How to Choose a Dehumidifier

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How to choose a dehumidifier

Humid air in a flat, house, cellar, or laundry room isn't just an unpleasant "heavy" feeling in the air. It's the primary cause of mould, odours, condensation on windows, and ultimately damage to building structures, furniture, and the health of the people living there. A dehumidifier is one of the cheapest and most effective solutions for getting rid of humidity - as long as you choose the right type and the right output. In practice, we most often see people buy the first dehumidifier they find in a shop, regardless of the room size or the real humidity level, and then are disappointed that "it isn't doing anything". This article shows you how to proceed step by step when choosing one, what types of dehumidifiers exist, how to calculate the output you need, and when it's better to reach for an air conditioner with a dehumidifying function instead.

Why dealing with air humidity matters at all

Optimal relative humidity in living spaces is between 40 and 60 %. If it consistently exceeds 65-70 %, it creates a favourable environment for mould and dust mites to grow, accelerates corrosion of metal elements (for example radiators, boilers, pipes), and wooden structures (parquet flooring, beams, furniture) absorb moisture and change dimension. In practice, we most often encounter high humidity in these situations:

New builds and flats after renovation - masonry, concrete, and plaster take months, even up to a year, to dry out, and during this time release large amounts of moisture into the air. Cellars and basements - low temperature combined with limited ventilation and contact with the ground means relative humidity here commonly reaches 80-90 %. Laundry rooms and drying rooms - drying laundry indoors without extraction can release as much as 1-2 litres of water into the air per day. Pool halls and windowless bathrooms - high evaporation from the water surface or shower. Cottages and cabins used seasonally - the building isn't ventilated or heated between stays, moisture builds up, and it's the cause of the typical "cottage" mouldy smell.

In all these cases, a dehumidifier is a rational solution - in many cases more effective and cheaper than structural work (additional waterproofing, replastering) or constant intensive ventilation, which in winter also raises heating costs.

Types of dehumidifiers and their characteristics

On the market you'll commonly find three fundamentally different technologies. Each has different strengths and suits a different type of space.

Compressor (refrigerant) dehumidifiers

These work on the same principle as a fridge or an air conditioner - air is drawn in over a cold heat exchanger, moisture condenses on it, and drains into a collection tank or out through a hose. This is the most widespread and generally the most energy-efficient type for ordinary living spaces. Typical capacity ranges between 10 and 30 litres of water per 24 hours, with a power draw of approximately 150-350 W. The downside is sensitivity to low temperature - below approximately 15 °C, efficiency drops significantly, because the heat exchanger quickly frosts over (better-quality models have an automatic defrost cycle that partially addresses this, but efficiency is still lower). This is why compressor dehumidifiers are ideal for occupied, heated spaces - living rooms, bedrooms, bathrooms, offices.

Adsorption (desiccant) dehumidifiers

Instead of cooling, these use a special rotor with a material (often silica gel) that absorbs moisture from the passing air, and then uses heat to release it and drain it into a tank. Thanks to this, they work reliably even at low temperatures, commonly rated by manufacturers down to -5 °C, making them the only sensible choice for unheated spaces during winter - cellars, garages, cottages, storage rooms. Capacity is typically lower, 6-20 litres per 24 hours, and power draw higher than the compressor type, commonly 400-650 W, since heating the rotor consumes more energy. They also tend to be somewhat noisier.

Thermoelectric (Peltier) mini dehumidifiers

Small, quiet, and cheap devices without a compressor, using a Peltier element. Capacity is only 1-3 litres per 24 hours, with a power draw of 20-50 W. They aren't suited to solving a real humidity problem across a whole room, but are practical as a supplement in a small enclosed space - a wardrobe, closet, pantry, small windowless bathroom, or a caravan.

How to calculate the dehumidifier capacity you need

The most common mistake when choosing is buying a dehumidifier based on price alone, regardless of how many m² and what humidity level it actually needs to handle. A dehumidifier's capacity is stated in litres of water the device can remove from the surrounding air over 24 hours under reference conditions (usually 30 °C and 80 % relative humidity - real-world output in a cooler, drier room will always be lower, so it's better to plan with a reserve).

An approximate recommendation based on common practice:

A room up to 20 m² with normal humidity (mildly elevated, up to approx. 65 %) - a capacity of around 10 litres per 24 hours is enough. A room up to 20 m² with high humidity or visible mould (above 75 %), or a small cellar - recommended capacity 14-16 litres per 24 hours. A room of 20-40 m² with normal humidity - 14 litres per 24 hours. A room of 20-40 m² with high humidity, for example a larger cellar or a laundry room where laundry is dried - 20 litres per 24 hours. Spaces over 40 m² or several connected rooms - 20 to 30 litres per 24 hours, or consider two smaller units instead of one large one (more even coverage of the space).

These figures are approximate - if you're not sure of the humidity level, it's worth investing in a cheap hygrometer, which costs only a few euros and gives you an exact value instead of a guess based on how the air feels.

Step-by-step selection procedure

To avoid getting lost in the range of dehumidifiers on offer, it's worth proceeding systematically instead of comparing just price and design. In practice, this procedure has worked well for us:

First, check the actual humidity and temperature in the space (with a hygrometer, and also based on whether the space is heated). Choose the technology based on temperature - above 15 °C a compressor type, below 15 °C or an unheated space during winter an adsorption type. Based on the room size and humidity level, use the table above to choose the required capacity in litres per 24 hours. Check practical parameters - the size of the collection tank (a larger tank means less frequent emptying), the option of permanent drainage via a hose (practical for cellars and laundry rooms where you don't need to check the device daily), noise level in decibels (important if running in a bedroom or living room overnight), and the energy class. Only at the end should you compare price and brand - two devices with the same capacity can differ in annual energy consumption by tens of euros.

Dehumidifier running costs

Running costs are a topic often underestimated with dehumidifiers - especially with the adsorption type, which runs continuously for longer than the compressor type. Let's give a specific example calculation at a model electricity price of €0.20/kWh:

A compressor dehumidifier with an average power draw of 250 W, running 8 hours a day, consumes 2 kWh per day, which at the stated price is €0.40 a day, or approximately €12 a month. An adsorption dehumidifier with an average power draw of 500 W, also 8 hours a day, consumes 4 kWh per day, i.e. €0.80 a day, which is approximately €24 a month - double that of the compressor type for the same running time. This is also why the adsorption type is worth it mainly where a compressor type simply can't work (low temperatures), and not as a universal choice for heated spaces.

When an air conditioner with a dehumidifying function is enough instead of a standalone dehumidifier

In occupied, heated spaces - a living room, bedroom, office - it's not always necessary to buy a separate dehumidifier. Most modern wall-mounted split air conditioners have their own "Dry" mode (dehumidifying), which lowers air humidity without significantly cooling the room. So if you're planning to install an air conditioner in the room anyway for summer cooling, the dehumidifying function is added value for free, and you no longer need a separate dehumidifier. This solution does have its limits, though - it requires a permanent installation of an outdoor and indoor unit (unlike a portable dehumidifier, which you can move between rooms), and it only works reliably within the normal room-temperature range, not in a cold, unheated cellar over winter. For occupied spaces where you're dealing with both summer heat and elevated humidity at the same time (for example a new build that's still drying out, but where you also want summer cooling), the "air conditioner with dry mode" combination is therefore a very practical choice.

From our range, this includes for example these Vaillant climaVAIR PRO units, which, besides cooling and heating, also offer a fully-fledged dehumidifying mode controlled via the remote:

VAIB1-025WN outdoor and indoor unit with remote control - the smallest output in the range (2.7 kW), suitable for smaller rooms up to approx. 25 m², where besides summer cooling you'll also appreciate the dehumidifying function outside the heating season. Price from €527.00.

VAIB1-035WN outdoor and indoor unit with remote control - mid output, suitable for an ordinary medium-sized living room or bedroom, where you want a single device to handle both summer cooling and ongoing dehumidifying during the transitional seasons. Price from €561.00.

VAIB1-050WN outdoor and indoor unit with remote control - the most powerful of the three, suitable for larger living spaces or open-plan layouts, where a single unit handles cooling, heating, and dehumidifying across a larger area at once. Price from €833.00.

However, if you're dealing with a cellar, garage, or cottage without permanent heating, or need a device you can easily move between rooms, an air conditioner won't replace that - there, a standalone portable dehumidifier remains the only sensible choice, ideally the adsorption type because of the lower temperatures.

Practical experience and tips from everyday use

Placement significantly affects a dehumidifier's real-world efficiency. The device should stand freely in the space, at least 15-20 cm from the wall and furniture, so it has enough airflow for both intake and outlet. Placing it tightly into a corner or behind furniture will noticeably reduce output. Doors and windows in the room should stay closed during operation - a dehumidifier works with an enclosed volume of air, and with an open window it's essentially "dehumidifying the outdoor air", which is a needless waste of energy. Regularly cleaning the air filter (usually once every 2-4 weeks under normal use) is key - a clogged filter reduces airflow, the device's output drops, and energy consumption per litre of water removed rises. For long-term operation (a cellar, laundry room), it's clearly worth using the option of permanent condensate drainage via a hose into the waste system instead of emptying the tank - you'll save yourself trips back and forth and also avoid the risk of a full tank overflowing and the device automatically switching off before you can intervene. Finally, when buying an adsorption type for a genuinely cold space (below 5 °C), always check whether the manufacturer states guaranteed functionality at that temperature - not all models on the market handle extremely low values equally well despite similar marketing labels.

Summary - what not to forget when choosing

In short, when choosing a dehumidifier: first check the temperature of the space (above or below 15 °C) and choose a compressor or adsorption type accordingly, then based on room size and humidity level determine the required capacity in litres per 24 hours (roughly 10 l for a smaller room with normal humidity, up to 20-30 l for a larger space or high humidity), check practical parameters such as noise level, tank size, and the option of permanent drainage, and finally consider whether, in a heated living space, an air conditioner with a dehumidifying function - which you're planning to install anyway for summer cooling - might be enough instead of a separate device.

Frequently asked questions

What's the difference between a dehumidifier and an air conditioner in Dry mode?

An air conditioner in Dry mode reduces humidity as a side effect of cooling air through a cold heat exchanger, and mostly drains the condensed water straight outside via the outdoor unit. A standalone dehumidifier is designed directly and exclusively for dehumidifying, can work more efficiently at a lower airflow, and doesn't require a permanent outdoor unit installation - so it's portable and more flexible between rooms.

Can I leave a dehumidifier running continuously even when I'm not home?

Yes, modern dehumidifiers have automatic shutoff when the tank is full, and many also have an adjustable target humidity level (a humidistat), after reaching which the device switches itself off and back on only when humidity rises again. For longer absences, it's safest to use permanent condensate drainage via a hose instead of relying on the tank.

Will a dehumidifier also help against existing mould on a wall?

A dehumidifier reduces air humidity, which slows further spread of mould and prevents it from returning after cleaning, but it won't remove an existing mould stain on a wall by itself - that needs to be treated mechanically and chemically on its own. A dehumidifier is therefore more of a preventive and supporting solution, not a replacement for wall remediation.

Is an adsorption dehumidifier suitable for a flat too, or is that overkill?

For a heated flat it's unnecessary in most cases, since a compressor type will be more powerful and more economical there at a lower price. An adsorption type is worth buying specifically where the temperature is permanently or seasonally lower than approximately 15 °C - i.e. cellars, garages, unheated cottages.

How long does it take for a dehumidifier to bring a room's humidity down to a normal level?

This depends on the room size, initial humidity, and the device's capacity, but with a correctly sized dehumidifier (per the table above), an ordinary room typically goes from high humidity (above 75 %) to a comfortable 50-55 % within 1 to 3 days of continuous operation. With an undersized device, this state may never be reached - humidity just stays just below its original value.

Is it worth buying a dehumidifier with wifi and an app?

Not essential for ordinary household use, but practical in cases where the device runs in a space you don't visit daily (a cottage, cellar, storage room) - you can remotely check the current humidity and tank status without having to physically go and check.

Do you have a question about this topic?

Can't decide, or dealing with a specific situation in your household? Write to us - we're happy to help.