Dimensions and thread types of radiator valves – what do the markings 1/2", 3/4", and EK mean
Dimensions and thread types of radiator valves – what do the markings 1/2", 3/4", and EK mean?
When browsing the range of radiator valves, you will encounter markings such as 1/2"xEK, 3/4", corner 1/2", or straight DN15. For someone not used to working with plumbing dimensions, this may look like a code. In reality, it is a quite logical system – once you understand it, you will immediately know what the valve does, where it fits, and what additional parts you need to buy. This article will cover the topic from the basics to practical details, including common mistakes when selecting and installing.
Inch threads – a historical foundation that still dominates
Radiator valves are mostly marked in inches (symbol "). This is the British measurement system, where 1 inch = 25.4 mm. Although metric dimensions dominate most of Europe, inch threads have remained in use in plumbing and heating – globally. The reason is simple: historical compatibility. Radiator piping was dimensioned in inch sizes even decades ago, and this tradition continues.
With valve threads, you will encounter two basic standards:
- BSP (British Standard Pipe) – also known as Whitworth or "G thread". This is the most common type of thread in radiator valves on the Slovak and Czech markets. It is a cylindrical thread (parallel walls).
- NPT (National Pipe Tapered) – an American tapered thread. It is almost non-existent in European heating systems, but you may encounter it with some imported components.
Therefore, when you see a marking such as 1/2" G or just 1/2" on a valve, we are referring to a BSP cylindrical thread with a nominal diameter of 1/2 inch. The actual outer diameter of the threaded part is, however, larger – approximately 20.9 mm. Why? Because in plumbing, the inch dimension originally indicated the inside diameter of the pipe, not the actual outer dimension. This is a source of many misunderstandings.
What does the marking 1/2" and 3/4" mean in practice?
The most commonly used size for radiator valves is 1/2". This is the size of the supply and return pipes for most apartment and house radiators installed since the 1970s. A valve marked 1/2" has an outer or inner thread with a BSP 1/2" dimension, which corresponds to the nut or coupling diameter for connecting to the radiator or pipe.
The size 3/4" appears on larger radiators (especially column and cast iron types) or on main distribution lines. It is less common in apartment installations, but regularly found in houses with higher performance. A 3/4" valve is physically larger, has higher flow capacity, and is suitable for circuits with higher flow rates.
The size 1" (one inch) is rarely used for valves themselves – more often for ball valves to isolate sections of the system or in special commercial applications.
In practice, I most often encounter 1/2" valves – including in panel buildings and new constructions. If you are buying a valve for an existing radiator and you are unsure of the size of your connection, the most reliable method is to remove the old valve and bring it to the store as a sample, or alternatively, measure the thread with a standard wrench or sliding caliper.
What is EK and why is it important?
EK is an abbreviation you will encounter with radiator valves – for example, in the marking 1/2"xEK. The abbreviation "EK" comes from the German "Euro-Kegel" or from the plumbing practice of the Central European market and refers to the European conical thread, more precisely the standardized internal thread M30x1.5 on the side of the thermostatic head.
Let us explain this with a concrete example: A thermostatic valve has two sides. One side connects to the pipe – here it has a BSP thread (1/2" or 3/4"). The other side of the valve is the preparation for mounting the thermostatic head – and this is where EK comes into play. Most thermostatic heads on the European market use a connection of M30x1.5 mm – that is, a metric thread with a diameter of 30 mm and a pitch of 1.5 mm. This is effectively a pan-European standard.
Therefore, when you see a product such as Thermostatic valve with dual control – 1/2"xEK; corner type, read it like this: the valve has a 1/2" BSP thread on the pipe side and a preparation with an EK (M30×1.5) thread for the thermostatic head. This is one of the most commonly used types in apartment installations.
Overview of common valve marking combinations
For better orientation, I list typical combinations that you will encounter in practice:
| Marking | Pipe connection | Head connection | Typical use |
|---|---|---|---|
| 1/2"xEK | BSP 1/2" (20,9 mm) | M30×1,5 | Standard apartments, panel buildings, new constructions |
| 3/4"xEK | BSP 3/4" (26,4 mm) | M30×1,5 | Larger radiators, detached houses |
| 1/2" (without EK) | BSP 1/2" | Manual handle | Manual valves, bathroom pipe systems |
| 1/2"×M30 | BSP 1/2" | M30×1,5 (explicitly) | Same meaning as 1/2"xEK |
| DN15 / DN20 | Nominal diameter (≈1/2" / ≈3/4") | Depending on the version | Technical design documentation |
Note on DN (Diameter Nominal): this is an international technical designation of the nominal pipe diameter used in projects and standards. DN15 corresponds to a diameter of 1/2" and DN20 corresponds to 3/4". When purchasing valves for standard installation, you will encounter DN less often – retailers and manufacturers use inch markings, as they are more commonly used in practice.
Differences between external and internal thread
Along with the thread size (1/2", 3/4"), the direction of the thread is also important – whether it is an external (male) or internal (female) thread. Valves are produced in various combinations depending on how the pipes are routed in the wall or floor and how the radiator head is shaped.
- External thread (G 1/2" M) – screwed into an internal thread on the opposite side. Typically on the side of the pipe coming out of the wall with a "nipple".
- Internal thread (G 1/2" F) – mounted on an external thread (e.g., on a radiator head). Less common for valves, more common for connectors.
The vast majority of thermostatic and angle valves have an external thread on the pipe side, which is inserted into a short adapter (head) welded or screwed to the radiator. This head usually has an internal thread of G 1/2". Therefore, always check what you have on the radiator: what kind of thread and in which direction.
Angle vs. straight valves – impact on dimensions and type of connection
The type of valve (angle or straight) is not directly related to the thread size, but it affects where the thread is physically located. In an angle valve, the inlet from the pipe and the outlet to the radiator form a 90° angle. In a straight valve, both openings are in one axis. We discuss this topic in detail in the article "Angle vs. straight valve – how to determine which type you need."
For now, it is important to note that with an angle valve, you come from the floor and the valve turns the flow to the side (or from above to the side), while with a straight valve, you come from the wall horizontally. The actual thread size (1/2" or 3/4") applies to both types equally.
Thermostatic heads and EK connection – compatibility in practice
A thermostatic head is mounted on the EK fitting (M30×1,5). Most common heads on the market are designed exactly for this standard. For example, Thermostatic head HPW large or Thermostatic head large – painted are intended for valves with an EK thread of M30×1,5 mm, which covers most standard installations.
In practice, I have encountered cases where a customer bought a valve from one manufacturer and a head from another, and the combination did not work. The most common reason was not a difference in the thread (M30×1,5 is standard), but the shape of the stop inside – some manufacturers use a different stem lift. Therefore, I recommend either buying sets from the same manufacturer or verifying compatibility with the seller before purchasing.
That is why complete solutions are popular, such as the HPW Set1 control kit or the HPW Set2 control kit – they include a valve and a head that have been mutually verified for compatibility and fit correctly. For more information on what control kits include and when they are worth buying, see the article "Radiator control kit – what it includes and when it is worth buying a set."
How to measure the thread of an existing valve or pipe
If you are renovating a bathroom or replacing valves on an existing system, you need to determine what thread size you have. There are several reliable methods:
Method 1 – Measuring the thread diameter with a caliper
Remove the valve and measure the outer diameter of the thread (for an external thread) or the inner diameter (for an internal thread) using a caliper. Then compare it with the following table:
- Outer diameter ~20.9 mm → BSP 1/2"
- Outer diameter ~26.4 mm → BSP 3/4"
- Outer diameter ~33.2 mm → BSP 1"
Method 2 – Using a wrench
If you have a threaded pipe fitting (pipe coupling), place an Allen key next to it or try to screw on a test nut. Experienced plumbers can estimate this at a glance – the head of a 1/2" BSP screw is about 27–30 mm wide.
Method 3 – Thread pitch
BSP threads have 14 threads per inch (for both 1/2" and 3/4"). If you run your finger along the thread and count the teeth, you can verify this. However, this is unnecessarily complicated in practice – it is simpler to measure the diameter.
Method 4 – Take the old valve to the store
The most reliable method, especially for a layperson. Remove the old valve (make sure to close the circuit first!) and bring it to the store in person. An experienced salesperson will immediately determine the type and recommend a compatible replacement.
Common mistakes when choosing a thread and how to avoid them
In practice, I repeatedly encounter the same types of mistakes. Here are the most common ones and how to avoid them:
Mistake 1 – Confusing 1/2" and 3/4"
A customer buys a 1/2" valve for a radiator with a 3/4" header. The attempt to install it ends with the thread "loose" – the valve cannot be tightened. Solution: always measure or bring a sample. Be careful, the difference in diameter is small when viewed with the naked eye – only about 5–6 mm of outer diameter.
Mistake 2 – A valve without an EK fitting for a thermostatic head
A customer buys a cheap simple valve (e.g., a ball valve) and then wants to attach a thermostatic head to it. A ball valve does not have an EK fitting and the head cannot be mounted. Solution: when planning for thermostatic control, always buy a thermostatic valve with the marking EK or M30.
Mistake 3 – Incompatible stem travel
A valve from one manufacturer and a head from another. Although both have M30×1.5, the stem travel differs by a fraction of a mm – the head does not fully close the valve or does not close it at all. Solution: buy a set from the same manufacturer or verify compatibility before purchase.
Mistake 4 – Confusing left-hand and right-hand threads
Most threading is right-hand. However, on some older cast iron radiators (especially on the left side), you may encounter left-hand threads. If the thread does not turn during installation, do not force it – check the direction of rotation.
Mistake 5 – Using an unsuitable sealing material
BSP threads are cylindrical, not conical. The seal between an external and internal BSP thread is not created "on metal," but by using sealing tape (Teflon, hemp fiber with paste). If you forget the sealing material or wind it in the wrong direction, the joint will leak.
Connection to standards – EN 215 and ISO 228
For those interested in technical details: BSP threads are standardized according to ISO 228 (or EN 10226 for conical variants). Thermostatic valves and heads must comply with the EN 215 standard in Europe, which, among other things, defines the minimum stem travel, regulation accuracy, and temperature range. This standard does not directly specify thread size, but manufacturers must comply with it to obtain the CE mark.
For the average customer in practice: if you buy a valve with a CE mark and from a reputable manufacturer, you can be sure that the EK fitting will be standard (M30×1.5) and compatible with other products certified according to EN 215.
Practical scenarios from installation practice
Scenario A – Panel building, central heating, valve replacement
Typical case: an apartment building from the 1980s, cast iron column radiators with wall connections (horizontal). The original manual valves are worn out, and the customer wants to install thermostatic ones. The wall connections are finished with an external thread of G 1/2". The radiators have headers with an internal thread of G 1/2". Solution: direct thermostatic valve 1/2"×EK, with a thermostatic head with an M30×1.5 connection. The result is a functional thermostatic control without any adapter.
Scenario B – New construction, floor distribution, corner valves
In new single-family homes, floor distribution is now standard. Two pipes (supply and return) protrude from the floor, finished with an external thread of G 1/2". A panel or bathroom ladder radiator has headers on the side. Solution: corner thermostatic valve 1/2"×EK + corner shut-off valve (on the return pipe) + thermostatic head.
Scenario C – Bathroom ladder radiator, larger diameter
Some bathroom ladder (heating) radiators have larger connections – G 3/4". The customer bought a G 1/2" valve and during installation discovered a mismatch. Solution: either replace the valve with a 3/4" version or use a transitional threaded piece (3/4"F × 1/2"M), but this is a solution for an emergency – it is better to have the correct size directly.
Scenario D – Two-way control valve in a two-pipe system
In modern two-pipe systems, both temperature (thermostatic head) and hydraulic balance (flow presetting) are adjusted at the valve node. Therefore, two-way control valves are used, where one component is used for thermostatic control and the other for hydraulic presetting. This is also the case with the mentioned corner two-way control valve 1/2"×EK. The dimensional marking is the same, only the function is extended. For more information on choosing the correct type, see the article How to choose the right radiator valve – type, connection and compatibility.
Adapters and reductions – rescue in case of incorrect dimensions
What if you have already bought a valve and find out that the dimension does not match? In plumbing stores, threaded reductions and adapters are available that can bridge different sizes. The most commonly used ones are:
- Reduction 3/4"F × 1/2"M – for the case when you have a 3/4" head and a 1/2" valve
- Extension nipple – when the pipe is embedded too deep in the wall and the valve cannot be pulled up to the wall
- Adapter M30×1.5 to another type of head – if you have an older head with a proprietary thread (e.g. Danfoss RA) and you want to mount a standard M30 head on it
These solutions are functional, but from a professional point of view, I consider them undesirable – each additional connection is a potential leak point. If time and situation allow, it is always better to choose the correct valve from the beginning.
Most frequently asked questions (FAQ)
What does the marking EK on a radiator valve mean?
EK indicates an European conic thread for a thermostatic head, specifically a metric thread M30×1.5 mm. It is de facto the European standard for connecting thermostatic heads. If you see the marking 1/2"×EK on a valve, it means that the valve has a BSP 1/2" thread on the pipe side and a M30×1.5 preparation on the head side.
Is 1/2" the same size as DN15?
Yes, in plumbing practice these markings are equivalent. DN15 is an international standard nominal pipe size, 1/2" is an imperial marking. Both refer to the same pipe and valves, just from different marking systems. The outer diameter of the BSP 1/2" thread is 20.9 mm.
Can I connect a thermostatic head M30×1.5 to any valve with EK?
In most cases yes – the M30×1.5 standard is unified. However, it is important that the valve stem lift corresponds to the range of movement of the head. If you buy a valve and a head from different manufacturers, I recommend checking compatibility before installation. Complete sets are a safe option, for example Regulation set HPW Set1 or Regulation set HPW Set2, where the components are verified to be compatible.
How can I find out what thread my radiator has, without removing the valve?
It is difficult, but not impossible, without removing the valve. Look at the thickness of the nut – if you see a nut with a key width of about 27 mm, it is likely 1/2". If the nut has a key width of about 32–34 mm, it is probably 3/4". A more reliable method is to remove the valve and measure the thread diameter with a caliper, or bring the old valve to the store as a sample.
Why does the outer diameter of the 1/2" thread have a value of almost 21 mm, when I would expect 12.7 mm (= half an inch)?
This is a historical issue. In the British BSP system, the inch dimension originally indicated the internal diameter (nominal size) of the pipe, not the outer diameter of the thread. The pipe wall and other construction factors added additional millimeters. Therefore, imperial plumbing threads are "larger than you would expect" – and therefore never compare 1/2" directly with the value of 12.7 mm.
Can I mount a standard thermostatic head on a two-way control valve, or do I need a special one?
If the two-way control valve has a standard EK preparation (M30×1.5), you can mount a standard thermostatic head on it – for example Large thermostatic head HPW. The two-way control refers to the hydraulic part (flow presetting), not the connection for the head. Always verify this in the technical specifications of the specific valve.
Conclusion – small numbers, big difference
The dimensions and threads of radiator valves may look like a technical detail at first glance. In reality, they are the basis of a functional and trouble-free installation. The difference between 1/2" and 3/4" is only a few millimeters, but with the wrong choice you will not be able to install anything at all. The abbreviation EK may seem unclear at first, but once you know it refers to M30×1.5 – a standard compatible with most thermostatic heads on the market – everything becomes clear.
Rule number one is: always measure first, then buy. Rule number two is: if you buy a valve and a head at the same time, go for verified sets from one manufacturer – you will save time, unnecessary returns to the store and possible leaks. And if you are still unsure, read other articles in the Knowledge Centre – for example, Installation of a thermostatic valve on a radiator or How to choose the right radiator valve – there you will find the entire procedure from selection to successful commissioning.
Do you have a question about this topic?
Not sure how to decide or dealing with a specific situation in your home? Write to us – we are happy to help.