How to choose a side-connected radiator – dimensions, output and room type
How to choose a radiator with side connection – complete guide to dimensions, power and room type
Choosing the right radiator is not as simple as it may seem at first glance. Customers often come in saying that they "need something for heating in the living room" – and that is, of course, a good start, but by no means sufficient. Each room has different thermal requirements, different shapes, different window glazing, different insulation. And to all of that come additional questions regarding connection to existing piping, the height of the sill, available wall space and many other factors.
In this article, we will focus specifically on radiators with side connection – their advantages, limitations, typical use and, above all, how to correctly choose the dimensions and power for a specific room. If you are not yet aware of the difference between types 11K, 21K and 22K, we recommend first reading the article What does the designation 21K, 22K, 11K mean – the difference between types of panel radiators, where this issue is discussed in detail. We will be working primarily with type 21K, which is among the most common in standard apartments and family homes.
What is a side connection and when to use it
A panel radiator with a side connection has the water supply and return located on the side (usually the right) side – specifically in the upper and lower corner. This solution was standard in older installations from the 70s to the 90s, when piping was run vertically along the walls and radiators were fed from the side. It is still the most popular option in original apartment blocks, when replacing radiators with newer types in panel buildings or in family homes with a similar layout.
Unlike central (bottom) connection, side connection does not require any floor cutting or running pipes underneath it. The piping comes from the side wall or from a window sill niche directly to the radiator connection. If you have existing piping with a side outlet in the room, the safest and most cost-effective choice is usually a radiator with the same type of connection.
More on the differences between side and central connections and when each is more suitable can be found in the article Side vs. central radiator connection – which is better and when to use which.
Radiator dimensions – what to measure before purchase
This is the point where the most misunderstandings arise. A customer measures the space under the window and comes with the number "I have 80 cm of space there". But that is not enough. When choosing a radiator, several dimensional parameters must be considered at the same time.
Radiator height and sill height
Radiator height is given without consoles and without valves. The mounting height (i.e. the total height including accessories and the necessary gap from the floor) is usually 5–10 cm higher. Radiators in the 300 mm range therefore have a nominal height of 300 mm, but the actual mounting height with consoles and a small gap from the floor will be around 360–380 mm.
Why does it matter? If you have a low sill – which is common in new buildings with large plastic windows – you must ensure that the radiator is not higher than the lower edge of the window. Otherwise, the radiator will cover the glazing and at the same time prevent proper airflow upwards along the glass (so-called thermal curtain). The 300 mm series is therefore an excellent choice precisely for low sills, where taller radiators (e.g. 600 mm) would simply not fit.
A more detailed comparison of widths in this height series can be found in the article Radiator dimensions 300 mm height – which width is suitable for my room.
Radiator width and available space
Radiator width directly affects its performance – the wider the radiator at the same height, the more heat exchange surface and the higher the performance. In practice, the width is chosen so that the radiator fills the space under the window or at least covers at least 60–70 % of the window width. A shorter radiator does not cover enough glazing width and in winter there may be condensation or even dew formation in the corners of the window.
Common available widths in the 300 mm range are, for example, 400, 500, 600, 700 and 800 mm. Each of these dimensions has a different thermal output and is suitable for a differently sized room:
- 400 mm – suitable for small rooms (bathroom, WC, closet, small pantry), narrow spaces, supplementary heat source
- 500 mm – medium-sized bathroom, child's room with good insulation, entrance hall
- 600 mm – 10–12 m² room with average insulation, smaller kitchen
- 700 mm – 12–16 m² room, study, bedroom in a standard apartment
- 800 mm – larger room, living room with one large window, kitchen with a dining area
Radiator depth and distance from the wall
Type 21K has a depth of usually 65 mm (with two panels and one rib section). This means that it protrudes approximately 8–10 cm from the wall (including consoles and an air gap, which is necessary for proper air circulation). If you have a vertical gas pipe on the wall or some protrusion, this must be taken into account and possibly use deeper consoles or a special forward mount.
Heater power – how to calculate it and why it matters
This is probably the most important part of the whole selection process. If you choose a radiator with too low power, the room will simply not be heated. If you choose one that is too powerful, your boiler will be constantly switching on and off (so-called cycling), which is energy inefficient and shortens the lifespan of the equipment.
A detailed procedure for calculating heat loss is discussed in a separate article What heater power do I need – calculation based on area and room heat loss. Here we summarize the basic rules that apply to standard apartments and houses of medium age (1970–2010).
Approximate calculation based on area
For a moderately insulated room with a ceiling height of 2.5–2.7 m, the following approximate rule applies:
- Well-insulated new build or after comprehensive renovation: 40–50 W/m²
- Standard panel building or masonry house without insulation: 70–90 W/m²
- Older buildings, weak windows, uninsulated walls: 90–120 W/m²
- Corner room, northern orientation, large window: add 15–25 %
Practical example: A kitchen in a panel building has an area of 11 m². Orientation is north, one plastic window from 2005 (replacing old wooden ones). Walls are not externally insulated. Area 11 m² × 85 W/m² = 935 W. This is the minimum power we need to cover in this room. If we have a maximum of 600 mm width available under the window and the window sill height is only 350 mm, we are limited to a height of 300 mm. One radiator 21K 300×600 with a power of 447 W is not enough – we will have to either choose type 22K (two panels, two rib sections = higher power), or place a radiator on the adjacent wall as well.
This specific scenario shows why the correct calculation is crucial even before you order anything.
Influence of heating system temperature on real power
Manufacturers specify the power of radiators under standard conditions: supply water temperature 75 °C, return temperature 65 °C, room air temperature 20 °C – i.e. at a temperature difference Δt = 50 K. Most modern condensing boilers operate at lower temperatures (55/45 °C or even 45/35 °C), which significantly reduces the real power of the radiator.
If you have a condensing boiler set to 55/45 °C, the real power of the radiator drops to approximately 65–70 % of the nominal power. At 45/35 °C, it is only around 40–50 % of the nominal power. Therefore, with condensing boilers, we always prefer radiators with a higher nominal power, or type 22K instead of 21K.
Room type and its influence on selection
Each type of room has its own characteristics that affect not only the required power, but also the appropriate dimensions and placement of the radiator.
Living room and dining room
The living room is usually the largest room in an apartment or house. It has large glazing, often a corner window or balcony doors, and sometimes an open space connected to the kitchen. Heat losses are highest here. For such a room, a typical solution is one larger radiator under the main window, supplemented by a smaller one under the balcony doors or on an adjacent wall.
A living room of 20 m² in a standard panel building requires a power of about 1,400–1,700 W. Radiator 21K 300×800 with a power of 596 W would not be sufficient on its own – you will need at least two of these or one wider radiator in a higher height series (e.g. 600 mm height).
Bedroom
The bedroom is a room where a higher temperature than in the living room is not required – 18–20 °C is sufficient instead of 22 °C. This reduces heat losses and allows the use of a radiator with lower power. A bedroom of 14 m² with good insulation requires a power of about 560–700 W. Radiator 21K 300×700 with a power of 552 W may be sufficient, or in combination with a thermostat set to a lower temperature.
Kitchen
The kitchen is an interesting room in terms of heating. It produces its own internal heat (cooking, oven, electrical appliances), so it does not require a very powerful radiator. On the other hand, there is increased humidity and the need for ventilation, which can increase heat losses. A typical kitchen of 9–12 m² can manage with a radiator of 298–447 W – that is, 21K 300×400 (298 W) for a smaller kitchen, or 21K 300×600 (447 W) for a larger one.
Children's room
A children's room requires a stable temperature of around 21–22 °C, so it is not advisable to underestimate the power. In addition, safety is important – radiators in children's rooms should have a thermostat with a temperature limiter to prevent the child from getting burned. A radiator of 300 mm with a lower surface is more suitable here than high types, because the surface under the window is closer to the children.
Bathroom
The bathroom is a special case – it requires a higher temperature (24–26 °C), but it is small. Panel radiators are used less often here (rather, finned ones), but if you use them, a small type is sufficient. Radiator 21K 300×500 with a power of 373 W will cover a standard bathroom of 4–6 m² in a panel building without any problems.
Entrance hall and corridor
The entrance hall is a room where heat losses are traditionally underestimated. It is the first zone between the exterior and interior, and at the door there is regular infiltration of cold air. A small radiator here also serves the function of quick heating when coming home. A type of 300×400 or 300×500 is usually sufficient here.
Practical scenarios from everyday customer practice
Over the years of sales, we have encountered many situations that repeat themselves. Here are several typical cases that will help you better understand the process.
Scenario 1: Reconstruction of an apartment in a panel building – replacement of old cast iron radiators
This is the most common case. The customer has cast iron radiators from the 80s in the apartment, which are in a poor condition (corroding, flaking). The piping is routed vertically in the walls, and the connection is side. The customer wants to replace the radiators with new panel radiators without any construction modifications.
Solution: Panel radiators with side connection, keeping the existing pipe spacing (usually 50 mm). It is crucial to measure the existing pipe spacing and ensure that the new radiators are compatible. Most standard panels have a 50 mm spacing (horizontal distance between the supply and return pipes). More on this in the article Connecting a radiator to existing piping – spacing, adapters, and sealing.
Scenario 2: New construction with a condensing boiler and low sills
The customer is building a family house with a condensing boiler set to temperatures of 55/45 °C. The windows are triple-glazed, and the sill is 25 cm from the floor. They want radiators under each window.
Solution: A height of 300 mm is ideal. Since the boiler operates at lower temperatures, the output is reduced to about 65 % of the nominal. For a living room of 22 m² with triple-glazed windows (heat loss of about 880 W), we need a radiator with a nominal output of at least 880 / 0.65 = 1 350 W – this means either a combination of two radiators or a switch to type 22K with a larger width.
Scenario 3: Adding heating in a hallway where there was no radiator before
The customer has a cold wall in the entrance and the heat from the living room is not enough. They want to add a small radiator. Piping is available only on one wall and it is routed side.
Solution: A small radiator 21K 300×400 or 300×500 depending on the size of the entrance. The installation is relatively simple, but attention must be paid to proper air venting of the new branch and hydraulic balancing of the entire system. Details can be found in the article Installation of a radiator with side connection – procedure, tools, and common mistakes.
Accessories and components you should not forget
Purchasing the radiator itself is not enough. Side connection requires several additional components, which are sometimes included in the price and sometimes not:
- Thermostatic valve – regulates water flow and room temperature. We strongly recommend it for every radiator in every room. Without it, you have no control and risk overheating and energy waste.
- Valve set (set of valves for side connection) – the set includes a thermostatic valve, a bottom regulating valve, and the necessary seals. For side connection, there are specific sets that are not interchangeable with sets for bottom connection.
- Brackets – load-bearing brackets are either delivered with the radiator or purchased separately. The number depends on the width of the radiator: 2 brackets are sufficient for widths up to 600 mm, while 3 brackets are needed for widths of 700 mm and more.
- Air vent cap – standardly delivered with the radiator. It is placed in the top left corner. More on proper venting can be found in the article Draining and cleaning a panel radiator – how to do it yourself.
- Blind cap – for the unused (bottom left) opening. It must be reliably sealed – in practice, we have seen leaks when the customer capped the opening loosely without a Teflon seal.
Most common mistakes in selection and installation
Over the years of sales, we know exactly where people most often make mistakes. Let's summarize them so you can avoid them.
1. Underestimating the output. "I have an old 6-section cast iron radiator there, so I will buy a similarly sized panel one." The problem is that a 6-section cast iron radiator has an output of about 750–900 W, while a panel 21K 300×600 has only 447 W. Replacing it with a similarly sized panel radiator may mean it will be cold in the room. Always compare outputs, not physical dimensions.
2. Ignoring the heating system temperature. As explained above, with condensing boilers, the real output is significantly lower than the nominal. Customers often forget this.
3. Forgetting the pipe spacing. With side connection, the distance between the upper supply and lower return is crucial. If this spacing is different from the standard (50 mm), you must use an adapter or bend the radiator with an adjusted spacing. More in the article Connecting a radiator to existing piping – spacing, adapters, and sealing.
4. Incorrect placement of the air vent. The air vent must always be at the highest point of the radiator – that is, in the top left corner (at the right side with side connection). Customers sometimes turn the radiator upside down and the air vent ends up at the bottom, which prevents proper air venting.
5. Neglecting hydraulic balancing. If you are adding a new radiator to an existing system, the system needs to be balanced. Otherwise, the new radiator might "steal" the flow from others, and those will become cold. If you are unsure, consult a heating technician. Typical problems are described in the article Common faults of panel radiators – cold spots, noise and leaks.
Tips for choosing the right width – a clear table
| Radiator (21K 300 mm) | Power (W) | Room area (75/65°C) | Room area (55/45°C) | Typical use |
|---|---|---|---|---|
| 21K 300 × 400 | 298 W | 3–4 m² | 2–3 m² | Toilet, closet, small entrance |
| 21K 300 × 500 | 373 W | 4–5 m² | 3–4 m² | Bathroom, entrance |
| 21K 300 × 600 | 447 W | 5–6 m² | 4–5 m² | Kitchen, children's room |
| 21K 300 × 700 | 552 W | 6–8 m² | 5–6 m² | Bedroom, study |
| 21K 300 × 800 | 596 W | 7–9 m² | 5–7 m² | Larger room, living room (supplementary) |
Note: The room area in the table assumes average insulation and a ceiling height of 2.6 m. For corner rooms, northern orientation, and older buildings, always choose a higher power or a wider radiator.
When one radiator is not enough – combining multiple units
In practice, it is not unusual that one room requires more than one radiator. A typical example is a living room with two windows or a room with a large balcony window. In such a case, it is much better to place a radiator under each window separately than to put one large radiator on one wall.
Why? Because a radiator under the window performs an important function of a thermal curtain – it prevents the cold air from the glass from entering the space. If you don't have a radiator under the window, you will feel the cold coming from the glazing in winter. This is why architects and heating technicians always prefer a radiator directly under the window over a radiator on the adjacent wall.
Two smaller units are also more hydraulically advantageous – each radiator can have its own thermostatic valve and be regulated individually, which increases comfort and saves energy.
Most frequently asked questions (FAQ)
Can I connect a panel radiator to an old side connection myself, without a heating technician?
Technically yes, if you have experience with plumbing work and have the necessary tools (wrenches, Teflon tape, adjustable wrenches). However, the work must be carried out on a turned off and drained heating system. In a panel building, the system is shared for the whole entrance, so you need to coordinate the shutdown with the building manager or maintenance company. If you are unsure, it is better to call a professional – a mistake in the connection can cause a water leak and significant damage. More details can be found in the article Installation of a radiator with a side connection – procedure, tools and common mistakes.
Why is my new panel radiator cold on one side and warm on the other?
The most common cause is air in the radiator. Panel radiators are more sensitive to air than cast iron radiators – air accumulates in the upper part and blocks the water flow. The solution is to bleed the air using the bleed valve in the corners of the radiator. If the problem persists even after bleeding, it may be due to poor flow (a poorly adjusted regulating valve) or dirt in the system. A detailed guide to solving the issue can be found in the article Common faults of panel radiators – cold spots, noise and leaks.
Does it matter whether I connect the fittings to the left or right side in a side connection?
Yes, most panel buildings and old pipe systems have the supply at the top and the return at the bottom on the same side (left or right, depending on how the pipe runs). A radiator with a side connection has standard connection holes on the right side, but there are also versions with left-side connections, or it is possible to change the side using an adapter coupling. Always measure from which side the pipe comes before purchasing.
I have a condensing boiler. Is a type 21K enough, or do I need a 22K?
It depends on the specific power you need. As we explained, at temperatures of 55/45 °C, the actual power of the radiator is only about 65 % of the nominal power. If your requirement is, for example, 500 W, and the nominal power is 596 W (21K 300×800) with an actual power of 388 W, it is not enough – you will need either a larger radiator or a type 22K (double panel, double fin), which has 30–40 % higher nominal power at the same dimensions. The differences between the types are explained in detail in the article What does the designation 21K, 22K, 11K mean – differences between types of panel radiators.
What pressure are panel radiators with side connections rated for, and can I use them in a panel building?
Standard panel radiators are certified for an operating pressure of 10 bar and a test pressure of 13 bar. In apartment panel buildings, the operating pressure is usually 3–4 bar, so the radiators are safe under these conditions with a large margin. A problem may arise in older systems where pressure tests are carried out at higher pressure – in such cases, consult the building manager.
How long does the installation of a panel radiator with a side connection take?
Replacing an existing radiator with a new one of the same type and dimensions takes an experienced plumber 1–2 hours, including turning off and restarting the system. If you are changing the dimensions or the type of connection, or if you are installing a radiator in a new location without existing piping, the time increases to half a day to a full day. You also need to add time for bleeding and checking the tightness after restarting the system.
Conclusion: What to take away from this article
Selecting a radiator with side connection is a combination of several factors: the physical dimensions of the available space, the thermal requirements of the room, the type of heating system, and the existing piping. None of these factors should be ignored, as each of them can be the reason why the radiator does not heat as expected.
For most standard apartments and family homes with a conventional hot water heating system and existing side piping, the 21K series in a height of 300 mm is a practical, cost-effective, and proven solution. Choosing the correct width depends on the size and character of the room – an approximate guide can be found in the table above. If you are unsure about the calculation, it is always better to choose a slightly more powerful radiator than to underdimension it – a thermostatic valve can always reduce the output, but an underdimensioned radiator cannot provide heat that it physically does not have.
Further practical information on common questions regarding installation and operation can be found in the article Frequently asked questions about radiators with side connection, which is also part of our Knowledge Center.
Do you have a question on this topic?
Having trouble deciding or dealing with a specific situation in your home? Write to us – we are happy to help.