Condensation on the windows in the morning, a musty smell in a wardrobe, or laundry that seems to take forever to dry can quickly make you think you need a dehumidifier. Sometimes that is the right answer. In other cases, the appliance only masks a leak, poor ventilation or a cold surface where moisture keeps condensing.
So the first question is not “how many litres per day should I buy?” but where is the moisture coming from, and how high is the humidity in reality?
Measure first, do not rely on how the room feels
A simple hygrometer is one of the most useful small tools in this situation. It shows the relative humidity in the room and helps you distinguish a temporary spike — after a shower or cooking, for example — from persistently high humidity.
As a general indoor-air guideline, the US EPA recommends keeping relative humidity below 60% where possible, ideally around 30–50%. This is not a universal building-code requirement for every home or climate, but it is a useful practical reference: if readings stay high for hours and you also see condensation, smell dampness or find wet surfaces, it is time to look for the cause.
A dehumidifier does not repair a leak
If moisture is coming from the roof, a pipe, failed waterproofing or water entering through a wall, a dehumidifier can reduce moisture in the air but it cannot remove the source of the water.
The same applies to a bathroom with poor extraction or a dryer that releases moisture into the room. In those cases, improving ventilation, exhausting moisture outdoors or fixing the underlying technical problem often comes first.
A useful rule of thumb is:
- visible leak or wet patch → fix the cause first;
- humidity rises mainly after showering or cooking → check ventilation and extraction;
- humidity stays high for long periods without an active leak → a dehumidifier can become a logical part of the solution;
- visible mould is present → do not rely on the appliance alone; the moisture source and affected materials also need to be dealt with according to the scale of the problem.
What “10, 20 or 30 litres per day” actually means
A dehumidifier's capacity describes how much water it can remove from the air in 24 hours under specified test conditions. It does not mean that a unit labelled 20 l/24 h will necessarily collect 20 litres every day in your home.
The real amount depends on temperature, starting humidity, room volume, air exchange, indoor laundry drying and any ongoing moisture sources.
That is why two rooms with the same floor area can need very different machines. A slightly damp bedroom does not load a dehumidifier in the same way as a basement with persistently damp walls or a room where laundry is dried every day.
Do not choose by floor area alone
Manufacturers often quote a maximum room size, but that is only a rough guide. At least four other questions matter:
1. How high is the humidity without the appliance? 2. What is the room temperature? 3. Is there a continuous source of moisture? 4. Do you want to maintain comfortable humidity, or deal with a heavy damp-load situation?
ENERGY STAR, for example, does not size portable dehumidifiers by space alone. Its guidance also separates slightly to moderately damp, very damp and wet conditions. That is a more useful way to think than “X square metres = Y litres”.
Different markets also use different units. US guidance commonly uses pints/day. One US pint is about 0.473 litres, so 20 pints/day is roughly 9.5 l/24 h. This is only a unit conversion — it does not mean that a US sizing table can be applied directly to every European model.
A little spare capacity is better than running at the limit
An undersized machine may run almost continuously and still fail to reach the target humidity. Some reserve is therefore sensible, especially when the room is cool, humidity is high or laundry is regularly dried indoors.
But “bigger is always better” is not a complete rule either. Larger units usually cost more, take more space and may be noisier. The goal is enough capacity for the real moisture load without buying an unnecessarily large machine for short-lived humidity peaks.
Pay attention to room temperature
Dehumidifiers do not perform identically at every temperature. If the appliance will be used in a cool basement, garage or unheated room, check the model's minimum operating temperature and its performance in cold conditions.
This matters particularly for refrigerant/condensing dehumidifiers because lower temperatures affect condensation on the heat exchanger. A colder space may require a model designed for that operating range or a different dehumidification approach.
Tank or continuous drainage
If you expect the appliance to run every day, continuous drainage is one of the most practical features. Instead of emptying the tank by hand, water can be discharged through a hose to a suitable drain.
In a bedroom or living room, however, noise may matter more than tank size. Check not only the maximum decibel figure but also whether the unit has a quiet mode and how the fan behaves while maintaining the set humidity.
Practical choices by scenario
You only get morning condensation in winter
Measure humidity at different times and check ventilation first. If the problem is concentrated on cold windows or external walls, the cause may be a combination of high indoor humidity and low surface temperature. A dehumidifier can help, but it should not replace solving poor air exchange or thermal-bridge problems.
You dry laundry indoors
That creates a real additional moisture load. A higher-capacity unit and a laundry mode can be useful, especially where there is no effective mechanical ventilation.
The basement always smells damp
Check for water ingress and wet walls before buying anything. If there is no active leak but humidity remains high, a dehumidifier with continuous drainage and adequate capacity is more practical than a small tank model that needs frequent emptying.
You simply want more comfortable indoor humidity
In that case, an automatic humidistat can matter more than maximum extraction capacity. You set the desired humidity and the unit cycles according to the measured level instead of running unnecessarily.
The shortest decision formula
Before buying, do three things:
1. measure humidity for at least a few days; 2. check for leaks or another continuous source of water; 3. choose capacity for the real humidity and room use, not floor area alone.
A dehumidifier is extremely useful when it is solving the right problem. But if water is entering through the roof, wall or pipework, even the best dehumidifier is only an aid — the repair is the real solution.
