Bottom vs. side radiator connection – advantages, disadvantages, and when it's worth it
Bottom connection of a radiator vs. side connection – advantages, disadvantages and when it pays off
When deciding to buy a new panel radiator, most people focus primarily on performance, dimensions and price. The type of connection usually comes into play only during installation – and it is precisely at this point that it becomes clear whether you have chosen correctly or not. Bottom and side connections are not just technical details: they significantly influence the aesthetics of the room, the ease of installation, the functionality of the heating system and long-term satisfaction with the result. In this article, we will thoroughly examine both types, compare them in all important parameters and show you when one solution is really better than the other.
Basic principle: what is hidden behind the term "type of connection"
A panel radiator is essentially a steel tank filled with hot water that transfers heat to the room. In order for the water to flow, it must have two connections to the pipe system – the supply (hot water enters the radiator) and the return (cooled water returns to the boiler). These two connections can be located at different places on the body of the radiator, and it is precisely according to this that we speak of the type of connection.
With a side connection, both openings – the supply and the return – are located on the side of the radiator, usually on the right or left, in the upper and lower corner. The pipes therefore come to the radiator from the side, either from the wall or from the floor through the side. This was the standard for many decades in panel apartment buildings built in the 70s–90s.
With a bottom connection, both openings are located at the bottom of the radiator, most often in the center or very close to each other. The pipes run directly from the floor or from the wall just above the floor and "disappear" under the radiator. This type of connection has become the dominant standard in new buildings since the end of the 90s and is increasingly popular in renovations of older apartments as well.
History and development: why bottom connection became the standard
In the 70s and 80s of the last century, side connection was the absolute norm. The reason was simple: the pipes ran along the walls, visibly, and the vertical pipes passed through the room. The radiator was simply connected to the vertical pipe with side-mounted fittings. The whole system was installation-friendly and repairable without demolition.
With the advent of new buildings with floor pipes (pipes hidden in the floor structure, without visible vertical pipes), the situation changed. The pipes began to "come out" from the ground just before the place where the radiator stands, and it was logical to have the connection points on the radiator at the bottom. This gave rise to and mass expansion of the bottom connection system.
Today, bottom connection is almost the exclusive standard in new buildings. In the renovation of old apartments, we increasingly encounter conversion – the customer wants to hide the pipes, move them into the wall or floor and at the same time replace the old radiator with a side connection with a more modern one with a bottom connection. This combination of work brings a cleaner result, but requires thorough planning.
How hydraulics works with bottom connection – one of the most common misunderstandings
Many installers and customers fear that bottom connection will be less hydraulically efficient than side connection. This is a understandable intuition: with side connection, hot water enters from the top (top side opening) and cold water leaves from the bottom, which corresponds to natural convection. With bottom connection, water enters from the bottom on one side and leaves from the bottom on the other side – and it seems that the water "does not rise" upwards.
In reality, it works differently. Modern radiators with bottom connection have a special hydraulic circuit inside (usually a steel insert or pipe) that distributes hot water along the entire width and height of the radiator. The water enters through the bottom opening, is guided by an internal pipe upwards and is distributed along the panel from top to bottom – effectively simulating the same flow as with side connection. Result: the difference in thermal efficiency between a properly dimensioned radiator with bottom connection and side connection is minimal, usually within 2–5 %.
Advantages of a radiator with bottom connection
1. Aesthetics and clean room appearance
This is the most convincing argument for most customers. With a bottom connection, the connecting pipes are hidden – either running in the floor structure and emerging directly under the radiator, or running in wall grooves and also disappearing under the bottom edge of the body. The result is that no vertical pipes, elbows or pipe bends are visible on the wall. The radiator looks as if it is "floating in the air" along the wall. In a modern interior, where minimalism and clean lines are emphasized, this is a significant advantage.
By contrast, with a side connection, you must expect that a visible elbow with a pipe bend will emerge from the side of the radiator, from where the pipe turns towards the wall or into the floor. Although this can be aesthetically solved (covers, cabinets, plastering), it is never as clean as a bottom connection with a floor distribution system.
2. Compatibility with modern floor distributions in new buildings
If you are buying a radiator for a new building constructed after the year 2000, you can be almost certain that plastic floor distributions (usually multilayer or PEX pipes) are laid in the floor, emerging from the ground at the radiator location. A radiator with a bottom connection can be connected directly and elegantly to these distributions. Buying a side radiator would mean either an aesthetically unpleasing solution with elbows or unnecessary extension of the distributions.
3. Simplicity when replacing and renovating
If you are replacing an old radiator in a room where the distributions already run from the floor (which is the case in most renovations of 90s apartments and newer buildings), replacing a radiator with a bottom connection is very straightforward. You remove the old one, install the new one, connect the bottom connections – done. You do not need to extend or move the distributions.
4. Hidden zone regulation and thermostatic valve
Modern radiators with a bottom connection come with an integrated bottom connection valve (so-called H-piece or combined valve), which hides the thermostatic valve, regulating insert and air vent. The entire regulating technology is thus hidden under the radiator or in a cover socket at floor level. This is a significantly more elegant solution than a visible thermostatic head valve protruding from the side of a side radiator.
5. Lower probability of damage from accidental contact
Side valves and elbows with a side connection are exposed to accidental impacts – during cleaning, moving furniture, or simply while moving around the room. With a bottom connection, the entire connecting part is hidden below, near the floor, where the risk of mechanical damage is lower.
Disadvantages of a radiator with bottom connection
1. More difficult air venting
This is one of the most frequently mentioned practical problems. With a side connection, the air vent is located in the upper side corner of the radiator – exactly where air naturally accumulates. With a bottom connection, air venting is solved either through an integrated insert in the bottom valve or through a separate air vent located in the upper corner of the radiator. If you do not have an automatic air vent, you will have to "reach" up for air venting, which is not a problem, but you need to think about it. A more detailed procedure can be found in the article Air venting and flow setting for radiators with bottom connection.
2. Higher sensitivity to correct installation
A bottom connection allows for fewer errors than a side connection. If the floor outlets are slightly shifted compared to the actual position of the radiator's central axis, the installation can be complicated. You need to carefully measure the position of the outlets before ordering the radiator and check whether the spacing of the radiator connections matches the spacing of the floor distributions. More on this topic can be found in the article Installation of a radiator with bottom connection step by step and also in the article Connecting a radiator to existing distributions – what you need to know before installation.
3. Slightly lower thermal output for the same size
As we mentioned in the section on hydraulics, the difference is small (2–5 %), but it exists. When dimensioning the output, you should take into account that the catalog outputs of radiators with bottom connection are given for standard conditions and usually already consider the hydraulics of the bottom connection. Nevertheless, if you are "on the edge" of output, it is reasonable to choose a slightly larger radiator. A detailed guide to the calculation can be found in the article What radiator output do I need – calculation based on room area and height.
4. Higher price compared to side connection
A radiator with a bottom connection is usually 10–25 % more expensive than a radiator of the same size with a side connection. The difference is caused by the internal hydraulic insert and the additional valve set (H-piece), which most manufacturers supply as part of the product or as an accessory. In a complex renovation, this is usually not a decisive factor, as you save on installation and distribution modifications.
Advantages of a radiator with side connection
A side connection still has its place and justification. It is not outdated – it just fits into a different context.
1. Compatibility with old distributions
In panel buildings and older family houses, the distributions are typically run as vertical pipes along the walls. The outlets are located at the radiator at a height of 30–60 cm from the floor, on the side. Here, a side connection is natural and appropriate – unnecessarily complicating the situation by switching to a bottom connection without relocating the distributions would be pointless.
2. Easier air venting and service
The air vent in the upper side corner is easily accessible and works reliably, as air actually rises there. The thermostatic valve on the side is also easier to access when replacing the head or adjusting it.
3. Lower price
Radiators with a side connection are cheaper, as they do not have an internal hydraulic insert. For standard distributions, this is a fully sufficient solution.
4. Greater variety of types and dimensions
Although the situation is improving, it is still true that a side connection is available in a wider range of manufacturers and types. Some special types of radiators (bathroom finned, design vertical) are available exclusively with a side or central side connection.
Disadvantages of a radiator with side connection
- Visible pipe on the side of the radiator, less aesthetically pleasing in a modern interior
- Necessity to run the pipe along the wall or to embed it – both solutions have compromises
- Thermostatic valve and valve head are visible and protrude from the side
- In new buildings with floor distributions, a side connection is unsuitable or requires extending the distributions
- More prone to mechanical damage to side valves
When is bottom connection worth it – specific scenarios from practice
Scenario A: New construction of a house or apartment
Almost always you will choose bottom connection. The heating designer will plan the piping in the floor or in the walls with outlets at floor level at the locations where radiators will be installed. Investing in radiators with bottom connection is fully justified here – the result is clean, modern and hassle-free.
Scenario B: Comprehensive apartment renovation with demolition work
If you are renovating an apartment "from scratch", tearing down walls, changing the entire layout, doing new flooring and electrical work, it is logical to relocate the heating piping into the floor and install radiators with bottom connection at the same time. The cost of relocating the piping will be "lost" in the overall renovation budget and the result will be unbeatably better.
Scenario C: Replacing one radiator in an apartment without further work
Here you need to consider. If the existing piping is side (outlet from the side of the wall or from the riser), replacing one radiator with a model with bottom connection will require relocating the piping – this is additional construction work, cutting grooves, moving valves. The costs may exceed the benefit. In such a case, it is more reasonable to stay with side connection or calculate precisely whether the change is worth it.
On the contrary, if the existing radiator has a bottom connection and you want to replace it with a larger or more modern one, replacing it with the same type is trivial: you remove the old one and install the new one of the same width/connection spacing.
Scenario D: Living room with a large glazed wall
Here aesthetics is especially important. Under a panoramic window with a view to the garden you want a radiator that looks clean and unobtrusive. Bottom connection is clearly the winner in such a case – no visible pipes, no elbows, no valve heads sticking out from the side.
Scenario E: Panel apartment with old risers
This is a classic case where you should seriously consider whether changing the connection type makes sense. If the piping is old steel risers running vertically along the wall and the outlets to the radiator are on the side at a height of about 30–50 cm, stay with side connection. Changing it would require overhauling the entire piping system, which is disproportionately costly. It is enough to replace the old sectional radiator with a new panel radiator – but with side connection.
Types 21VK – what does it mean and why the panel type matters
Radiators in the bottom connection category are available in various type series. The designation 21VK is a standard coding notation for steel panel radiators:
- 2 – number of heating panels (two)
- 1 – number of convective fins between panels (one)
- VK – Ventil Kompakt, i.e. model with built-in valve section for bottom connection
Type 21VK is one of the most widespread and versatile – it offers a good balance between compactness, performance and price. It is suitable for most living rooms.
The performance of a radiator mainly depends on its height and width (length). For a better idea: Radiator 21VK 300 × 400 with a power of 298 W covers a small room or a hallway, while Radiator 21VK 300 × 800 with a power of 596 W is suitable for a larger room or a room with poor thermal insulation.
For medium-sized rooms with an area of 10–15 m², popular dimensions are Radiator 21VK 300 × 600 with a power of 447 W or Radiator 21VK 300 × 700 with a power of 522 W. The exact procedure for calculating the required performance is described in detail in the article What radiator power do I need – calculation according to room area and height.
The selection of the correct dimensions (height 300 mm is just one of the options, there are also heights of 400, 500, 600 mm) is thoroughly discussed in the article Dimensions of 21VK radiators – how to correctly choose the height and width of the panel.
Practical tips before purchase – what not to forget
From long-term customer experience we know that most problems with the installation of radiators with bottom connection do not arise from a bad product, but from incorrect measurement or misunderstanding of the system. Here is a list of things to check before ordering:
- Connection spacing from the floor: The standard connection spacing for most radiators with bottom connection is 50 mm. Check whether your outlet pipes from the floor fit this dimension, or what adapter (H-piece) is delivered with the radiator.
- Height of the connections above the floor: The outlet pipes should ideally protrude 0–20 mm above the finished floor. If they protrude higher, it needs to be solved with shortened connections or a special H-piece.
- Minimum distance from the floor: The radiator must be installed with a minimum clearance from the floor (usually 10–15 cm) for proper air convection. Do not install the radiator directly on the floor.
- Minimum distance from the sill: The radiator should be at least 10 cm from the sill or window from above.
- Direction of supply and return: On most H-pieces it is possible to set which outlet is the supply and which is the return – this allows flexibility during installation. Nevertheless, check in advance from which side the hot water comes.
- Type and size of mounting brackets: The brackets must match the height of the radiator. A height of 300 mm requires different brackets than a height of 600 mm. Most manufacturers include the brackets in the package.
The complete installation procedure including all dimensions is covered in the article Installation of a radiator with bottom connection step by step.
Hydraulic balancing and control – bottom vs side
Modern heating systems require hydraulic balancing – each radiator in the system must receive the correct amount of water, no more, no less. With side connection, the control valve is usually on the side (as a thermostatic valve with adjustable preset), while with bottom connection, the control element is integrated in the H-piece or bottom valve.
Functionally, both systems are equally capable of hydraulic control. The advantage of the bottom system is that all control equipment is hidden and protected – nobody accidentally changes it while dusting or playing.
On the other hand, with side connection, the thermostatic head is in a visible location, which makes manual adjustment easier. Some users prefer to have the head within reach, while others find it aesthetically displeasing.
Frequently asked questions (FAQ)
Can I replace a radiator with side connection with a radiator with bottom connection without rerouting the piping?
In most cases, this is not straightforward. If you have side outlets from the wall or riser, you will need to either reroute the outlets to the floor (construction work) or use elbow adapters that redirect the outlet downward. Some installers use custom-shaped connecting pieces, but the result is rarely aesthetically perfect. If you are planning a renovation with demolition, it is reasonable to reroute the piping properly and install a new radiator with bottom connection.
Is bottom connection suitable for low-temperature or heat pump systems?
Yes, radiators with bottom connection are fully compatible with low-temperature systems (heat pumps, condensing boilers with lower temperatures). However, with low-temperature heating, more careful sizing is required – at lower water temperatures (e.g., 45/35 °C instead of standard 75/65 °C), the radiator's heat output is significantly lower. Catalogue outputs are usually given for DT = 50 K; for low-temperature systems, use correction factors or contact the seller.
What is the lifespan of a radiator with bottom connection compared to side connection?
The lifespan of a steel panel radiator mainly depends on the quality of the water in the system, pH, oxygen content, and regular maintenance – not on the type of connection. Properly treated water, a closed system without air access, and regular air venting ensure a lifespan of 20–30 years without problems, regardless of the type of connection. More about maintenance can be found in the article Maintenance and cleaning of panel radiators – how to extend their lifespan.
Can I connect an electric insert (hybrid heating) to a radiator with bottom connection?
Yes, most modern panel radiators with bottom connection allow installation of an electric heating insert (rod) into the air vent hole in the top corner. This radiator can then be operated either from the heating system or electrically, or in combination. This is a practical solution, for example, during a transitional period when the boiler is not yet in operation, or for holiday homes.
What if my outlet pipes from the floor are shifted from the center of the radiator – can this be solved?
Yes, there are special H-pieces with eccentric or adjustable outlets that allow shifting the position of the connection point by several centimeters. Some types of H-pieces can be adjusted within a range of ±3–5 cm. If the shift is larger, you need to use extension fittings or reroute the outlets. Details can be found in the article Connecting a radiator to existing piping – what you need to know before installation.
Is type 21VK with bottom connection suitable for a bathroom?
Steel panel radiator 21VK is not primarily intended for the wet environment of a bathroom – for this, it is better to use bathroom ladder radiators made of stainless steel or with special surface treatment. In a dry WC or bathroom with sufficient ventilation and no excessive humidity, a panel radiator may be acceptable, but manufacturers usually do not recommend it. Always choose types specifically designed for bathrooms in wet areas.
Conclusion – which type of connection is "better"?
There is no unequivocal answer – and anyone who tells you otherwise is either selling a specific product or does not know the full range of customer needs. The correct answer depends on the condition of your existing piping, the type of renovation you are undertaking, your aesthetic preferences, and your budget.
If you are building a new building or doing a comprehensive renovation, bottom connection is clearly the choice today. A cleaner result, hidden technology, modern appearance, and compatibility with current piping – these are the arguments that decide in practice. Radiators such as 21VK 300 × 500 with output 373 W or 21VK 300 × 600 with output 447 W are proven models that meet the needs of a typical living room and can be installed without compromises.
If you are only replacing one radiator in an apartment with existing side piping and you are not planning further demolition work, consider whether switching to a bottom system is worth it – sometimes it is more sensible to stay with side connection and avoid unnecessary complications.
In any case, a well-planned installation with a radiator of the correct output and the correct type of connection is one you will forget – because it works without problems for decades. And this is exactly the result every customer should strive for. More information on selection for specific rooms can be found in the article How to choose a radiator with bottom connection for every room, or also in Frequently asked questions about radiators with bottom connection, where the most common practical questions from customer practice are answered.
Do you have a question about this topic?
Can't decide or are you dealing with a specific situation in your home? Write to us – we are happy to help.