Dehumidifiers for basements, flats and bedrooms: Which size is right?
In short: According to the Federal Environment Agency, indoor humidity should be between 40 and 60 per cent. If it remains consistently above this level and ventilation is insufficient, an electric dehumidifier can help. For a bedroom or bathroom of up to around 15 m², the manufacturer recommends a unit with a capacity of 10 litres per day; for larger rooms and cellars, one with a capacity of 20 or 30 litres. In cool cellars, compressor-based units operate more slowly. A dehumidifier does not address the cause of the damp, such as a leaking pipe.
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When is the air too humid?
A dehumidifier extracts water from the air in a room and collects it in a tank. This helps when the air is consistently too damp: in a cellar, in a flat where washing is drying, or in a room where the windows are misting up. This page explains whether you need one and what size is suitable.
- Target range: According to the Federal Environment Agency, the relative humidity in rooms should be between 40 and 60 per cent to minimise the risk of mould.
- If this persists: According to Stiftung Warentest, if the humidity remains above 60 per cent for a prolonged period, there is a risk of mould.
- Measuring: A simple thermo-hygrometer displays temperature and relative humidity. The Consumer Advice Centre recommends keeping an eye on the readings using this device.
Ventilate first, then perform dehumidification
According to the Federal Environment Agency, regular and adequate ventilation prevents mould growth in many cases. A dehumidifier is no substitute for this; it helps where ventilation alone is not sufficient.
Important: A dehumidifier removes water from the air, but according to Stiftung Warentest, it does not tackle the root cause. If the damp is coming from a leaky roof, a burst pipe or a damp wall, this source must be identified and rectified. According to the Consumer Advice Centre, mould can only be permanently removed once the root cause has been rectified.
Which size for which room?
The capacity of a dehumidifier is specified in litres per day. The appliances in the range, all listed under ‘dehumidifiers’, according to the data sheet:
| Power | power consumption | Tank | Room size according to the manufacturer | Sound pressure |
|---|---|---|---|---|
| 10 litres a day | 205 W | 2.0 to 2.3 l | up to 15 or 16 m² | 46 dB(A); for a model, state only the sound power (43 dB) |
| 20 litres per day | 300 to 325 W | 3,7 l | 20 to 32 m² | 46 dB(A) |
| 30 litres per day | 460 W | 4,0 l | 25 to 30 m² | 53 dB(A) |
Information current as at: ACTEC product range, October 2026. The litres per day represent the maximum capacity (a 10-litre unit specifies 11.8 litres in the data sheet under the test conditions stated there); the actual amount collected by a unit depends on the temperature and humidity in the room.
- Bedroom and bathroom: According to the manufacturer, a 10-litre unit is designed for rooms up to 15 or 16 m². In the bedroom, noise level is a key consideration; a timer switches the unit on or off after a set time of between 1 and 24 hours.
- Flat, living room, laundry: For larger rooms or if laundry is regularly dried indoors, a 20-litre appliance is suitable. According to Stiftung Warentest, laundry should be dried on a washing line wherever possible. If it is dried indoors, the dehumidifier absorbs the moisture it releases. Some instruction manuals exclude use in the laundry room; please check your appliance’s manual.
- Cellars and outbuildings: Here, the rooms are often large and the humidity high. A unit with a 20- or 30-litre capacity is suitable for this; where the instructions describe continuous drainage via a hose, this eliminates the need for constant emptying of the tank.
In a cool cellar
The appliances in the range are compressor-based dehumidifiers: they cool the air against a cold component, causing the water to condense and drip into the tank. According to Stiftung Warentest, such condensation dehumidifiers work best at temperatures of around 10 °C or above; for colder rooms, it recommends adsorption dehumidifiers as an alternative.
The instruction manuals for the devices in the range specify an operating range of 7 to 35 °C; the device will not start below 5 °C. At low temperatures, the cold component ices over, and the appliance defrosts it regularly: according to the manual, every 30 minutes at 5 to 12 °C and every 45 minutes at 12 to 20 °C. It does not perform dehumidification during this time. In a cool cellar, therefore, an appliance collects less water than in a warm living room.
In summer, a cellar can become more damp due to incorrect ventilation: warm outside air cools down against the cold cellar walls, and the moisture condenses there. The Federal Environment Agency advises that cellars should not be ventilated during the day in summer, but rather at night or in the cool morning hours.
Continuous operation, hygrostat and installation
- Continuous drainage: Instead of emptying the tank, a hose can be connected to drain the water into a drain. According to the instructions, it must be positioned at a slight downward angle so that the water can drain away continuously. According to the instructions, the hose is not included in the scope of delivery (internal diameter 10 mm).
- Hygrostat: You set the desired relative humidity, and the device switches off as soon as it is reached. This saves electricity.
- Installation: Position the unit away from walls and furniture to allow air to flow freely in and out; the instructions for the appliances in the range specify a clearance of 20 cm all round.
How much does it cost to run?
Consumption is the power consumption multiplied by the running time. A device with a power rating of 300 W that runs for 8 hours a day consumes 2.4 kWh per day; at an example price of 35 cents per kWh, this amounts to 84 cents. If, thanks to a hygrostat, it only runs when the air is too humid, the consumption will be lower. Use your own electricity price.
The power consumption of the dehumidifiers in the range (205 to 460 W) is lower than that of the air conditioners in the range. If the room is not only damp but also too warm, an air conditioner that also performs dehumidification whilst cooling will help; for more on this, see ‘Portable air conditioner without an exhaust hose?’ and ‘Air conditioning: power consumption and costs’.
