How to choose an air conditioner for a flat or house

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Why choosing an air conditioner deserves extra time

An air conditioner belongs to that category of appliances we buy rarely but use for years - often 10 or even 15 seasons. That's exactly why it's worth spending more than just a few minutes comparing prices. Choosing the wrong unit output means either needlessly high investment and operating costs, or, conversely, a room that still doesn't cool properly in August even running at full power. In heating technology we regularly come across customers who ask "which air conditioner is the best", when the right question is "which air conditioner is best for my particular room, orientation, insulation and usage pattern". In this article we go step by step through everything that leads to the right decision - from the type of unit, through calculating output, energy class, to practical installation experience that's often left out of typical guides.

This text is aimed at both the average flat or house owner deciding for the first time, and at someone who already has one air conditioner and is now dealing with another room. We won't pretend there's a single universal recommendation - instead we'll show you a way of thinking that you can apply to your own space.

Types of air conditioners - split, multi-split, portable

Before we get to output, we need to clarify what type of unit we're actually talking about. On the market we commonly come across three basic designs:

Wall-mounted split (single split)

The most common solution for a single room - a flat, bedroom, living room, office. It consists of an outdoor unit (compressor, heat exchanger) placed on the facade or balcony, and an indoor wall-mounted unit, connected by refrigerant piping. The advantage is quiet indoor operation, since the noisier components (compressor, outdoor unit fan) are outside the living space. This solution is also the core focus of this article, as we look at it using specific products below.

Multi-split

One outdoor unit powers two to five indoor units in different rooms. The advantage is only needing a single spot on the facade for the outdoor unit (important especially in apartment buildings, where facade space is limited or subject to approval by the homeowners' association). The disadvantage is that if the outdoor unit fails, all connected rooms lose cooling at once, and the sum of the indoor units' outputs usually somewhat exceeds the real output of the outdoor unit - meaning if all units run simultaneously at full output, the system won't reach the sum of their rated values.

Portable (free-standing) air conditioner

A unit on wheels with a hose routed through a window or wall. The main advantage is zero installation requirement and the ability to move it between rooms. The disadvantage is lower efficiency (warm air from the hose partly mixes with the air drawn from the room, which reduces the real cooling output compared to a split system of the same rated value) and noise, since the compressor runs directly in the room. For a seasonal or temporary solution (a rented flat, one hot room occasionally) it can be a reasonable choice, but it's not worthwhile long-term for year-round living.

For a comparison of wall-mounted and portable air conditioners in terms of cost, noise and practicality, we've prepared a separate detailed article - see the related topics section below. In this text we'll now focus mainly on wall-mounted split units, since for permanent living this is the most common and generally the most advantageous solution.

How to calculate the required air conditioner output

The most common mistake when choosing is to go solely by the room's area in square metres and a simple "so many watts per square metre" conversion. This calculation is only a rough guide - the actual required output is influenced by far more factors:

Compass orientation and daylight exposure - a room with large windows facing south or west heats up significantly more in summer than a north-facing room with small windows.
Quality of insulation - a new building with an insulated exterior envelope and quality windows needs less output than an older, uninsulated building of the same area.
Ceiling height - with higher ceilings (typically in older apartment buildings or houses with open-plan spaces), it's the volume of air that counts, not just the floor area.
Number and output of heat sources in the room - a computer, TV, several people present at once, a kitchen stove in a combined kitchen-living room - all of this adds to the heat load that the air conditioner has to remove.
Floor level - a flat under a flat roof or on the top floor with insufficient roof insulation heats up noticeably more in summer than a flat in the middle of the building.

As a practical rule of thumb, however, most retailers and manufacturers use a simple rule: with average insulation and a standard ceiling height (up to approx. 2.7 m), you allow roughly 100 W of cooling output per square metre; with poorer insulation, larger glazing or a south-facing orientation, this figure rises to 120-140 W/m². This calculation gives three common size categories, which we'll also show in the diagram below and which correspond exactly to the range of commonly available units:

Smaller bedroom or home office up to approx. 25 m² - a unit with around 2.5 kW cooling output is sufficient.
Medium living room or bedroom up to approx. 35 m² - a unit with around 3.5 kW output is suitable.
Larger living space, combined kitchen and living room up to approx. 50 m² - here we already need a unit with around 5.0 kW output.

These three values (2.5 kW, 3.5 kW, 5.0 kW) aren't random - they correspond exactly to three commonly available units in the Vaillant climaVAIR pro range, which we'll present in more detail below as a specific example of how unit output translates in practice into room size.

Diagram: guide output by room size

Note: this is a guide calculation for average insulation; with poorer insulation, significant glazing or a south-facing orientation, allow for higher output within the same area.

A specific choice - the Vaillant climaVAIR pro range

As a practical example, let's take a closer look at three units from a single range whose output exactly covers the three size categories described above. All three are single-split units with both an outdoor and indoor part and a remote control included, so they form a complete solution for one room.

VAIB1-025WN outdoor and indoor unit with remote control - the smallest unit in the range, suitable for a smaller bedroom, children's room or home office up to approx. 25 m². It's precisely in smaller rooms that an oversized unit becomes the biggest needless extra expense, so it's worth choosing exactly this category instead of a larger one "just in case". Price from EUR 527.00.

VAIB1-035WN outdoor and indoor unit with remote control - the mid-range variant, suitable for an average living room or larger bedroom up to approx. 35 m². It's the most universal choice for an average Slovak flat, which is why it's also the best-selling in practice. Price from EUR 561.00.

VAIB1-050WN outdoor and indoor unit with remote control - the most powerful of the three, suitable for a combined kitchen and living room or a larger open-plan space up to approx. 50 m². In open-plan layouts with a stove and several people present, this is usually the minimum sufficient choice, not an unnecessary luxury. Price from EUR 833.00.

Diagram: comparing the three units in the climaVAIR pro range

Energy class and operating costs

Rated output is only half of the equation - the other half is how much electricity the unit consumes to achieve that output. This is where energy class comes in (stated separately on the energy label for cooling and for heating, if the unit has that function), along with the efficiency coefficients SEER (for cooling) and SCOP (for heating). The higher these coefficients are, the less electricity the unit consumes for the same cooling or heating effect.

In practice this means two units with the same rated output (for example, both 3.5 kW) can have noticeably different electricity consumption over the course of a season - the difference between older technology and a modern, higher-class inverter unit typically amounts to tens of a percent in real operation. Over a full-season operation (cooling in summer, plus possibly supplementary heating in spring and autumn for units with a heating function), this difference shows up more noticeably on the electricity bill than the difference in purchase price might first suggest.

We therefore recommend never looking only at the purchase price when comparing offers, but also factoring in the expected operating consumption over the equipment's whole expected service life. You'll find a detailed breakdown of operating costs with specific examples in a separate article in the related topics section below.

Diagram: from measuring the room to booking installation

The last, sixth step (left out of the diagram for clarity) is booking professional installation - more in the separate article on installation below.

The heating function - when it's worth it

Most modern split air conditioners, including the climaVAIR pro range mentioned above, work as an air-to-air heat pump - so they can not only cool but also heat, generally effectively down to an outdoor temperature of roughly -15 to -20 °C depending on the specific model. This makes an air conditioner an interesting supplementary heat source, especially in spring and autumn, when the main heating system (boiler, heat pump) is no longer/not yet running but the room is still cold.

It's important to stress the word supplementary - an air conditioner with a heating function shouldn't replace the main heating system in a typical house or flat during the winter months, since its efficiency and output drop at low outdoor temperatures while electricity consumption rises. As a full-fledged, efficient year-round heat source, air conditioners with a lower operating temperature range, or directly air-to-water heat pumps connected to the heating system, are better suited. We cover this topic in more detail in a separate article on air conditioners with a heating function, linked below.

Noise level - an often underestimated factor

When choosing an air conditioner for a bedroom or children's room, noise level is at least as important as output. The indoor units of modern split systems commonly achieve around 19-22 dB(A) in quiet night mode, comparable to the quiet hum of a library - in practice a virtually inaudible background. The bigger issue tends to be the outdoor unit, whose noise level (typically 45-55 dB(A) depending on output and fan speed) can be disruptive, especially if poorly placed close to a neighbour's bedroom window or in a narrow courtyard shaft, where sound reflects.

A tip from experience: when choosing where to place the outdoor unit, think not only of your own windows but also your neighbours' - in apartment buildings this is a frequent source of disputes that can be completely avoided by simply relocating the unit by a metre or two. If the unit runs in quiet night mode for most of the year, manufacturers commonly let you enable this restriction directly via the remote control or app.

Diagram: indoor and outdoor unit noise level

Values are indicative and vary by specific model and chosen operating mode; the key factor is correctly placing the outdoor unit away from bedroom windows (both your own and your neighbours').

Other parameters worth checking

Inverter technology

Practically all modern units designed for households today use an inverter compressor, which smoothly varies its speed according to current demand instead of the simple on/off cycling of older technology. The result is a more stable room temperature, lower electricity consumption, and a longer compressor service life thanks to fewer start-ups.

Air filtration

A basic washable dust filter is a standard part of indoor units. Higher-end model ranges add finer filtration layers as well (for example for pollen or odours), which allergy sufferers in particular appreciate. The filter needs regular cleaning - a clogged filter reduces airflow, lowers real output and increases consumption. We cover the practical cleaning and maintenance procedure in a separate article below.

WiFi/app control

More and more units (either as standard, or via an optional WiFi module) allow control through a mobile app - switching on before you get home, setting a weekly schedule, or a notification when the filter needs cleaning. When deciding between two similar models, this feature can be a practical deciding factor.

Operating temperature range

If you plan to also use the unit for cooling during extreme summer heat (above 35 °C outside) or for heating during hard frosts, check the technical sheet for the specific range in which the manufacturer guarantees full output - with cheaper or older models, efficiency can drop significantly outside this range.

Common selection mistakes - what keeps coming up in practice

Buying the cheapest unit regardless of output - an undersized air conditioner runs continuously at full output, doesn't reach the desired temperature during greater heat, and wears out faster.
Conversely, needless oversizing - a significantly oversized unit cycles (switches on and off frequently), which worsens air dehumidification and comfort, and is also a needlessly higher initial investment.
Underestimating room orientation - the same square metre on the south side with large glazing needs noticeably more output than a north-facing room with a small window.
Ignoring noise for a bedroom - especially with cheaper models or a poorly placed outdoor unit.
Underestimating installation - even the best-quality unit loses output and service life if installed incorrectly (insufficient vacuuming of the circuit, poorly secured piping, incorrect condensate drain slope). We cover this topic in detail in a separate article on installation.

Summary - how to proceed step by step

If we were to summarise the whole process into a few steps that we recommend at every visit or phone consultation: first measure the room's area and height and note the window orientation. Then assess the quality of insulation and any other heat sources in the room (electronics, number of people, a kitchen stove in a combined space). Based on this, choose an output category - for a typical room up to 25 m², a unit around 2.5 kW is sufficient; for a medium living room up to 35 m², go for around 3.5 kW; for a larger combined space up to 50 m², around 5.0 kW. Then compare energy class and noise level between models within that output category, and finally book installation with a professionally qualified company - correct installation matters at least as much for output and service life as the choice of unit itself.

Frequently asked questions

Does the outdoor unit have to be larger than the indoor unit?

The outdoor unit contains the compressor and is indeed physically bulkier and heavier than the indoor wall-mounted unit. When choosing a spot on the facade or balcony, allow for sufficient space (generally tens of centimetres of clearance around the unit for proper airflow) and for the load-bearing capacity of the structure the unit will be mounted on.

Can an air conditioner be used for heating instead of a heating system?

Yes, for models with a heating function (air-to-air heat pump), this is common especially in spring and autumn as a supplementary heat source. During hard frosts in the middle of winter, however, efficiency drops and consumption rises, so it's not recommended as the sole year-round heating source for a typical house - more in a separate article on this topic.

How often does an air conditioner need servicing?

Basic cleaning of the washable filter can be done by the owner, recommended at least once every 4-6 weeks during the season under normal use. Professional servicing (checking refrigerant pressure, cleaning heat exchangers, checking electrical connections) is usually recommended once a year, ideally before the start of the summer season. You'll find a detailed procedure in the article on air conditioner maintenance and cleaning.

Is a multi-split system worth it instead of several separate split units?

It depends on the specific situation. A multi-split system saves facade space (important especially in apartment buildings), but if the outdoor unit fails, all connected rooms lose cooling at once, and the sum of real outputs when all indoor units run simultaneously is usually lower than the sum of their rated values. For separate rooms with different requirements (for example a bedroom used only at night and a living room during the day), separate split units may actually be more advantageous operationally.

Is professional installation required, or can an air conditioner be installed DIY?

Installing a split air conditioner involves working with the refrigerant circuit (brazing or press-fitting the piping, vacuuming, charging with refrigerant), electrical wiring, and often working at height on the facade. This work should be carried out by a professionally qualified company with the relevant licence - incorrect installation is the most common cause of complaints and reduced service life, even with a quality unit. More detail in the article on air conditioner installation.

Does choosing an air conditioner also affect the room's air humidity?

Yes, every air conditioner dehumidifies the air while cooling, since water vapour condenses on the cold heat exchanger. In rooms with permanently high humidity (for example a windowless bathroom, basement spaces), however, the air conditioner alone may not be enough, and a separate dehumidifier may be a better choice - we cover this topic in the article on choosing a dehumidifier.

Have a question on this topic?

Not sure what to decide, or dealing with a specific situation in your home? Write to us - we're happy to help.