Frequently asked questions about bottom-connected radiators
Frequently asked questions about bottom-connected radiators – a comprehensive guide for DIY enthusiasts and professionals
Bottom-connected radiators are now the most common standard in new builds and renovations. Despite this, there are countless questions surrounding them – from the basic ("Why are the angled hoses so short?") to the technically complex ("What impact does the position of the thermostatic valve have on hysteresis?"). In this article, we have gathered the most practical and frequently asked questions from everyday sales and installation practice and answered them as specifically as possible – with numbers, dimensions and real examples from customer projects.
If you are still planning to choose a radiator, we recommend reading the article How to choose a bottom-connected radiator for every room and What radiator power do I need – calculation based on room area and height. Here we focus purely on questions that arise from installation, operation and problem-solving practice.
1. What does "bottom connection" actually mean and how does it differ from side connection?
Bottom connection (in English bottom connection, abbreviation VK in radiator designation) means that both connection ports – the supply and return – are located in the lower part of the radiator body, usually next to each other with a spacing of 50 mm. Hot water enters the radiator from below on one side, circulates through the panels and lamellas upwards and returns to the boiler system through the other port, also located at the bottom.
Side connection, on the other hand, is a solution where the supply enters the upper side port and the return exits from the lower side port (or vice versa depending on the connection). This system was the standard in older apartment buildings with continuous steel pipes running visibly along walls or floors. Today, it is more commonly found in older buildings or in specific installations where existing side piping must be preserved.
A more detailed comparison can be found in the article Bottom connection radiator vs side connection – advantages, disadvantages and when it is worth it. In short: bottom connection is more aesthetic (pipework is hidden in the floor or in a floor strip), installation is faster and with modern thermostatic valves integrated into the radiator body, you achieve better hydraulic balance of the system.
2. What type of piping do I need to connect a bottom-connected radiator?
This is probably the most common question during renovations. Bottom-connected radiators are designed for floor-run piping (or floor strip) with vertical pipes rising from the floor. The spacing of the connection ports is standard at 50 mm (measured from center to center). The connection ports have a 1/2" thread (external or internal depending on the connecting set used).
In practice, the following piping solutions are most commonly used:
- Copper pipes ø15 mm – a classic used from the 90s to today, durable, can be soldered or crimped
- Multi-layer (PEX-AL-PEX) pipes ø16 mm or 20 mm – most popular today, can be shaped cold, no soldering required
- Plastic-metal (PE-RT/AL/PE-RT) – similar properties to PEX-AL-PEX, more resistant to UV radiation
Important: pipes from the floor must emerge at the exact correct spacing and in the correct location under the radiator. A common mistake during renovations is that you are off by 20–30 mm in position, which the installer then solves by bending the pipes – sometimes elegantly, sometimes quite awkwardly. We recommend precisely measuring the pipes before pouring the screed according to the actual dimensions of the radiator, including the distance from the wall and from the floor. More about this procedure can be found in the article Connecting a radiator to existing piping – what you need to know before installation.
3. What is a connecting (H) set and is it really necessary?
Yes, the connecting set (commonly called an "H-set" or "Anschluss set") is an essential part of the installation of every bottom-connected radiator. Without it, you would have the ports connected, but no regulation, no air venting and no way to close the radiator without draining the entire system.
A standard connecting set includes:
- Thermostatic valve – regulates the flow of hot water according to the position of the thermostatic head, built directly into the body of the set
- Closing valve (roháček) on the return branch – used to set the hydraulic resistance and to close the radiator
- Air vent – usually at the top of the radiator, manual or automatic
- Plugs – for unused ports (usually on the side ports, if not used)
- Seals and adapters for connecting to different types of pipes
There are sets in various configurations – straight (for pipes coming out of the wall) and angled (for pipes coming out of the floor). For bottom-connected radiators, you almost exclusively use angled sets with a spacing of 50 mm. Sets from different manufacturers (Heimeier, Oventrop, Danfoss, Herz) are interchangeable, as the port dimensions are standardized.
4. What radiator power is right for my room?
This is a question for which there is no universal answer without knowing the specific parameters of the room. As a general rule: a well-insulated new build requires 50–70 W/m², older buildings with poor insulation may require 80–120 W/m² or more. For an accurate calculation, we recommend the article What radiator power do I need – calculation based on room area and height.
To give you a concrete idea, here are some examples from real customer orders:
- Bathroom 4 m², older panel building, double-glazed window: required power approx. 320 W → suitable for example Radiator 21VK 300 × 400 with power 298 W or Radiator 21VK 300 × 500 with power 373 W
- Children's room 12 m², new building with heat recovery: required power approx. 600–720 W → Radiator 21VK 300 × 800 with power 596 W or a pair of smaller radiators
- Corridor 6 m² with an external wall: required power approx. 400–480 W → Radiator 21VK 300 × 600 with power 447 W is usually sufficient
Important note: radiator power is stated under standard test conditions (supply temperature 75 °C, return 65 °C, room temperature 20 °C – so-called Δt 50 K according to EN 442 standard). If you have a condensing boiler set to lower temperatures (e.g. 55/45 °C), the actual radiator output will be significantly lower – typically only 55–60 % of the nominal output at Δt 30 K. This must be taken into account when choosing a radiator!
5. How to correctly choose the height and width of the 21VK panel radiator?
21VK series radiators are panel radiators with one panel and one convectional baffle (designation „2" = 2 layers, „1" = 1 baffle). They are a compromise – they are not as shallow as type 11 (one plate, no baffles), but they are also not as thick and powerful as type 22 (two plates, two baffles). For standard use in new buildings and well-insulated renovations, they are ideal.
Height of 300 mm is popular for a simple reason: a radiator of this height can be placed even under a low window sill (standard height 400–450 mm from the floor), while maintaining the required clearance of 100–120 mm from the floor and at least 50 mm from the sill. This height is therefore suitable where vertical space is limited. If you have more space available (e.g. a wall without a window), you can go for a height of 400, 500 or 600 mm and achieve a higher output at the same width.
Width is chosen according to the required output and available wall space. An overview of outputs for the 21VK series at a height of 300 mm:
| Width (mm) | Output (W) at Δt 50 K | Typical application |
|---|---|---|
| 400 | 298 W | Small bathroom, WC, corridor |
| 500 | 373 W | Bathroom up to 5 m², entrance hall |
| 600 | 447 W | Corridor, smaller room under a window |
| 700 | 522 W | Children's room, bedroom, study |
| 800 | 596 W | Larger room, main living room as an additional radiator |
More about choosing specific dimensions can be found in the article Dimensions of 21VK radiators – how to correctly choose the height and width of the panel.
6. At what height from the floor is a radiator with bottom connection mounted?
This is a question where many people make a mistake and then have to shorten or extend the pipes. The correct installation height is determined by two conditions:
- Clearance from the floor (final walking surface): minimum 100 mm, recommended 100–120 mm. Less would restrict air convection under the radiator, which reduces performance and causes dust to settle.
- Clearance from the window sill (from above): minimum 50 mm, ideally 60–80 mm. With a smaller clearance, warm air cannot freely rise upwards, and the radiator does not heat efficiently.
Practical example: window with a sill at 420 mm from the floor, radiator 21VK 300 in height. Bottom of the radiator = 110 mm, top = 110 + 300 = 410 mm. This is 10 mm below the sill – tight! In such a case, either lower the installation to 100 mm and you will have 10 mm below the sill (insufficient), or choose a lower panel (e.g. height 200 mm, if available), or move the radiator away from the window to a free wall with greater height.
Attention: the height of the pipe connections (where the pipes come out of the floor) must correspond to the actual position of the connections on the radiator. In the 21VK 300 series, the connections are at the bottom of the radiator body, i.e. about 10–20 mm from the bottom of the radiator. If the bottom of the radiator is at 110 mm from the floor, the connections are approximately at 115–125 mm from the floor. The pipes from the floor must be long enough to reach this height. A detailed procedure can be found in the article Installation of a radiator with bottom connection step by step.
7. Why is the bottom-connected radiator not heating at the top, even though the bottom is warm?
This is a classic problem that we encounter with customers quite regularly. There may be several causes:
Air in the radiator – the most common cause. Air accumulates in the upper part of the radiator panel, where it hinders water circulation. Solution: bleed the radiator using the bleed valve at the top. A detailed procedure can be found in the article Bleeding and flow adjustment of bottom-connected radiators.
Too low water flow – if the flow rate is set too low on the return valve (the valve is closed too much), the water cannot circulate through the entire volume of the radiator, and the upper part remains cold. Solution: open the valve on the return pipe more (or have it adjusted by a plumber during system balancing).
Scale and corrosion inside the panel – in older systems with unsuitable water pH, deposits can form that block the channels in the upper part of the radiator. In such cases, system flushing or radiator replacement may help.
Incorrect setting of the balancing valve – in multi-radiator systems, this particular radiator may be hydraulically "suppressed" by another radiator with lower resistance. The solution is hydraulic balancing of the entire system.
8. Can I replace an old radiator with side connections with a new one with bottom connections?
Yes, but it requires intervention in the piping. If you have side pipes running visibly or in wall grooves, you will need to either:
- Transfer the pipes into the floor (ideally during a complete reconstruction of the floor structure)
- Use an adapter kit that allows connecting a bottom-connected radiator to vertical side pipes – not an elegant solution, but technically functional
- Cover the side pipes with a baseboard or box and keep the side connection
- Continue using a radiator with side connections
Important: when replacing a radiator with one of the same size (same width, same height), it is usually not necessary to change the brackets or the holes for the pipe connections. The brackets have standardized dimensions for individual radiator heights (300, 400, 500, 600 mm), so replacing with the same type is usually straightforward. When changing the height (e.g., from 600 to 300 mm), you will need to drill new holes for the brackets at the new height.
9. Is it necessary to use sealing tape (Teflon) during installation, or are rubber seals sufficient?
It depends on the specific location and type of connection. In general:
- Metal threaded connections (e.g., the set on the radiator neck): use only flat rubber seals (O-rings), which are part of the set. Teflon is not suitable here and can cause leaks.
- Tapered threaded connections (e.g., pipe fitting to a valve): Teflon or hemp fiber with paste is the correct choice here.
- Compression connections (PEX-AL-PEX pipes): no sealing is required – the connection is mechanical and secured by crimping with crimping pliers.
Never mix types of seals: for example, Teflon on a flat seal only adds thickness, does not improve sealing, and can deform the rubber ring. From experience: most water leaks after installation occur precisely due to the wrong type of seal or a loose connection – not due to lack of sealing.
10. How does a thermostatic head work and where should it be mounted?
A thermostatic head (thermostatic valve actuator) is a device that mechanically (using a temperature-sensitive filling) or electronically regulates the water flow through the valve in the set connection. When the air temperature in the room rises above the set value, the head closes the valve and reduces the water flow. When the temperature drops, the valve opens again.
The head is mounted directly on the body of the connecting set (thermostatic valve). It is important that it is placed in such a way that it "senses" the actual room temperature – that is, not covered by a curtain, not behind a radiator cover, and not directly in the sun. If the mounting location is unsuitable, there are extension capillaries that allow the sensor of the head to be placed in a more suitable location (e.g., 2 m from the radiator in the center of the room).
Electronic programmable thermostatic heads (e.g., from Danfoss, Honeywell, Tado) allow you to set a daily heating schedule and reduce the temperature at night or during absence. From an energy-saving perspective, this is a very effective component – savings can reach 15–25 % of heating costs.
11. What to do if water is leaking from the radiator?
Water leakage from a bottom-connected radiator usually occurs at one of these locations:
- Connection of the connecting set to the radiator neck – loose or cracked rubber seal. Solution: tighten the nut (carefully, without excessive force) or replace the rubber O-ring.
- Connection of the set to the floor pipes – loose crimping sleeve or loose nut. Check visually, and if necessary, tighten or recrimp at the pipe location.
- Bleed valve – clogged or cracked seal. Replace the bleed valve (threaded 1/2", available for a few euros on the market).
- Radiator body itself – pierced by corrosion or mechanical damage. In this case, the only option is to replace the radiator. Corrosion usually occurs due to long-term acidic or improperly softened water in the system.
Important: in the case of any water leak, first close the valve on the return pipe and the thermostatic valve (turn to "0" or "Frost"). This will disconnect the radiator from the system without the need to drain the entire system. You can then calmly repair the seal or replace the component.
12. How long does it take to replace a radiator with bottom connection?
An experienced plumber can replace a radiator with bottom connection (while keeping the existing floor piping) in 30–60 minutes, including bleeding and checking the seal. If the connecting set also needs to be replaced, add another 20–30 minutes. The total work on one radiator therefore usually takes less than 1.5 hours.
If you are renovating the entire apartment and replacing several radiators at once, an experienced plumber can handle 4–6 units in one working day, depending on the complexity of the piping and the availability of radiators for installation.
DIY installation is also possible if you have basic experience with home repairs. The complete step-by-step procedure can be found in the article Installing a radiator with bottom connection step by step. The most important tip: before starting, prepare all the necessary components – set, seals, wrenches, a bucket and a cloth – nothing is more frustrating than realizing in the middle of installation that you are missing an adapter.
13. What is the lifespan of a panel radiator with bottom connection?
With proper operation and maintenance, a high-quality steel panel radiator should last 20–30 years. Key factors that affect lifespan:
- Water quality in the system – correct pH (7.5–9.0) and low oxygen content are essential. Corrosion from the inside is the most common cause of premature radiator failure.
- Bleeding – air in the system accelerates corrosion. Regular bleeding (at least once a year) significantly extends the lifespan.
- System pressure – operating pressure should not exceed 6 bar continuously. Most radiators are tested at 8–10 bar, but long-term operation at higher pressures strains the material.
- Mechanical damage – impacts or improper handling during installation can damage the thin panel sheets.
More on proper maintenance can be found in the article Maintenance and cleaning of panel radiators – how to extend their lifespan.
14. Can a radiator with bottom connection be used in a system with a heat pump?
Yes, but with an important note on sizing. Heat pumps operate with significantly lower fluid temperatures (40–55 °C on the supply, 35–45 °C on the return), which are much lower than with boilers. The radiator's performance is significantly lower at this lower Δt – at Δt 25 K (supply 50 °C, return 40 °C, room 20 °C), the panel achieves only 40–50% of its nominal output.
What this means in practice: a radiator that would be sufficient in a boiler system can be significantly undersized when used with a heat pump. For heat pump systems, it is recommended to size radiators at least double the calculated heat loss, or alternatively choose panel type 22 (double panel) or 33 (triple panel), which provide significantly higher performance at the same dimensions.
An alternative is panel radiators with a built-in fan (fancoils), which are specifically designed for low-temperature systems and achieve sufficient performance even at 40/35 °C.
15. What is the difference between types 11, 21, 22 and 33 in panel radiators?
Panel radiator types are standardized and directly describe the construction of the body:
- Type 10 – one smooth panel, no fins. Minimum performance, aesthetic, suitable for bathrooms and design interiors.
- Type 11 – one panel, one convection fin. Good performance-to-thickness ratio, common standard for smaller rooms.
- Type 21 (e.g., Radiator 21VK 300 × 700) – two panels, one fin. Higher performance with relatively small thickness (about 63–70 mm).
- Type 22 – two panels, two fins. Significantly higher performance than type 21, thickness about 100 mm. Most commonly used type for larger rooms in standard buildings.
- Type 33 – three panels, three fins. Maximum performance, thickness up to 155 mm. For very well insulated spaces or low-temperature systems.
The thickness of the radiator has a practical impact on installation: the thicker the radiator, the more it protrudes from the wall, and the more careful you need to be when placing furniture. A radiator of type 22 and width 1200 mm is a truly bulky piece of furniture that needs to be included in the room layout.
Most frequently asked questions (FAQ)
Can I turn a radiator with bottom connection upside down (pipes up)?
No, radiators with bottom connection are designed so that water enters from the bottom and exits from the bottom, with circulation occurring naturally by convection through the panels upwards. Turning the radiator upside down would cause incorrect water circulation and air could not be bled out in the usual way. If you need top connection, choose a radiator with side or top connection.
What is the recommended operating pressure for panel radiators?
Most panel radiators (including the 21VK series) are tested at a maximum test pressure of 13 bar and the permitted operating pressure is 10 bar. In real home systems, pressure typically ranges between 1.5 and 2.5 bar, which is well below the maximum. Pressure issues are more likely to occur due to an incorrectly set expansion tank or a faulty boiler safety valve.
How often should I bleed the radiator in a year?
In normally functioning closed systems, it is sufficient to bleed once a year – ideally at the beginning of the heating season, when you turn on the heating after summer. If the radiator buzzes, gurgles, or its upper part is noticeably cooler than the lower part, bleed it immediately. In new installations, bleed after the first filling and again after 48–72 hours of operation, when the air has settled.
Is it possible to connect a radiator with bottom connection without a thermostatic head?
Yes, a thermostatic head is not technically mandatory – you can leave the thermostatic valve fully open (set to maximum) and regulate only with a lockshield valve or with a central boiler controller. However, this is not ideal from an energy efficiency perspective: without individual regulation, you have no control over the temperature in individual rooms and unnecessary heating consumes more energy. For bathrooms, where electronic control is not desired, there are special waterproof thermostatic heads with IP 44 protection.
Why is my new radiator noisy when I start it for the first time?
Noise when starting a new radiator for the first time is normal and has several causes: metal expansion when heated (light cracking), air in the system (gurgling or hissing), and possible deposits from the pipes that move during the first flow. Most of these noises disappear completely after bleeding and a few days of operation. If the noise persists for a long time, check whether the system is insufficiently bled or whether the flow is too high (characteristic noise from too fast flow – solution: slightly close the lockshield valve).
How can I determine if the radiator's output is sufficient if it is still cold in the room?
First, check whether the radiator is actually heating – touch it with your hand at the top: if it is the same temperature as the bottom, the circulation is working properly. If the upper part is cold, the problem is in the flow or bleeding. If the radiator is warm over its entire surface, but it is still cold in the room, the radiator's output is insufficient (or the room's heat loss is higher than expected – for example, due to unsealed windows or insufficient insulation). In such a case, we recommend either adding another radiator or replacing the existing one with a more powerful type (22 instead of 21, or a larger width). For example, replacing the Radiátor 21VK 300 × 600 (447 W) with the Radiátor 21VK 300 × 800 (596 W) will increase the output by a third at the same height and with minimal changes to the piping.
Conclusion: what to remember about bottom-connected radiators
Bottom-connected radiators are now the de facto standard of modern heating – and not without reason. Hiding the piping in the floor, easy installation, compact appearance, and reliable operation make them a choice you will appreciate for years without unnecessary worries. At the same time, it is true that quality installation and correct output sizing are the foundation of a satisfactory result.
If you came across a question while reading this article that we did not have time to answer in detail, check out other topics in our Knowledge Center – for example, Common faults in bottom-connected radiators and how to eliminate them or Bleeding and flow setting for bottom-connected radiators. The entire category of bottom-connected radiators on atria.sk also offers practical filters by output, height, and width, so finding the right unit for your room is just a few clicks away.
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