How to choose a radiator with side connection
What is a radiator with side connection and why it is still the most commonly used type
A radiator with side connection is a classic type of heating unit, where the inlets and outlets for the heating water are located on the side of the panel – usually at the bottom, on one side (most often on the left, on some installations on the right). This is the oldest and still the most widespread type of radiator connection in apartments and family houses in Slovakia, because it corresponds to the classic routing of pipes along the wall or in the floor towards the corner of the room, under the window.
Unlike radiators with central (bottom) connection, where both connections are concentrated in the center of the bottom edge of the panel, side connection requires the pipe to come from the side – typically from under the sill or from behind the corner of the room. This very property makes side connection an ideal choice in renovations, where the old pipe is already running to the original radiator, and also in new buildings, where the designer routes the pipes along the perimeter wall.
In practice, we encounter this type when replacing old cast iron sectional radiators or panel radiators with new steel panel radiators – in about ninety percent of cases, the original pipe was connected to the side of the body, and therefore it is simpler and cheaper to replace the old radiator with a new one of the same connection type, rather than to rework the entire pipe routing to central.
Side vs. central connection – how it works technically
In order to make the choice clear even for a layperson, let's look at how the water flows inside the panel in both types of connection. In the case of side connection, the hot water enters one bottom corner of the panel, flows through internal channels upwards to the top collector, spreads across the entire width of the panel, and returns downwards as cooled water on the same side where the inlet is located (connection 1/2" or adapted for a thermostatic head and shut-off valve).
On the other hand, with central connection, the water enters the center of the bottom edge, divides symmetrically into both halves of the panel, and returns back through the center as well. This system is typical for radiators intended for floor-mounted pipes running perpendicular to the wall, where the pipe comes up from the floor directly under the center of the body. More about when central connection is more advantageous is discussed in a separate article: Side vs. central radiator connection – which is better and when to use which.
From the point of view of thermal performance, there is no fundamental difference between the two types of connection – it is more about a mechanical and installation solution adapted to the specific pipe routing in a given room. The decisive factor is always the condition and direction of the existing pipes, or the design of a new building.
When side connection is the right choice
From dozens of realizations we commonly encounter, several typical situations emerge where side connection is clearly the best and most cost-effective option:
- Replacement of an old cast iron radiator – if the pipes were routed to the side of the original body (which is almost standard in older apartments and family houses), a new radiator with side connection can be connected without any intervention in the masonry and without the need to cut the floor or wall.
- Renovation of an apartment in a panel building – the vertical pipes in panel buildings are often routed along the wall towards the window, where they connect to the side of the radiator.
- New construction with classic floor-mounted pipe routing under the sill – the designer often plans the pipe to come to the corner of the room, from where a side valve can be easily connected.
- Spaces where it is necessary to preserve the aesthetics under the window – side connection allows the radiator to be placed exactly under the sill without the need for symmetrical pipe placement in the center of the window.
On the contrary, if it is a completely new installation without existing limitations and the designer has free rein, central connection is often chosen due to the simpler routing of the pipe directly from the floor. However, the decision should always be based on the actual condition of the pipes, not on theoretical preference.
Type designation 21K, 22K, 11K – what it means and how it affects performance
The majority of panel radiators with side connection are sold in three basic construction types, which determine the number of panels and the number of finned (convective) plates on the back side:
| Type | Construction | Typical use |
|---|---|---|
| 10K / 11K | 1 panel + 1 row of fins (11K), or without fins (10K) | Smaller rooms, bathrooms, hallways, supplementary heating |
| 21K | 2 panels + 1 row of fins between them | Universal use – living rooms, bedrooms, offices |
| 22K | 2 panels + 2 rows of fins | Rooms with higher heat losses, corner rooms, older buildings without insulation |
| 33K | 3 panels + 3 rows of fins | Spaces with very high heat losses or limited wall space |
The number indicates the combination of panels and finning – the first digit is the number of panels, the second is the number of rows of convective fins between them. The higher the number, the higher the performance for the same size, but also the greater the depth of the body (for example, 21K has a depth of about 100 mm, while 22K is about 105 – 110 mm and 33K up to 155 – 160 mm). More detailed differences are discussed in the article What does the designation 21K, 22K, 11K mean – differences between types of panel radiators and in the article Difference between 21K radiator type and other types (10K, 11K, 22K, 33K).
For a typical living room or bedroom, the 21K type is usually sufficient in most cases – it offers a good balance of performance, depth (does not take up too much space in the room), and price. That is why 21K radiators in a height of 300 mm are one of the most popular categories – for example, Radiator 21K 300 x 400 performance 298W or the larger Radiator 21K 300 x 600 performance 447W.
How to calculate the required radiator power
Before selecting a specific size, it is necessary to know the approximate heat loss of the room. In practice, the most common method is a simplified estimate based on the room area and building type, or a more accurate calculation based on actual heat losses (perimeter walls, windows, orientation, insulation thickness).
Approximate values of the required power per square meter of floor area:
| Type of building | Required power (W/m²) |
|---|---|
| Newly built, well insulated (low-energy house) | 40 – 60 W/m² |
| Standard insulated building (after reconstruction, insulated panel building) | 60 – 80 W/m² |
| Older non-insulated building | 90 – 120 W/m² |
| Very old building without insulation, high ceilings | 120 – 150 W/m² |
Example calculation: a room with an area of 16 m² in a standard insulated apartment with a ceiling height of 2.6 m requires approximately 16 × 70 W/m² = 1,120 W of total heating power in the room. If there is only one radiator under the window in the room, it should have a minimum power of 1,100 – 1,200 W, which corresponds to a width of approximately 1,600 – 1,800 mm for a type 21K with a height of 300 mm, or it is more advantageous to choose a higher type (22K) with a shorter width.
For a smaller room, for example, a children's room of 10 m², the required power is approximately 700 W, which is well covered by, for example, Radiator 21K 300 x 700 power 552W in combination with a second smaller radiator, or one radiator of greater width. The exact calculation procedure with several examples can be found in the article "What radiator power do I need – calculation based on area and heat loss of the room" and "What radiator power do I need for a room (W/m²)."
Dimensions – what height and width is suitable for my room
A height of 300 mm is among the most practical dimensions for apartment and residential installations – it fits well under lower sills, under windows with a sill board at a height of around 60 – 70 cm above the floor, and at the same time provides sufficient power even with shorter widths. With this height, we most commonly encounter the following widths and corresponding powers in practice (type 21K):
| Dimension (h x w) | Power | Approximate use |
|---|---|---|
| 300 × 400 mm | 298 W | Small bathroom, hallway, WC, supplementary radiator |
| 300 × 500 mm | 373 W | Smaller room, office, kitchen corner |
| 300 × 600 mm | 447 W | Children's room, medium-sized bedroom |
| 300 × 700 mm | 552 W | Bedroom, study, combination with another radiator |
| 300 × 800 mm | 596 W | Larger room, living area of a smaller apartment |
Specific products in this dimension range can be found directly in the category, for example, Radiator 21K 300 x 400 power 298W, Radiator 21K 300 x 500 power 373W or Radiator 21K 300 x 800 power 596W. A complete overview of dimensions with recommendations according to room type can be found in the separate article "Radiator dimensions 300 mm height – which width is suitable for my room" and "What radiator dimensions to choose according to room size."
When choosing the width, it is important to consider not only the power, but also the actual free wall length under the window – the radiator should ideally be slightly narrower than the window opening (at least 10 – 15 cm from both sides of the window opening), to leave sufficient space for mounting the brackets and to ensure that the flow of warm air along the wall is not blocked by curtains or furniture.
Installation of a radiator with side connection – step-by-step procedure
The installation itself is not complicated, but it requires precision in measuring and adherence to a few rules to ensure the radiator functions reliably and without unnecessary leaks. The basic procedure is as follows:
1. Aiming and preparation
First, the exact position of the existing piping is determined (during renovation) or the position of the connections is marked according to the plan (during new construction). It is important to verify the distance between the inlet and outlet, which must correspond to the distance of the connections on the radiator – with a side connection, both connections are close to each other, usually at a distance of 50 mm, which is standardized by most manufacturers.
2. Mounting the brackets
The radiator is hung on wall brackets (supports), the number and placement of which depends on the width and weight of the radiator – for widths up to 600 mm, two lower brackets are usually sufficient, while for larger widths (over 1000 mm), it is recommended to add an upper support bracket. The brackets must be anchored into the load-bearing structure of the wall, and in the case of drywall partitions, special anchors or backing plates must be used.
3. Hanging and connecting
After hanging, the radiator is connected to the system using angled or straight radiator valves (in the case of thermostatic control with a thermostatic head on one side and a shut-off – draining valve on the other side). The connections are sealed with Teflon tape or hemp fiber with sealing paste, depending on the type of thread.
4. Bleeding and pressure test
After filling the system, the radiator must be bled through the bleed valve in the upper corner (on the opposite side from the connection) and then a pressure test should be performed, ideally at 1.3 – 1.5 times the operating pressure, to verify the tightness of all connections before covering the pipes or before painting.
A detailed guide including recommended tightening torques, types of seals and common mistakes by installers can be found in the article "Installation of a radiator with side connection – procedure, tools and common mistakes."
Most common mistakes when selecting and installing a side connection
Over the years of sales and advisory services, we repeatedly encounter several typical mistakes that can be easily avoided:
- Underestimating the power – the customer selects a radiator only based on the original dimensions of the old radiator, without calculating the actual heat loss. If the room has been insulated in the meantime or, on the contrary, the original radiator was undersized, the power must be recalculated.
- Wrong connection spacing – when ordering a replacement radiator, it is necessary to verify that the new radiator has the same or compatible connection spacing as the original piping, otherwise complicated modifications to the piping may be required.
- Mixing up the side of the connection – the side connection can be on the right or left side; before ordering, check on which side the piping comes out of the wall or floor.
- Forgetting the bleed valve – when ordering, it is advisable to purchase a set of accessories (valve, plug, bleed valve, brackets) at the same time, if they are not included in the package.
- Placing too close to the sill or floor – the recommended minimum distance from the floor is 100 – 120 mm and from the sill 100 mm, so that the air flow (convection) works properly and dust does not accumulate directly under the sill.
Practical examples from everyday practice
Example 1 – renovation of an apartment in a panel building: The customer is replacing the original steel radiator under the window in the living room of 18 m², with piping running in the floor and connecting to the side of the wall on the right. After calculating the heat loss (insulated panel building, approx. 65 W/m²), the required output is approximately 1,170 W. Due to the window width of 1.8 m, a radiator type 21K with a height of 300 mm and a width of 1,400 mm was selected, supplemented by a smaller radiator in the kitchen corner.
Example 2 – children's room in a family house: A room with an area of 11 m², a new build with insulation, requiring approx. 50 W/m² = 550 W. The selected radiator was Radiator 21K 300 x 700 output 552W, which exactly covers the calculated loss without unnecessary overdimensioning.
Example 3 – small bathroom with supplementary heating: Next to the stair radiator, it was necessary to add a small radiator under the window with a heat loss of approx. 300 W. A Radiator 21K 300 x 400 output 298W was used, which with its compact width fit into the narrow space between the door and the corner of the room.
Example 4 – older building without insulation: A room of 14 m² in a masonry house from the 1960s without facade insulation, with an estimated heat loss of 110 W/m² = 1,540 W. Due to the limited wall width (only 1 m of free space), instead of 21K, the more powerful type 22K of the same width was selected to achieve the required output without the need to extend the radiator beyond the available space.
How to measure correctly before ordering
Before ordering a specific radiator, we recommend checking and recording the following data directly at the installation site:
- The exact clear width of the wall or space under the window where the radiator is to be placed.
- The height from the floor to the lower edge of the sill (for lower sills, choose a radiator height of max. 500 – 550 mm; with a height of 300 mm, there will be sufficient reserve).
- The exact position of the pipe outlets from the wall or floor (distance from the corner, height above the floor).
- The spacing between the inlet and outlet pipes (for side connection usually 50 mm).
- The side of the connection (left/right) depending on which side the pipe comes out.
These data significantly speed up the selection of the appropriate model and reduce the risk that the radiator will need to be returned or the piping modified additionally.
Frequently asked questions (FAQ)
What is the difference between side and central connection in terms of performance?
In terms of thermal performance, there is practically no difference between side and central connection – both types of the same panel type (21K, 22K, etc.) and the same size achieve the same declared output. The difference is purely in the location of the connection to the piping and in the mounting method.
Can I replace an old cast iron radiator with a panel radiator with a side connection without modifying the piping?
In most cases yes, provided the same spacing and connection side are maintained. We still recommend measuring the distance between the inlet and outlet pipes precisely before ordering and comparing it with the technical parameters of the selected model.
Which type of radiator (21K, 22K, 33K) is suitable for an older, non-insulated house?
With higher heat losses, we recommend type 22K or 33K, which provide significantly higher output at the same width due to two or three rows of convection fins. If sufficient wall length is available, a 21K in a larger width may also be sufficient.
How do I know which side to order the connection – left or right?
The side is determined by which side (when looking directly at the radiator) the pipe comes out of the wall or floor. In a renovation, it is easiest to keep the same side as the original radiator had.
Is it necessary to purchase separate valves and brackets for the radiator?
It depends on the specific model and packaging – some radiators are sold with a mounting kit (brackets, air vent, plugs), others only as the body itself. We recommend checking the contents of the package in the product description before ordering.
What is the minimum recommended distance of the radiator from the floor and the sill?
The recommended distance from the floor is 100 – 120 mm and from the lower edge of the sill approximately 100 mm, to ensure sufficient air circulation (convection) and easy access for cleaning.
Summary
A radiator with a side connection is a reliable and proven solution for most apartment and family house installations, especially during renovations where the piping is already running to the side of the wall. The key to the correct selection is accurate measurement of the space and existing piping, correct calculation of the room's heat loss, and choosing the appropriate panel type (21K, 22K, or 33K) according to the available wall width. A complete range of 21K radiators in a height of 300 mm with various widths and outputs can be found directly in the category Radiators with side connection, where the selection can be further filtered according to the exact dimensions and required output.
Do you have a question about this topic?
Not sure or dealing with a specific situation in your home? Write to us – we are happy to help.