Side vs. central radiator connection – which is better and when to use which

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Side vs. central radiator connection – which is better and when to use which

When a customer chooses a panel radiator, most of the attention goes to performance, dimensions and type (11K, 21K, 22K...). The type of connection – that is, the place where the radiator is connected to the piping – is often considered last, sometimes only during the actual installation. And it is precisely at that moment that it turns out the choice was not entirely lucky: the plumber has to improvise, the piping does not fit, adapters need to be bought, or – in the worst case – the entire job is stuck waiting for another piece.

This article deals with precisely this topic in detail and from multiple angles. We will explain what side and central connections actually mean from a hydraulic and installation point of view, when each of them has a practical advantage, where people make mistakes, and how to decide in specific situations – during renovation, new construction, replacing an old cast iron radiator, or with underfloor piping.

What is a side connection radiator

Side connection (in English side connection, manufacturers sometimes also refer to it as lateral or side) means that both connecting threads – the supply and return – are located on one side of the radiator. Typically at the bottom right (supply) and top right (return), or the opposite depending on the flow direction in the specific system. On the left side, they are usually sealed with plugs.

The vast majority of panel radiators in Europe are delivered with side connection as standard. It is a historically established format that suits the most common installation, where the piping runs along the wall or under the window and rises vertically to the radiator from the side.

The threads are standard G 1/2" (inch), the same as on standard valves, manifolds and thermostatic heads. The spacing between the supply openings is not a key dimension for the side type – important is the height of the radiator and where exactly the connections come out on the Y-axis. For a height of 300 mm, the axis of the supply typically comes out at about 50–55 mm from the bottom edge and the axis of the return at about 250–255 mm from the bottom edge. These are values to consider when preparing the piping.

What is a central connection radiator

Central connection (in English central connection, bottom central connection) places both connecting threads – the supply and return – on the bottom side of the radiator, mostly near the center or slightly to one side, but always at the bottom edge. Both threads are next to each other, typically 50 mm apart (some manufacturers 50–55 mm).

This solution is common today especially in underfloor heating in combination with panel radiators – when the piping comes directly up from the floor under the radiator. It is also standard for so-called compact radiators with an integrated valve (type Compact with bottom connection), where the entire hydraulics is hidden under the radiator body.

The 50 mm spacing between the central connections is not much and requires precise termination of the piping – it does not tolerate large deviations. If the piping from the floor deviates by more than 5–8 mm from the planned position, a problem arises and compensating fittings must be used.

Hydraulic behavior – how water flows in each type

From the perspective of fluid dynamics, there is a fundamental difference between side and central connections in how hot water spreads through the internal space of the radiator. This has a direct impact on the uniformity of heating and thus on the actual performance.

Side connection – natural counterflow

With a side connection, hot water enters through the supply openings at the bottom of the side and passes through horizontal distributors into the entire body of the radiator. The return removes the cooled water from the opposite end (for wider radiators) or from the top side thread. The result is so-called counterflow or diagonal flow, which ensures even temperature distribution across the entire surface.

With wide radiators (over 1 000 mm), there may be a slight cooling in the middle part if the flow is low – but in a normally dimensioned system with appropriate flow, this is not a problem. A greater influence on the uniformity of heating has proper air venting (more on that in the article Ventilation and cleaning of panel radiators – how to do it yourself).

Central connection – flow from the center to the edges

With a central connection, water enters through the central supply at the bottom and spreads symmetrically to both sides of the radiator. The cooled water also returns symmetrically to the central return. This flow is uniform in a symmetric radiator, but has one weakness: if the radiator is very long (over 1 200–1 400 mm), the ends of the radiator can be significantly cooler than the center. For shorter and medium-length units (up to 1 000 mm), the central connection is hydraulically equivalent to the side connection.

Manufacturers of compatible valve kits for central connections solve this property with integrated distributors in the body of the bottom connection set, thereby redistributing the flow. However, these sets are tied to specific types and are not always available for all heights and widths.

When is side connection a better choice

Experience from dozens of projects shows that side connection makes sense in these situations:

1. Existing installation with side distribution

This is the most common scenario. You have a panel apartment, a family house or a commercial space, where the piping runs along the wall or behind the cladding, ending with a side connection at the radiator. Replacing an old cast iron radiator or the original panel radiator with a new model with side connection is straightforward in such a setup – you unscrew the old one, install the new one, and adjust the valves. No additional piping work, no demolition.

This situation occurs in the vast majority of apartments and older family homes built before 2005. In a comprehensive renovation, where the customer is changing the entire apartment interior and planning a new piping system, it is possible to consider switching to a central connection – but only if the customer explicitly requests it and the architect has incorporated it into the design.

2. Wider radiators (over 800 mm)

As explained above, for widths over 1 000–1 200 mm, side connection is more advantageous from a hydraulic perspective, as the diagonal flow better distributes heat to the outer segments. A central connection on long bodies without special distributors can cause the outer parts to remain noticeably cooler.

3. Vertical pipe routing along the wall

If the piping rises vertically along the wall and the radiator is mounted on the wall in front of this piping (typical in corridors, entrance halls, bathrooms), a side connection with typical elbows and straight valves is the best fit. The piping simply branches off to the radiator connections without any complex bends.

4. Flexibility of connection placement

A standard radiator with side connection has four threads: two on the right (active) and two on the left (blind). This means you can freely choose from which side the piping runs – simply swap the plugs and valves. This flexibility is valuable, for example, when the piping comes out of the corner of the room where the radiator is located, and it is more practical to connect it from the left than from the right.

Real-life example from practice

A customer is renovating a living room in a panel apartment from the 80s. The original 500 x 1 000 mm cast iron radiator is to be replaced with a modern panel radiator. The piping comes out of the side at a standard height. Choice? Definitely a radiator with side connection, height 500 mm, width according to the power calculation. No renovations, minimal installation costs. If you want a concrete example of a smaller unit, for example in an entrance hall or bathroom with a lower ceiling, take a look at Radiator 21K 300 x 600 power 447W – 300 mm height with side connection works perfectly in these conditions.

When is central connection a better choice

Central connection has its irreplaceable place in these situations:

1. Floor distribution – piping comes up from the floor

This is an increasingly common scenario in new builds and completely renovated apartments, where floor heating with supplementary radiators was chosen, or where the heating distribution runs in floor grooves and the piping comes up directly under the radiator. Here, central connection is not only an advantage, but practically a necessity – a side connection would require bending the piping under the floor or in the wall, which complicates installation and increases costs.

2. Aesthetic installation – hiding the piping

In interior-sensitive projects (design apartments, residential projects, hotels), there is a desire to hide the piping as much as possible. A central connection with a bottom valve allows a situation where the only visible elements are the floor covers and a compact valve set directly under the radiator. The piping runs exclusively in the floor and when looking at the front of the radiator, you see nothing technical. A side connection in such a requirement forces you to either embed the side pipes in the cover or accept visible pipes.

3. Compact spaces with no space on the sides

Sometimes the radiator is mounted between two walls (for example, in a niche under a window with built-in sides), where there is simply no access from the side to the valves. In such a case, a central connection from the bottom is an elegant solution – the valves are accessible from the front bottom, not from the side behind the wall.

4. New build with precisely defined drilling

In a new build, where the project precisely defines the position of each radiator and the plumber knows the exact coordinates of the pipe outlet from the floor with millimeter precision, a central connection is ideal. Planning eliminates the weakness of this solution (sensitivity to the exact position of the floor outlets).

Real-life example from practice

A developer is building an apartment block, where floor heating distribution was chosen. The designer designed panel radiators under each window, with the pipes coming up exactly in the center of the planned position of each radiator. Installation proceeds so that the plumber installs the bottom valve sets, connects the piping, and mounts the radiator – a clean and quick job. Result: a flat wall without visible pipes, a modern look.

Comparison in numbers – summary table

Criterion	Side connection	Central connection
Type of distribution	Side (along the wall)	Floor (from the floor)
Standard in Slovakia	Yes – very common	Increasingly common in new builds
Connection spacing	Depends on radiator height (approx. 200–450 mm)	Typically 50 mm (standardized)
Flexibility of side	Yes – left or right side	No – fixed in the center from below
Aesthetics	Visible side valves	Hidden under the radiator
Sensitivity to accuracy	Low – valves compensate	High – precise coordinate of the outlet
Hydraulics at width over 1 200 mm	Better – diagonal flow	Worse – cold ends
Availability of valve sets	Very wide range	Depends on the manufacturer, fewer variants
Installation cost (with existing distribution)	Lower	Higher (pipe modifications)

Technical details – threads, valves and adapters

Both types of connection use G 1/2" thread (external or internal, depending on the specific location – valves are usually for external thread, radiator connections for internal thread). This is important to know when purchasing valve sets.

For side connection, the standard set includes:

Thermostatic valve (supply, temperature regulation) with a thermostatic head
Lockshield valve (return, flow regulation, hydraulic balancing)
Drain valve (corner, on the upper side thread) – either manual or automatic
Plug for unused thread (typically upper left and lower left position)

For central connection, the entire set is replaced by one integrated connecting set – a bottom block, which contains the supply and return, thermostatic valve, adjustable elements and air vent, all in one compact housing. The advantage is cleanliness, the disadvantage is dependence on the availability of a specific set for a given type and size of radiator.

If you are solving the transition from side to central connection or vice versa on an existing radiator, there are transitional adapter sets available. For example, a radiator with side connection can be converted to a central bottom connection using an internal distribution set (so-called bottom connection kit) – but not every manufacturer supports this for every type and not every body allows it structurally. Always check compatibility with the specific product range.

Height 300 mm – specifications for side connection

Radiators with a height of 300 mm are popular in spaces with low sills or where the height under the window cannot be used for a taller unit. In terms of side connection, it is important to know that at a height of 300 mm, the vertical spacing between the supply ports is significantly smaller – typically 200–210 mm. This means that the pipe coming from the side must be run very close together, which sometimes requires precision in the preparation of vertical pipes.

Availability in this height category is very good for side connection. For example:

Radiator 21K 300 x 400 power 298W – compact unit for small spaces, hallways and bathrooms
Radiator 21K 300 x 500 power 373W – suitable for medium-sized bathrooms or smaller living rooms
Radiator 21K 300 x 800 power 596W – sufficient power for a larger room while maintaining a low height

All these units are supplied with a standard side connection G 1/2" and are compatible with standard valves and thermostatic heads. A more detailed selection of dimensions for a specific room can be found in the article Radiator dimensions 300 mm height – which width is suitable for my room and What radiator power do I need – calculation based on area and heat loss of the room.

Most common mistakes when choosing the type of connection

From practice, there are several recurring mistakes that increase the cost of installation or complicate operation:

Ordering a radiator with central connection for an existing side installation: The customer chooses a nice compact radiator with central connection in the catalog, but forgets to check it. The installer arrives at the site and discovers the problem. Either the unit has to be replaced or an expensive adapter is needed, which barely fits under the radiator. Solution: always check the type of connection when ordering and compare it with your installation.
Incorrect coordinate of pipe outlet from the floor for central connection: The construction company measures the position 30–40 mm differently than in the project. The pipe comes out of the floor outside the axis of the radiator. Result: necessary pipe bending, an unattractive solution, sometimes cutting the floor. Solution: coordinate the position of the pipe during rough construction, not afterwards.
Forgetting the air vent for side connection: The customer or installer installs a thermostatic valve and lockshield, but forgets the drain valve. The radiator fills with air and does not work fully. Solution: always include the air vent. More in the article Air venting and cleaning of panel radiator – how to do it yourself.
Loss of flexibility when covering side connections: The customer tiles the wall with wooden panels right next to the radiator and covers the side valves so that the thermostatic head is not accessible or not in the air stream. Result: the thermostat does not work properly because it measures the temperature in an enclosed space. Solution: always leave access to the valves and an air space in front of the radiator when tiling.

Practical selection process – decision scheme

Combination of types in one heating system

Sometimes the question arises whether radiators with side and central connection can be used simultaneously in one heating system. The answer is yes – there is no problem from a hydraulic point of view. Each radiator is its own node in the system and the type of its connection to the valve has no effect on the other radiators. It is important that each radiator has a properly installed thermostatic valve, lockshield (for side connection) or the appropriate regulating element (for central connection), and that the system is hydraulically balanced as a whole.

This combined situation often occurs during partial renovation – for example, when a new bathroom with a floor distribution is added to an apartment with original side-connected radiators, and a radiator with a central connection is installed there.

Influence on the selection of valve fittings

The type of connection directly affects the selection of valve sets. This is practically important because a mistake here costs time and money.

For side connection, the following are commonly available:

Corner thermostatic valves (for vertical pipe turning into a horizontal direction towards the radiator)
Straight thermostatic valves (for horizontal pipe running in the same plane)
Lockshield corner or straight (return pipe)
Complete valve sets (two valves + head + air vent + plugs)

A more detailed description of the installation, including tools and steps, can be found in the article Installation of a radiator with side connection – procedure, tools and common mistakes and also in the article Connecting a radiator to an existing pipe – spacing, adapters and sealing.

For central connection, the valve sets are typically:

Compact bottom connecting set (includes both threads, regulation and air vent)
Some sets allow flow adjustment and closing without draining the entire system
Some sets integrate a thermoelectric actuator for controlled regulation

What says the standard and design practice

From the point of view of technical standards (STN EN 442, STN EN 12831), the type of connection is not prescribed – the standard specifies the performance of the radiator under standard conditions (supply 75 °C, return 65 °C, room temperature 20 °C with a temperature difference of 50 K – so-called ΔT50). Manufacturers test performance precisely with side connection, as it is the most widespread variant. With central connection, the actual performance may be 2–5 % lower in some sizes and types – but in practice, this deviation is usually negligible and manufacturers do not consider it critical in a properly dimensioned system.

For an accurate calculation of the required radiator performance for a specific room, we recommend the article What radiator power do I need – calculation based on area and heat losses of the room, where you will also find formulas for different types of buildings.

Summary and recommendations

If we were to summarize the whole topic in a few sentences: side connection is a proven, flexible and for most Slovak households and renovations more suitable format. It is compatible with existing distribution systems, easy to install, there are enough valve variants, and for radiator widths up to 1 500 mm it is fully hydraulically suitable.

Central connection makes sense in new buildings with floor distribution, when emphasizing the aesthetics of hidden distribution, or where the architect/designer has incorporated this technology into the project from the beginning. When converting a side distribution to a central connection, always calculate with the effort and verification of compatibility.

A good starting point when choosing a specific radiator with side connection for your room is Radiátor 21K 300 x 700 výkon 552W – a balanced choice for a room with an area of around 14–18 m² with standard insulation. The entire category can be found clearly on the page radiátory s bočným pripojením.

Frequently asked questions (FAQ)

Can I connect a radiator with side connection to a pipe coming out of the floor?

Yes, but it requires the use of special corner valves or adapter elbows that turn the direction of the pipe from vertical (from the floor) to horizontal (into the side thread of the radiator). A solution exists, but it is not as aesthetic as central connection and requires protruding valves on the side. If the pipe from the floor is firmly terminated, consider either special fittings or switching to a radiator with central connection.

Is the performance of a radiator with side connection higher or lower than with central connection?

The performance of a radiator depends mainly on its surface area, type (11K, 21K, 22K) and the thermal parameters of the system – not on the type of connection. Standard tabular performance values (ΔT50) are measured with side connection. With central connection, the performance may be slightly lower by a few percent in very long pieces, but for common sizes up to 1 200 mm, the difference is practically negligible. The selection of performance should be based on the heat losses of the room, not on the preferred type of connection.

What is the connection spacing and why is it important for central connection?

Connection spacing is the distance between the centers of the supply and return. With central connection, it is typically 50 mm (some manufacturers 55 mm). This is a critical dimension because the pipe output from the floor must exactly match this value. If the dimensions do not match, the valve set cannot be installed without modifications. Always check before ordering what spacing your chosen bottom valve set supports and compare it with the floor distribution plan. More about spacing and adapters in the article Connecting a radiator to an existing pipe – spacing, adapters and sealing.

Can a radiator with side connection be connected from the left side instead of the right?

Yes, a standard radiator with side connection has four threads – two on the right (active, with valves) and two on the left (blinded). If you prefer the connection from the left, simply move the valves (valve, lockshield, air vent) to the left side and blind the right threads. This flexibility is one of the practical advantages of side connection. Just make sure to assign the supply and return correctly (usually supply at the bottom, return at the top) according to the flow direction in your system.

Why is my radiator with side connection cold in the middle?

If the side parts are warm, but the central part is cold, it is almost always an air pocket inside the radiator. Air most often accumulates in the upper central part of the body. The solution is simple: use an air vent (corner valve with manual or automatic venting) to release the air until water starts to flow. A step-by-step procedure can be found in the article Draining and cleaning a panel radiator – how to do it yourself. If the problem persists after venting, it may be due to deposits or a hydraulic issue – see the article Common faults of panel radiators – cold spots, noise and leaks.

Can I check myself whether a radiator has side or central connection before buying?

Yes. In the product description, look for information about the type of connection – manufacturers usually mention it as "side", "side connection", "side connection" or on the contrary "central", "central bottom connection", "bottom central connection". With side connection, the side body threads will be visible in the product images, and with central connection, you will see the threads centrally on the bottom edge. If in doubt, contact the seller before ordering – it is much easier than solving incompatibility after the product has been delivered. More questions and answers can also be found in the article Common questions about radiators with side connection.

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.