What does the designation 21K, 22K, 11K mean – the difference between types of panel radiators

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What does the designation 11K, 21K and 22K mean – the complete guide to panel radiator types

When you first look at the range of panel radiators, you will come across designations such as 11K, 21K or 22K. For someone unfamiliar, these numbers and letters may seem like a mysterious code. In reality, it is a systematic technical designation that immediately tells you what the radiator is made of and how it works. Understanding this system will save you a lot of time when choosing and will help you avoid mistakes that I have seen very often among customers over the years – for example, buying a too weak radiator for a larger room or, on the contrary, an unnecessarily large and expensive one for a small bathroom corner.

In this article, we will go into the designations in detail, explain the construction of each type, compare their thermal outputs, advise which type belongs in which room, and show concrete examples from practice. If you are interested in how to calculate the required radiator output, see our separate article What radiator output do I need – calculation according to room area and heat loss.

Panel radiator designation system – how to read the numbers and the letter K

The designation of a panel radiator consists of two digits and the letter K (sometimes the letter C or S is used instead, depending on the manufacturer, but the principle is the same). Let's break down each part separately.

First digit – number of heating panels

The first digit indicates the number of metal heating panels, i.e. hollow steel plates, through which hot water from the heating system flows. These panels are the carriers of performance – the more of them there are, the more heat the radiator can deliver to the room. In practice, we encounter values of 1 and 2 (rarely 3 in special types).

1 – single-panel radiator (one heating panel)
2 – double-panel radiator (two heating panels in a row)
3 – triple-panel radiator (three panels, less common, for demanding applications)

Second digit – number of convection fins (lamellas)

The second digit tells you the number of sets of convection fins, i.e. corrugated steel lamellas, which are attached between the individual panels. These fins significantly increase the total surface area of the radiator and allow for more efficient heat transfer by air flow (convection). Hot air rises upwards, cold air is drawn in from below and circulates through the lamellas.

0 – without convection fins (purely radiant radiator)
1 – one set of convection fins
2 – two sets of convection fins

Letter K – what does it mean?

The letter K comes from the German word Kompakt and indicates that the radiator has a steel cover (grid) on the top side, which covers the convection fins. This cover serves not only aesthetically – it evenly distributes the outgoing air and also protects the fins from mechanical damage and the settling of larger dust. Radiators without a cover are marked without the letter K and are nowadays rather exceptional, usually these are older or specialized types.

Type 11K – single-panel radiator with one set of fins

The 11K radiator is the simplest and thinnest version of a panel radiator. It consists of one heating panel and one set of convection fins, covered by a top cover. The depth (thickness) of such a radiator is typically around 61 mm, making it an ideal solution for spaces with limited depth in a recess or for places where minimal protrusion from the wall is important.

From the point of view of thermal performance, the 11K is the weakest of the standard types. A significant portion of the heat is transferred by radiation – the heat radiates directly from the surface of the panel into the surrounding space. Convection does work as well, but with only one set of fins, it is less intense than in double-panel types. The result is a pleasant, more even heat, but at the expense of a lower overall output for the same front panel area of the radiator.

Where is 11K used in practice?

From my experience, the 11K type most often appears in the following scenarios:

Entrances and corridors – spaces that do not require high performance, but the customer wants a radiator that protrudes as little as possible and does not hinder passage
Summer kitchens and garage extensions – spaces with lower comfort requirements, where cost is more important than maximum performance
Recesses with limited depth – for example, under a staircase or in historic buildings with thick walls, where the recess for the radiator is shallow
Supplementary heating – as a second, auxiliary radiator in a room, where the primary heat source is another (e.g. underfloor heating)

Type 21K – double-panel radiator with one set of fins

Type 21K is an interesting hybrid. It has two heating panels, but only one set of convection fins, which is placed between the two panels. Thanks to the two panels, it has a higher radiant output than the 11K, while the convection component remains medium. The depth of a 21K radiator is typically around 77–80 mm.

Exactly this type is very popular for standard living rooms, where you want a higher output than the 11K, but you do not want a radiator that is too thick. The heat from the 21K is characterized by a combination of radiation from the front and back panels and medium convection through the fins – many customers find this feeling of heat more pleasant and less "aggressive" than the purely convective type 22K.

An example from our range is the Radiátor 21K 300 x 600 výkon 447W, which is well suited as a base radiator for a medium-sized living room or a child's bedroom. For smaller spaces, such as a wardrobe or a small bathroom, the Radiátor 21K 300 x 400 výkon 298W or the Radiátor 21K 300 x 500 výkon 373W will suffice.

Where is 21K used in practice?

Living rooms – rooms where a pleasant, not too intense radiant output is important
Children's bedrooms – for the same reasons, the warm surface of the front panel is less hot than that of a 22K with the same output
Medium-sized living rooms – when we want a radiator of reasonable thickness but with sufficient output
Office spaces – where even, not too turbulent heat is important
Renovations of older apartments – where old cast iron radiators are still in use and the customer wants to retain the character of "radiant" heat but switch to a more modern panel

Type 22K – double-panel radiator with two sets of fins

The 22K radiator is the performance king among standard panel radiator types. It has two thermal panels and two sets of convection fins – each set is placed directly behind one of the panels. The depth of this radiator is typically around 100 mm.

Thanks to the two sets of fins, the convective component is significantly stronger – air flows more intensively, which results in faster room heating after turning on the heating. This is appreciated by everyone who has an intermittent heating system or who enters a cold room and wants to warm it up quickly. On the other hand, stronger convection means more movement of air masses, which may be less suitable in some cases (sensitive individuals, a lot of dust) than the radiant type.

From a comparative point of view: if we took an 11K radiator of dimensions 600 × 1000 mm and compared it to a 22K radiator of the same dimensions, the 22K will have a performance roughly 80–90 % higher, which is a huge difference. This means that the 22K can cover a significantly larger room at the same external dimensions.

Where is 22K used in practice?

Living rooms and large social spaces – where high output is needed without the need to install an oversized radiator
Ground-floor apartments and corner rooms – these have increased heat losses through the floor and side walls, 22K easily covers the deficit
New buildings with strict energy standards – where work is done with lower system temperature parameters (55/45°C instead of the old 75/65°C) and it is necessary to compensate for the lower temperature with a larger fin area
Kitchens – fast air heating is welcome, convection does not matter because in the kitchen, steam and odors move quickly anyway
Spaces with frequently opened external doors – entrance halls of family houses, receptions, shops

Other types – 10K, 20K, 33K and special versions

Although 11K, 21K and 22K are by far the most common in standard residential construction, it should be known that the system also allows for other combinations:

10K – single-panel without fins, purely radiant. Today it is used exceptionally, mostly in historical buildings or for decorative applications
20K – double-panel without fins. Same logic – larger radiant surface, no convection
33K – triple-panel with three sets of fins. The most powerful standard type, depth over 150 mm, used in industry, workshops, large halls
Type C (some manufacturers) – same construction as K, only without side covers. Today mostly replaced by K-types in production as well.

How the type of radiator affects performance – specific numbers

To stay within the realm of real values and not just abstract percentages, let's look at a specific series from our range. A height of 300 mm with different types and widths gives us a tangible idea of how many watts each type provides.

Type	Height × width	Power (W)	Approximate use
21K	300 × 400	298 W	Bathroom, WC, hallway up to 8 m²
21K	300 × 500	373 W	Entryway, cloakroom, small bathroom
21K	300 × 600	447 W	Children's room, bedroom up to 12 m²
21K	300 × 700	552 W	Bedroom, study up to 14 m²
21K	300 × 800	596 W	Larger bedroom, studio up to 16 m²

The performance values are measured according to the EN 442 standard under standard conditions (supply water temperature 75°C, return 65°C, room temperature 20°C – so-called ΔT50 conditions). In real practice of a modern low-temperature system (e.g. 55/45°C), the actual performance will be lower, usually by 25–40 %. If you are working with a modern condensing boiler or heat pump, treat the catalog values as approximate and calculate with a correction factor. More about calculations can be found in the article What radiator power do I need – calculation based on area and room heat loss.

A concrete example from practice: a customer was to be equipped with a 21K 300 × 700 with a power of 552 W for a bedroom with dimensions 3.5 × 4 m in a well-insulated house. With room heat loss calculations around 450 W, this radiator provided enough reserve, while being compact enough and not taking up too much wall space under the window.

Radiation versus convection – which type of heat is better for you?

This is one of the most common questions I receive from customers who want to seriously consider their choice and not just buy anything. Both heat transfer mechanisms are legitimate and safe for health, but each has different properties.

Radiant heat – heat spreads in the form of infrared radiation directly from the radiator surface to surrounding objects and surfaces (walls, floor, furniture, people). These objects then radiate heat into the room themselves. The result is an even temperature throughout the room without significant air movement. People with a tendency to dust allergies will appreciate this type, as dust is raised much less with radiant heating. Types with a higher proportion of radiation: 10K, 20K, and to a lesser extent 11K and 21K.

Convective heat – heat is transferred by air flow. Cold air comes to the fins, heats up and rises, spreading through the room. This mechanism is faster – the room heats up sooner. The drawback is more intense air movement, which can stir up dust. In rooms with plush carpets and plush toys, this is noticeable. The type with the highest proportion of convection: 22K, 33K.

In practice, these mechanisms are never pure – each panel radiator transfers heat by both methods simultaneously, just in different proportions. For families with small children and asthmatics, I recommend types with a higher proportion of radiation (11K or 21K). For spaces where you need to warm up quickly after coming in from outside (entryway, living room with a frequently used entrance), the faster 22K is a better choice.

Dimensional advice and heights – what else the numbers in the name tell you

Besides the designation 11K/21K/22K, there are two more numbers in the radiator name – height and width in millimeters. For example, Radiator 21K 300 x 800 power 596W tells us that it is a type 21K, height 300 mm and width 800 mm.

Height of 300 mm is one of the most commonly used in the series of panel radiators with side connection – it fits well under low sills, in spaces with low windows or where we don't want a too bulky radiator. If you are looking for details about which width is suitable for your room at a height of 300 mm, see the article Radiator dimensions 300 mm height – which width is suitable for my room.

A general rule when choosing dimensions: height should take precedence over width. A taller radiator at the same width will always be more powerful and will have a larger radiating surface. The width is adjusted to the available space under the window or on the wall. If the type 21K is not sufficient to cover the heat loss at the available width, consider switching to type 22K.

Practical decision-making – recommendation table by room

To help you navigate better, I have prepared a brief recommendation table for typical home rooms. These recommendations are based on experience from standard customer implementations in insulated houses and apartments with gas boilers or heat pumps.

Room	Recommended type	Reason
Bedroom	21K	Radiant heat, quiet operation, healthier air
Children's room	21K	Less hot surface, less dust disturbance
Living room	22K	Higher output, faster heating, larger space
Kitchen	22K or 21K	Depends on area and heat loss, convection is not a problem
Bathroom	21K or bathroom radiator	Small area, special solution for towel drying
Entryway / hallway	11K or 21K	Low output is sufficient, thin construction is important
Study	21K	Comfortable heat for long stays, quiet operation
Entrance hall, reception	22K	Compensation for heat loss at frequently used entrance

How radiator type relates to connection type

Types 11K, 21K and 22K are available both in side connection and central (bottom) connection versions. In the category radiators with side connection you will find radiators where the inlets and outlets are located on the side of the radiator – usually on the lower right and upper right side, or it is possible to turn the valve set to the left side.

Side connection is ideal for existing pipe systems where the pipe comes out of the wall in the side part under the radiator. It is also more common in renovations where an old radiator is replaced with a new one. On the contrary, central connection (so-called "H" connector) is typical for new buildings where the pipe comes directly from the ground under the center of the radiator. More about this decision can be found in the article Side vs. central radiator connection – which is better and when to use which.

From the point of view of performance, the type of connection has no effect on the radiator's thermal output – it is purely a hydraulic issue. The output is determined by the construction (type K), dimensions and temperature parameters of the system.

Warranty and material parameters – what to consider when comparing different manufacturers

When comparing offers from different manufacturers (Korado, Purmo, Kermi, Radik, Vogel&Noot and others), pay attention to several technical parameters that are not included in the 11K/21K/22K designation, but affect the quality and durability:

Maximum working pressure – standard is 10 bar for apartment installations, in some panel buildings with central heating 13 bar may be required. Check this before buying a cheaper radiator with lower pressure resistance.
Maximum water temperature – standard is 120°C, which is more than enough for all standard systems. Problems may arise only with steam systems, which are rarely used today.
Steel sheet thickness – cheaper radiators may have thinner sheet metal, which negatively affects corrosion resistance and lifespan. Reputable manufacturers usually specify panel thickness of 1.2–1.5 mm.
Surface treatment – most radiators have powder coating, but quality depends on the manufacturer. Resistance to mechanical damage and yellowing at higher temperatures are relevant criteria.
Manufacturer warranty – ranges from 2 to 10 years depending on the manufacturer and model. A longer warranty usually indicates higher production quality.

Installation and mounting – brief overview by type

The installation of radiators of types 11K, 21K and 22K does not differ significantly – all are mounted on the wall using brackets (supports), which are either part of the delivery or purchased separately. It is important to consider the depth of the radiator when mounting the brackets – for 22K you must ensure that the radiator is sufficiently spaced from the wall (usually at least 30–50 mm from the wall to the back panel, so that air can freely flow from below).

When replacing an old radiator with a new one, pay attention to the distance between the connections. The standard distance for side connection is fixed by the dimensions of the radiator, but with adapter valves and reductions you can elegantly solve most situations. A more detailed procedure can be found in the articles Installation of a radiator with side connection – procedure, tools and common mistakes and Connecting a radiator to existing piping – spacing, adapters and sealing.

One practical note: most panel radiators with side connection are delivered with plastic plugs on all four connections. These plugs should be removed only from the openings that are actually used – the remaining ones can be left or replaced with metal blind plugs with sealing.

Most frequently asked questions (FAQ)

Is type 22K always better than 21K because it has higher output?

No, higher output does not automatically mean a better choice. Type 22K is thicker (about 100 mm compared to 77 mm for 21K), more convective and more expensive for the same dimensions. For rooms where lower output is sufficient (bedroom, children's room), 21K is ideal – it is quieter, has a more pleasant heat character and is less prone to dust disturbance. Type 22K makes sense where you really need higher output for a given front panel area of the radiator.

Can I connect a 21K radiator in place of a 22K radiator and vice versa?

Hydraulically yes – the connections are in the same place for the same height and width. The problem may be the output: if you replace a 22K with a 21K of the same dimensions, you will lose about 25–30% of the output. This may be noticeable in cold winter. Conversely, replacing a 21K with a 22K increases the output, which is usually welcome, but you must have enough space for the thicker radiator (100 mm depth instead of 77 mm).

What impact does type K have on energy consumption – is one type more energy efficient?

Not directly. All types deliver the same amount of energy that you put into them from the boiler. The difference is in the speed of heat delivery – 22K delivers heat faster, the room heats up sooner, but the thermostat stops regulation sooner as well. With a well-regulated system using thermostatic heads, the consumption is practically the same for all types under the same conditions.

Can I combine different types in one apartment?

Yes, and it is very common in practice. Each room has different thermal needs, different dimensions and different requirements for the type of heating. A 21K in the bedroom, a 22K in the living room, and an 11K in the entryway – such a combination is perfectly fine. It is hydraulically problem-free as long as the system is properly balanced with thermostatic valves and possibly regulating heads.

Why can a 21K radiator have the same performance as a smaller 22K radiator?

Because performance is a function of the total heat exchange surface, not just the type. A wider or taller 21K can have a larger panel and fin surface than a shorter 22K. For example, a 21K 600 × 1200 can have a higher total output than a 22K 300 × 600. That is why it is always important to look at the specific watt values in the catalog, not just the type.

Where can I find answers to further questions about side connection radiators?

In our Knowledge Center, we have a whole series of articles covering the topic from selection through installation to operation. Specifically, I recommend: How to choose a radiator with side connection – dimensions, output and room type, Depressurization and cleaning of panel radiator – how to do it yourself, and also Common problems of panel radiators – cold spots, noise and leaks. If you are interested in a comprehensive overview of the most common dilemmas when choosing, also see Common questions about radiators with side connection.

Conclusion – which type to choose?

The 11K, 21K and 22K labeling system is logical and, once you understand the basic principles, it will tell you most of what you need to know on its own. The first digit = number of panels (= output and thickness), the second digit = number of fin sets (= intensity of convection), the letter K = sealed

Do you have a question about this topic?

Having trouble deciding or dealing with a specific situation in your home? Write to us – we are happy to help.