Step-by-step installation of a bottom-connected radiator

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Bottom connection radiator installation step by step – complete professional guide

Installing a panel radiator with a bottom connection is not rocket science, but it's definitely not a trivial task that can be handled "by eye". I've seen dozens of installations where the customer or their helper saved a few euros by skipping the professional and the result was leaking joints, the radiator hanging off by a few centimeters, or – worst of all – the system could never be properly bled because the valve ended up on the wrong side. This article will show you the entire process from measurements and pipe preparation to the first heating cycles, with specific values, practical tips and warnings I've collected from real customer jobs.

If you're still considering which radiator to choose, I recommend reading the companion article How to choose a bottom connection radiator for each room and also What radiator power do I need – calculation based on room area and height, where you'll find the procedure for thermal calculation. This article assumes you've already selected a specific model and size and now you want to know how to install it reliably.

What is a bottom connection radiator and why the installation is a bit different from side connection

Panel radiators with a bottom (ventil compact, English "bottom connection" or "VK") connection have both the supply and return hot water connections routed through one integrated valve block at the bottom of the panel, usually on the right side. The entire piping can then run under the plaster or in the floor and the radiator's front remains clean without visible pipes on the sides.

Exactly because the valve block combines an expansion valve, a control valve and a shut-off valve in one compact body, it's important to understand the internal logic: the supply (hot water from the boiler) goes into one opening, the return (cooled water back) goes into the other. Swapping the supply and return won't cause an immediate failure, but the radiator will be significantly weaker and the regulation will work inaccurately. More on this topic in the article Connecting a radiator to existing piping – what you need to know before installation.

Types of VK panels differ in the number of plates and convectors: type 11 (one panel, one convector), type 21 (two panels, one convector), type 22 (two panels, two convectors), etc. In practice, I most often encounter type 21VK in standard rooms and bathrooms, which offers a good performance/depth ratio. For example, Radiator 21VK 300 × 600 with a power of 447 W is a popular choice for a smaller bathroom or a bathroom with underfloor heating as an additional heat source.

Tools and materials you will need

Before starting the installation, prepare the following. Nothing is worse than stopping work halfway because you're missing a piece of Teflon or the right wrench.

Water level (minimum 60 cm, ideally 80–100 cm) – the radiator must be perfectly horizontal; a deviation greater than 2–3 mm per meter causes problems with bleeding
Tape measure and pencil – for measuring height and width, marking the position of brackets
Hammer drill + masonry bits (diameter according to anchors, typically 8 mm)
Screwdriver or electric screwdriver
Adjustable wrench or set of open-end wrenches (wrenches 24, 30, 32 are common for radiator valves)
Service wrench for the bleed valve (square head 5 mm – usually included with the radiator)
Teflon tape (PTFE) or hemp fiber with sealing paste – for threaded joints
Bucket and hose – for draining water from the system when replacing on an existing layout
Bracket template (most manufacturers include it with the radiator or you can make it from cardboard)
Replacement gaskets (flat rubber G1/2" for connection points)

Preparation before installation – measurements, calculations, radiator location

Where exactly to place the radiator on the wall

The classic recommendation says: radiator under the window. And it applies to bottom connection as well – the window is the coldest place on the wall and the radiator below it creates a thermal curtain barrier against cold air flowing down along the glass. Practice, however, sometimes dictates compromises: in modern passive houses with triple glazing and insulated construction, the difference compared to another wall is not dramatic.

More important rules for choosing the location:

The bottom edge of the radiator should be at least 10–12 cm above the floor (for air circulation under the radiator). Standard practice is 10–15 cm.
The top edge should be at least 10 cm below the sill, ideally 15 cm, so that the thermal flow can rise freely.
Leave at least 5 cm from the side wall, better 8–10 cm.
When calculating the position of the brackets, don't forget that the brackets support the radiator through the mounting strip on its back side – check the documentation of the specific model to see exactly where the mounting points are.

Height and distance of connections from the floor

This is a critical dimension, because the position of the floor (or wall) outlets must match the position of the radiator's valve block. The VK panel valve block is standardized: the axes of both connections (supply and return) are 50 mm apart and are usually located 50–60 mm from the right edge of the panel. The height of the connection axis above the floor depends on the radiator's installation height.

Practical example: We are installing Radiator 21VK 300 × 800 with a power of 596 W in a living room. The panel height is 300 mm. We want to install it with the bottom edge 110 mm above the floor. The valve block axis is 60 mm from the bottom edge. Therefore, the connection axis will be at a height of 110 + 60 = 170 mm from the finished floor. The pipe outlets from the floor or wall must be at this height.

Step-by-step installation procedure

Step 1 – Draining water from the system (when replacing an existing radiator)

If you are replacing an old radiator with a new one, you must first drain the water from at least that part of the system. With shut-off valves on each radiator, it is sufficient to close both valves (supply and return) and drain the water from the radiator only through the bleed valve in its upper corner. If you do not have valves or they are not reliable, drain the entire system through the drain cock at the boiler.

Warning: with older panels and corroded connections, it often happens that the shut-off valves on the radiator do not hold – check this before disassembly. Having a bucket and a hose handy is always wise.

Step 2 – Preparation of the piping

If it is a new build or a renovation, the piping should be installed at the correct height and distance according to previous calculations. Use protective caps or plastic protectors on the pipe ends until it is time for the final connection – this prevents contamination of the threads.

With copper or steel pipe, check whether the thread is not damaged. If it is, cut it off and re-thread. With pressed systems (e.g. multicooler, press-fitting), proceed according to the technology of the given system – these connections must not be made with Teflon, but with original sealing in the pressing sleeves.

Step 3 – Marking and drilling for brackets

Most VK panels are delivered with brackets that are attached to the top mounting strip of the radiator. Some models also have bottom supports. Procedure:

Place the template (or the radiator itself turned upside down) against the wall and mark the position of the holes for the brackets with a pencil.
Check with a spirit level whether both brackets are at the same height.
Drill the holes – minimum depth 60 mm into the solid part of the wall (not just the plaster), diameter according to the anchors.
Insert the anchors and fix the brackets with screws. Tightening torque: sufficiently tight, but do not overdo it – in brick walls you may crush the anchor.
Suspend the radiator on the brackets and check the level from both sides.

Note from practice: with drywall walls the situation is more complicated. Standard anchors in drywall are not sufficient to support the weight of a radiator full of water (a 300 × 800 radiator full of water weighs about 12–15 kg). Use metal expansion anchors or ideally hit the load-bearing profile in the drywall construction. If you are not sure, it is better to hang the radiator on a masonry wall behind the partition, or consult an installer.

Step 4 – Installation of the valve block and fittings

Modern VK panels (including the entire 21VK range) have the valve block integrated directly from the manufacturer. You do not need to install it separately – it is part of the radiator. What you must do, however, is:

Check the direction of the supply and return – standard is supply (H = heat supply) on the left connector of the block and return (R = return) on the right, but with some manufacturers or special configurations this may be the opposite. Always verify in the documentation.
Install the thermostatic head or manual regulating head on the appropriate output of the valve block – this step can be done before hanging the radiator, it is more convenient.
Install plugs on all unused openings (usually the left side of the panel, where there is no valve block) – they usually come with the radiator. Tighten them well with sealing.

Step 5 – Connection to the piping

This is a critical step where mistakes are most commonly made. The connections must be tight and correctly oriented.

With threaded connections (G1/2" external thread on the valve block) proceed as follows:

Wind 3–5 layers of Teflon tape in the direction of the thread (in the direction of tightening) or use hemp fiber with sealing paste.
Mount the connecting valves (angled or straight valves, or connecting elbows) on the connectors of the valve block. Tighten with a wrench – not too much, but firmly (approx. 30–40 Nm for G1/2").
Connect the piping. Check that you are not pulling on the pipe – if the pipe is under tension and you are trying to force it to connect to the radiator, the reliability of the connection will be lower and you may break the pipe from the base.

Step 6 – Filling the system and pressure test

After the connection, let the system be filled with water. The pressure in the system should typically be 1.0–1.5 bar (cold system) during filling. Monitor:

Whether water is dripping from any connection
Whether the pressure gauge on the boiler shows a stable pressure (not decreasing – this would indicate a water leak)
Whether air bubbles appear on visible parts of the piping

For renovations in older buildings, I recommend keeping the pressure at 1.5–2 bar for at least an hour before the first heating. It is not comfortable, but it ensures safety.

Step 7 – Bleeding the radiator

Bleeding is a mandatory operation, without which the radiator will be colder in the upper part and you will hear characteristic gurgling. VK panels have a bleed valve in the upper corner (usually the left one) with a square head of 5 mm. Procedure:

Fill the system, start the circulation pump, and let the system heat up.
Insert a radiator key (5 mm square) into the bleed valve and turn it about 1/4 turn to the left.
Listen – first air will escape (hissing), then water will start to drip.
Close the valve immediately after water appears.
Check the pressure in the system – the pressure may have changed due to filling and bleeding.

More on this topic, including flow setting and equithermal regulation, can be found in the article Bleeding and flow setting for radiators with bottom connection.

Step 8 – Setting the thermostatic head and balancing

The thermostatic head on the valve block of VK panels regulates the room temperature based on the ambient temperature. Set it to the desired value (usually 1 = ~12 °C, 3 = ~20–21 °C, 5 = ~28 °C, but it depends on the manufacturer). A common setting for a bedroom is 2–2.5, for a living room 3, for a bathroom 3–3.5.

In addition to the thermostatic head, the valve block VK also allows flow adjustment – this is known as pre-setting (presetting). In a larger system with multiple radiators, it is essential that each radiator receives the correct flow – this is hydraulic balancing. Without it, the radiator near the boiler will be overheated and the distant one will be cold. The pre-setting adjustment is usually performed by an installer with project documentation, but in a smaller system with 3–5 radiators, you can leave it fully open and the system will function acceptably.

Dimension tables and performance parameters – a general overview

To correctly select the size of the radiator and plan the installation, it is good to have an overview of the dimensions and performance. Below is an overview of the 21VK series at a height of 300 mm – a popular format for rooms with low sills or for supplementary heating:

Model	Width (mm)	Height (mm)	Output (W)	Typical use
21VK 300 × 400	400	300	298	WC, hallway, small bathroom
21VK 300 × 500	500	300	373	Bathroom, bedroom, hallway
21VK 300 × 600	600	300	447	Bedroom, bathroom, supplementary panel
21VK 300 × 700	700	300	522	Living room, study
21VK 300 × 800	800	300	596	Living room, larger bedroom

Note: the outputs are given for a temperature drop of 75/65/20 °C (supply/return/room). In modern low-temperature systems (55/45/20 °C or 45/35/20 °C with a heat pump), the actual outputs are 30–50 % lower. More on this can be found in the article What radiator output do I need – calculation based on room area and height.

Special situations and complications from practice

Installation of a radiator on a tiled wall (bathroom)

Drilling into ceramic tiles requires special drills (diamond drill or flat carbide drill without hammer function). Never use the hammer function when drilling into tiles – the tiles will crack. First, drill through the tile with a standard ceramic drill, then continue drilling into the wall with the hammer function. The brackets will be sufficiently secure only if they are anchored in the wall behind the tiles, not just in the grout under the tiles.

Radiator installation during plastering and renovation – timing

This is a question that renovators almost always ask during every renovation: when to install the radiator? The answer is clear – pipe runs should be embedded and plastered before tiles and flooring, but the radiator itself should be installed only after all wet processes (plastering, underfloor heating, tiling) are completed. Otherwise, you risk damaging the lacquer, corrosion from lime moisture, and damaging the thermostatic head.

Practical solution: after plastering, leave the pipes sealed with plugs or valves. Connect and install the radiator in the final phase – before final cleaning and after fitting the baseboards.

Installation in a system with a heat pump

With a heat pump, the system operates at a lower temperature (typically 35–50 °C). This directly affects the choice of radiator – you must select a higher output or larger size to compensate for the lower temperature drop. Bottom connection works just as well as in a boiler system, but pay attention to the flow pre-setting adjustment – higher flow is more important in low-temperature systems. About the comparison of bottom and side connections in various scenarios, you will read in the article Bottom radiator connection vs side – advantages, disadvantages and when it is worth it.

Replacing a radiator without draining the entire system

If each radiator has its own shut-off valves (as it should), you can replace the radiator without draining the entire system. Procedure:

Close both radiator valves (supply and return).
Drain the pressure and water from the radiator through the air vent at the top into a bucket.
Unscrew the connecting fittings from the valve block.
Remove the radiator from the brackets.
Mount the new radiator, connect it, and open the valves.
Bleed the radiator.

This procedure works reliably if the shut-off valves are in good condition. In practice, it can happen that an older valve, after years of inactivity, leaks after tightening – in that case, there is no other option than to drain the system and replace the valve.

Most common installation errors – what to avoid

Over the years in the industry, I have seen these recurring mistakes:

Mixing up the supply and return connections – performance drops by 20–30 %, regulation is inaccurate. Always verify which connector is H (hot water from the boiler) and which is R (return to the boiler).
Insufficient sealing of threaded connections – Teflon tape wrapped in the wrong direction, too few layers, or missing gasket under the nut. Result: a leaking connection that may only appear after several days.
Radiator not horizontal – air cannot be properly bled, the top edge remains cold.
Brackets anchored only in plaster – plaster cannot support the weight of a full radiator. Brackets must be embedded in the wall structure (masonry).
Missing plugs in unused holes – typically the left side of the panel. Result: a geyser when filling.
Thermostatic head set to maximum and left that way – the system is overloaded, regulation does not work. Set the heads according to the room and let them do their job.
Insufficient bleeding – the most common reason for complaints like "the radiator does not heat up even though the pipe is hot". Always bleed after the first start-up and again after 1–2 days of operation.

Post-installation check – what to verify before handing over

Before you mark the job as complete (or before the customer pays), check:

No visible water leaks at the connections, plugs, or bleed valve
Radiator is securely mounted and does not move
Radiator heats evenly – top and bottom, left and right edges
Thermostatic head turns smoothly across the full range
System pressure is stable (boiler pressure gauge does not show a drop)
No loud noises are heard (gurgling = air, tapping = thermal expansion in an unrestrained pipe)

Frequently asked questions (FAQ)

Do I need to drain the entire system when installing a bottom-connected radiator?

Not necessarily. If each radiator in the system has its own shut-off valves (which they should in new installations), it is enough to close the valves at the radiator being replaced, drain the water from it via the bleed valve, and replace it without affecting the rest of the system. The entire system must be drained only if shut-off valves are missing or leaking.

Which side is the supply and which is the return on a VK panel?

By default, the supply (H – hot water from the boiler) is the left connector of the valve block and the return (R – return to the boiler) is the right connector, when viewed from the front of the block. Always verify the specific documentation for the model – some manufacturers have it the other way around or the block is adjustable. Mixing them up will not immediately damage the radiator, but will reduce performance and regulation accuracy.

Can I rotate a bottom-connected radiator and have the valve block on the left side?

It depends on the model. Some VK panels allow the valve block to be moved from the right to the left side – simply remove the block and relocate it. Other models do not allow this and are sold separately as "left-side" or "right-side" units. When ordering, check which side your piping is on and choose the model accordingly. The strength of bottom connections lies precisely in the fact that with a well-planned layout, you won't need to rotate the radiator at all.

How many layers of Teflon tape are enough for a radiator thread connection?

For a G1/2" thread (standard for VK panel connections), 3–5 layers of 12–15 mm wide Teflon tape, wrapped in the direction of the thread (i.e., clockwise when viewed from above), are sufficient. Stretch the tape slightly and make sure it does not extend beyond the end of the thread. An alternative is hemp fiber with Fermit or Loctite 577 paste – both methods are equally reliable when used correctly.

Why is the top part of my new radiator cold?

The typical cause is air trapped in the radiator. Bleed it using a square key (5 mm) on the bleed valve in the top corner of the panel – turn it a quarter turn to the left, wait until water starts to flow, then close it. If the radiator still does not heat evenly after bleeding, check the flow: if the valve block is set too low, the flow may be insufficient. For more information, see the article Bleeding and flow adjustment for bottom-connected radiators.

What distance should be left between the radiator and the floor?

At least 10 cm, ideally 12–15 cm. This gap is necessary for proper air circulation under the radiator – cold air flows in from below, passes through the convectors (fins) between the panels, heats up, and rises. If the radiator is too low or if the carpet is placed directly under it, radiator performance drops and the carpet may overheat.

Conclusion and practical summary

Installing a bottom-connected radiator is manageable even for a skilled DIYer, but each step requires attention. The most important prerequisite for success is thorough preparation: accurately measured bracket and connection positions, verifying the direction of supply and return, quality sealing of threaded connections, and thorough bleeding after system start-up.

For those interested in a deeper understanding of installation-related issues, I recommend reading the articles Dimensions of 21VK radiators – how to correctly choose the height and width of the panel and Common faults of bottom-connected radiators and how to eliminate them, where you will find solutions to problems you may encounter even after what seems to be a correct installation. And if you are unsure about choosing the right power, don't miss Common questions about bottom-connected radiators – a compact overview of answers to the issues customers most often ask.

A properly installed and adjusted bottom-connected radiator will operate reliably for decades without any intervention – and that is exactly the goal worth achieving by doing the installation properly from the start.

Do you have a question on this topic?

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