8 ºC to 32 ºC and warns that outside those limits ice forms on the evaporator.
Capacity: litres per day
DECO PROteste (April 2026):
- Small units: for rooms up to about 25 m², extraction up to 16 L/day, depending on how damp the room is.
- Large units: for rooms above 20 m², more than 16 L/day. Small rooms with very high humidity and poor ventilation may need more.
- In its 2024 test: 10 to 12 L/day for about 20 to 25 m², 16 to 20 L/day for about 35 to 40 m² (indicative values, DECO PROteste says).
DECO PROteste notes that a unit too small for a big room runs longer and can use more electricity. ENERGY STAR (USA) says it is better to oversize than undersize.
Consumption and label
The European Commission confirms that dehumidifiers fall outside the energy-labelling regulations it cites, and that the EU has not defined an energy rating for them. There is no A to G letter to compare. DECO PROteste scores water extracted per energy used, speed in lowering humidity from 80 % to 60 %, and standby consumption.
You can estimate your own cost: power in watts × hours running ÷ 1000 = kWh, times the kWh price on your bill. We give no monthly cost: no source we read provides one.
Where to put it and how to use it
- In the room you want to dry, with doors and windows closed while it runs (ENERGY STAR).
- Away from walls and furniture so air can circulate. The De’Longhi manual asks for at least 50 cm around it.
- Humidistat: digital models switch off at the set value (DECO PROteste). FOPH suggests 30 % to 50 % relative humidity in winter.
- A small tank means frequent emptying; a continuous-drain hose avoids it (DECO PROteste).
- In basements: a conservation guide from Ohio (Ohio History Connection, 2002, about old US buildings) advises a fan or a dehumidifier, not both, in very damp periods. See basements and garages.
Cleaning
DECO PROteste (22 November 2024) says to clean regularly, especially with frequent use:
- Unplug it.
- Tank: empty when full and wash with water and neutral detergent; rinse and air-dry.
- Filter: vacuum the dust; if very dirty, wash in water up to 40 ºC, no chemicals, and air-dry.
- Outside: dry or slightly damp cloth. Never spray water or cleaners directly on the unit.
The De’Longhi manual asks for weekly filter cleaning. Follow your model’s manual.
Painting over a damp wall fails: fix the cause and let it dry. See anti-damp and anti-mould paint.
What we could not confirm
- Monthly running cost in Portugal: no source we read gives it.
- Full DECO PROteste test results (which model is worth it): behind a paywall, not read.
- The compressor versus desiccant comparison comes from a manufacturer; we found no independent or official Portuguese source.
- DGS guidance on dehumidifiers and timescales for lowering a home’s humidity: not found.
Sources (read on 4 November 2026)
- DECO PROteste, Que tipo de humidade tenho em casa?, 21 March 2026.
- DECO PROteste, Desumidificadores grandes: melhores modelos de 2026, 8 April 2026.
- DECO PROteste, Desumidificadores pequenos, 9 April 2026.
- DECO PROteste, Como testamos, 6 December 2024, and Como limpar o desumidificador, 22 November 2024.
- FOPH (Switzerland), Bolor em espaços habitáveis, Portuguese edition, February 2021.
- European Commission, Is there any system of energy rating for dehumidifiers? (undated FAQ).
- ENERGY STAR (USA), Dehumidifiers.
- Meaco, When to buy a desiccant dehumidifier and when to buy a compressor dehumidifier, updated 20 October 2025.
- De’Longhi, DH40 manual (old model, undated).
- Ohio History Connection, Judith L. Kitchen, FastFacts: About Wet Basements, PDF, 2002.