Steel Pipe and Fitting Diameters: Which Size Do I Need for My System
Steel pipe and fitting diameters: which dimension do I need for my system
One of the most common causes of complaints, rework, and unnecessary site visits is simple: a confused pipe or fitting dimension. It seems trivial until you're standing in front of a partially assembled boiler room with the wrong material in your hand. This topic is more complicated than it appears at first glance – steel pipe has several different dimension systems that historically overlap, and the confusion between them is not always intuitive. In this article, we will go through the entire issue systematically, from basic terminology to concrete examples from practice.
Why is the steel pipe dimension system so confusing
If you bought a pipe labeled as "15 mm" and measured its outer diameter with a caliper, you would get a value of 15.0 mm. If you did the same with a copper pipe labeled "15 mm", you would also get 15.0 mm. So far, so good. But if you bought a threaded steel pipe labeled "1/2 inch" and measured it, the outer diameter would be around 21.3 mm – and the inner diameter would be around 16 mm. No inch is to be found there by any means.
The reason is historical. The English inch system for threaded pipes (so-called Whitworth thread, later Gas or BSP – British Standard Pipe) was originally derived from the inner diameter of the pipe, when pipes had much thicker walls. When the walls were thinned, the inner diameter changed, but the labeling remained. The result is that a "1/2" pipe has an outer diameter of 21.3 mm and the actual inner diameter varies according to the wall thickness.
Parallel to this, there is a metric system for press pipes (where 15 mm really means an outer diameter of 15 mm), the German DIN standard, the European EN standard, and a whole range of older Slovak and Czechoslovak standards (ČSN). In practice, you can encounter all of them on a single job – an older steel distribution DN15 with BSP threads, a new press branch 18 mm, and a boiler with connections Rp 3/4".
Metric press pipes: what do 15, 18, 22, 28, 35, 42 and 54 mm mean
In modern heating installations, the press system with metric outer diameter labeling dominates. The series of steel press pipes in heating includes these common dimensions:
- 15 mm – the smallest common dimension, typically for radiator connections, manifold branches, small floor heating circuits
- 18 mm – very common for distribution branches in apartment buildings, horizontal distribution to radiators, secondary circuits
- 22 mm – main branches in smaller family homes, connections to TÚV storage tanks, inlet lines to boilers
- 28 mm – main branches in apartment buildings, primary circuits of larger systems
- 35 mm – machine rooms, boiler rooms, larger commercial buildings
- 42 mm – industrial boiler rooms, small-scale district heating distribution
- 54 mm – main distribution branches in professional boiler rooms
Each of these dimensions has a defined wall thickness according to the EN 10255 standard (or older ČSN 42 6710). For pipes up to 54 mm outer diameter, the following nominal wall thicknesses apply:
| Pipe OD (mm) | Wall thickness (mm) | Inner diameter (mm) | Typical use in heating |
|---|---|---|---|
| 15 | 1.0 / 1.2 | 12.6 – 13.0 | Radiator connections, small circuits |
| 18 | 1.0 / 1.2 | 15.6 – 16.0 | Distribution branches, secondary circuits |
| 22 | 1.2 / 1.5 | 19.0 – 19.6 | Main branches for family homes, boiler inlet |
| 28 | 1.2 / 1.5 | 25.0 – 25.6 | Primary circuits of apartment buildings |
| 35 | 1.5 | 32.0 | Boiler rooms, commercial buildings |
| 42 | 1.5 | 39.0 | Industrial boiler rooms |
| 54 | 2.0 | 50.0 | Main branches of boiler rooms |
Inch threaded pipes and DN system: what is behind the number
Older installations and many devices (boilers, valves, manifolds) still use inch threaded pipes or at least connections with BSP threads. Here is the key:
DN (Diameter Nominal) is a nominal diameter – a commercial designation that does not directly refer to either the outer or the inner diameter. It is just a reference number. DN 15, DN 20, DN 25, DN 32, DN 40, DN 50 – these are common values for heating. The outer diameter of the corresponding threaded pipe is always larger:
| Inch designation | DN | Pipe OD (mm) | BSP thread (Rp/R) |
|---|---|---|---|
| 3/8" | 10 | 17,2 | Rp 3/8" |
| 1/2" | 15 | 21,3 | Rp 1/2" |
| 3/4" | 20 | 26,9 | Rp 3/4" |
| 1" | 25 | 33,7 | Rp 1" |
| 1 1/4" | 32 | 42,4 | Rp 1 1/4" |
| 1 1/2" | 40 | 48,3 | Rp 1 1/2" |
| 2" | 50 | 60,3 | Rp 2" |
Key takeaway: press pipe 15 mm is not the same as threaded pipe DN 15. They have different outer diameters (15.0 mm vs. 21.3 mm) and cannot be interchanged without transition fittings. This is a major source of confusion on construction sites.
Threads Rp, R, G: what they mean and how to distinguish them
On boiler connections, valves, fittings and accessories, you will encounter three basic types of pipe threads according to ISO 7-1 and EN 10226 standards:
- Rp (internal cylindrical thread) – sealing depends on the sealing surface or Teflon tape/fiber; the thread axis is parallel, the profile is parallel. This is the most common internal thread on heating fittings.
- R (external conical thread) – conical profile, when tightened, the thread self-seals to some extent, but in heating it is still supplemented with sealing material.
- G (external/internal cylindrical thread) – so-called "gas" thread, cylindrical, historically most widespread in older installations.
In practice: when you buy a connecting coupling with an internal thread of 15 mm – Rp1/2", it refers to a press pipe with an outer diameter of 15 mm and a fitting with an external thread of R 1/2" (or G 1/2"). Rp is the internal thread of the fitting – cylindrical, into which the external thread R or G on the fitting fits. This is a standard combination, for example, when connecting a thermostat, drain valve or pump to press piping.
Connecting couplings: where the press system meets the threaded world
Connecting couplings are exactly the category of fittings where the metric press world intersects with the inch-threaded world. These are reducing elements that clamp the press pipe on one side and have a thread – either external (G/R) or internal (Rp) – on the other side.
There are many combinations and it is precisely here that the most confusion arises. Take a look at a few specific examples:
- Connecting coupling 15 mm – Rp 1/2": press pipe 15 mm on one side, internal thread Rp 1/2" on the other. Typical use: connecting a 15 mm press pipe to a radiator valve with an external thread R 1/2" or to a boiler fitting.
- Connecting coupling 15 mm – Rp 3/4": press pipe 15 mm on the press side, internal thread Rp 3/4" – used, for example, when connecting a smaller pump or when the fitting has a larger connection than the pipe.
- Connecting coupling 18 mm – Rp 1/2": press pipe 18 mm with a 1/2" thread – common in a secondary circuit 18 mm distribution to fittings with a smaller thread.
- Connecting coupling 18 mm – Rp 3/4": press pipe 18 mm with a 3/4" thread – typically an input/output of a manifold or pump body in 18 mm circuits.
- Connecting coupling 22 mm – Rp 1": press pipe 22 mm with a 1" thread – most commonly used at the input and output of a boiler or tank, where the boiler has R 1" connections and the primary branch is 22 mm press.
As you can see, when choosing a connecting coupling, you must know exactly two things: the outer diameter of the press pipe (15, 18 or 22 mm) and the size of the thread on the fitting (Rp 1/2", Rp 3/4", Rp 1"). Only with both values can you choose the correct piece. We discuss this topic in more detail in the article How to choose the correct combination of pipe diameter and fitting thread in our Knowledge Center.
How to correctly measure existing pipe on the site
This is a practical situation: you arrive at a job, need to extend a branch or replace a section. You need to find out what you are working with. Procedure:
1. Caliper (sliding rule): Measure the outer diameter of the pipe. If you get 15.0 mm, it is a 15 mm press pipe. If you get 21.3 mm (or slightly less due to wear/paint), it is a threaded pipe DN 15 / 1/2". If you get 26.9 mm, you have DN 20 / 3/4".
2. Look at the wall shape: Press pipe has a flat, smooth wall without thread and without forging folds. Threaded pipe has a visible thread or couplings with internal thread at the ends.
3. Color and surface: Steel press pipe is usually without protective coating or with a thin phosphatizing coating. Galvanized threaded pipe has a visible silver zinc surface. Black threaded pipe (without zinc) is dark gray to dull black.
4. Presence of press joint: If you see a characteristic hexagonal or circular embossed shape of the fitting on the pipe (without thread, without welding), you are in a press system.
5. Valves and their connections: If you need to replace a valve or connect a new one, do not forget to measure the thread on the valve (internal or external) and its size. A wrench or thread gauge (thread templates) are more reliable than estimating by eye.
An experienced technician always has at least a caliper with them. How many times have I seen a company leave the first site visit and order materials based on an estimate, only to return for another visit with the wrong fittings. Measuring takes thirty seconds and saves hours.
Typical examples from practice: what dimension for what system
Example 1: Single-family house with condensing boiler and radiators
A condensing boiler usually has R 3/4" or R 1" connections on the inlet and outlet. The primary branch from the boiler to the manifold is mostly 22 mm press (for houses up to 20 kW). Horizontal runs to radiators are usually 15 mm or 18 mm press. Radiator valves have R 1/2" or R 3/4" thread depending on size. To connect the boiler, you need connecting adapter 22 mm – Rp 1" (if the boiler has inch connections R 1"). To connect radiators from 15 mm press pipe to R 1/2" valves, you need connecting adapter 15 mm – Rp 1/2".
Example 2: Apartment building, reconstruction of old riser system
The old system is a threaded black pipe DN 25 (outer diameter 33.7 mm). You want to replace it with 28 mm press and a modern steel press system. For the transition between the old threaded system and the new press system, you need a press transition fitting 28 mm × G 1" (or R 1"). Branches to apartments are DN 15 (old pipe), new branches are made in 15 mm press. Note: if the old supply valve of the apartment thermostat has R 3/4" thread, you use connecting adapter 15 mm – Rp 3/4" to connect the new press branch to the old valve.
Example 3: Floor heating with manifold
A floor heating manifold is usually steel or brass with G 1" connections on the inlet and outlet and G 3/4" on individual circuits. The supply and return pipe to the manifold is usually 22 mm press (for larger buildings 28 mm). To connect 22 mm press pipe to the manifold connection G 1", you need an adapter 22 mm – Rp 1". Individual floor heating circuits are usually plastic (PEX or PERT), but with steel supply to the manifold you will work with a press system for 22 mm.
Example 4: Technical room with TÚV tank
A TÚV tank has R 3/4" or R 1" connections. The primary circuit (heating the tank from the boiler) is usually 22 mm press, the secondary circuit (hot water distribution to outlets) is 18 mm press. To connect the 18 mm press pipe to the tank connection R 3/4", use connecting adapter 18 mm – Rp 3/4".
Most common mistakes when selecting dimensions and how to avoid them
Over the years of practice, I have seen these recurring mistakes:
- Mixing press 15 mm with DN 15 / 1/2": The most common. Press 15 mm and threaded pipe DN 15 have completely different outer diameters. A threaded pipe will not fit into a press fitting (outer diameter 21.3 mm is too large for a press sleeve 15 mm). Always verify the system type and current outer diameter.
- Negative thread reduction: A technician takes a connecting adapter 18 mm – Rp 1/2" instead of 18 mm – Rp 3/4". The thread is smaller than the valve connection – result: either the fitting does not fit at all, or they force it in and the valve leaks.
- Wrong type of thread (Rp vs. R): Internal thread Rp is paired with external thread R or G. Two internal threads (Rp + Rp) or two external threads (R + R) cannot be connected. At first glance, this seems obvious, but during purchase, confusion often arises.
- Ignoring wall thickness when calculating cross-section: When designing pipe dimensions, you must work with the internal diameter, not the external one. A pipe 18 mm × 1.2 mm wall has an internal diameter of only 15.6 mm – which is less than you might have expected.
- Mixing standards of different manufacturers: Press fittings from different manufacturers (e.g. Viega, Eurotubi, Rautitan) are not mutually compatible, even if they have the same dimensions. The sealing series, geometry of the insertion profile, and in some cases even the wall thickness of the pipe are manufacturer-specific. Always install a fitting and pipe from the same system.
Hydraulic dimensioning: what diameter for what power
Along with physical compatibility, there is also the hydraulic aspect – the selected diameter must handle the required flow without excessive pressure loss. This is a topic for a separate article on hydraulic dimensioning, but briefly, the following approximate values apply to heating systems with Δt = 10 to 20 °C:
- 15 mm press (ID ≈ 13 mm): maximum recommended flow approx. 0.3–0.5 m³/h, corresponds to power approx. 3.5–6 kW at Δt 10 °C or 7–12 kW at Δt 20 °C
- 18 mm press (ID ≈ 16 mm): max. flow approx. 0.6–1.0 m³/h, power approx. 7–12 kW (Δt 10 °C) or 14–23 kW (Δt 20 °C)
- 22 mm press (ID ≈ 19 mm): max. flow approx. 1.2–2.0 m³/h, power approx. 14–23 kW (Δt 10 °C) or 28–46 kW (Δt 20 °C)
- 28 mm press (ID ≈ 25 mm): max. flow approx. 2.5–4.0 m³/h, power approx. 29–46 kW (Δt 10 °C)
Note: these values are approximate and depend on the required pressure loss per meter (recommended value is 100–150 Pa/m). For accurate dimensioning, a hydraulic calculation is necessary, or consultation with a designer. More detailed information can also be found in the article How to choose the right press fittings for steel pipe in heating.
Pressure and temperature limitations of steel press pipes
Steel press pipes for heating are certified for these common parameters according to EN 10305-3 and the manufacturers' internal standards of press systems:
- Maximum operating pressure: standard 16 bar for diameters up to 54 mm (some systems up to 25 bar)
- Maximum operating temperature: 110 °C for continuous operation, short-term peaks up to 120 °C
- Minimum temperature: typically -20 °C (depends on the sealing in the press fitting)
For standard hot water heating (max. 90 °C, max. 6 bar), these parameters are significantly oversized, which is good – the system has safety margins. For industrial use or systems with higher pressure, it is necessary to verify the parameters of the specific press system manufacturer.
Common questions (FAQ)
What is the difference between a 15 mm press pipe and a DN 15 threaded pipe?
These are two completely different pipes despite the similar name. A 15 mm press pipe has an exact outer diameter of 15.0 mm and is connected by pressing (press). A DN 15 (1/2") threaded pipe has an outer diameter of 21.3 mm and is connected by a threaded system or welding. They are not directly compatible – to switch between them you need a special transition fitting.
How can I determine the thread type of my valve if it is not marked?
Use thread templates (a set of measuring templates for BSP threads), or measure the outer diameter of the thread with a caliper: 21.3 mm = G/R 1/2", 26.9 mm = G/R 3/4", 33.7 mm = G/R 1". For internal threads, measure the diameter of the hole. If you are unsure, take the valve to a store and ask for advice – mismatched threads are a source of leaks and unnecessary complaints.
Can I combine press pipes and fittings from different manufacturers?
Generally NO. Press systems from different manufacturers (Viega Profipress, Eurotubi, Rautitan and similar) have different sealing geometries and crimping profiles. Although the pipes may look the same (same outer diameter, same wall thickness), a fitting from one manufacturer crimped onto a pipe from another manufacturer may not be airtight. Manufacturers explicitly prohibit this and such a connection loses its warranty. Always install a complete system from one manufacturer.
What diameter of press pipe should I use for a 20 kW condensing boiler?
For most condensing boilers up to 25 kW, the primary branch (boiler inlet/outlet) is 22 mm press. The boiler will have G 3/4" or G 1" connections – depending on the manufacturer and model. If the boiler has G 1" connections, you will use a 22 mm – Rp 1" connecting adapter. Horizontal distribution branches to radiators are usually done in 15 mm or 18 mm press, depending on the number and power of the radiators on that branch. Optimal dimensioning will be provided by a hydraulic calculation or a designer.
Can a press fitting be opened again after crimping?
No – a press connection is permanent. After crimping, it is not possible to open it without damage. If you need to make a change, you must cut off the press fitting and replace it with a new one. This is why it is extremely important to check the correct size, position of the seal and insertion depth of the pipe before crimping. We write in detail about the installation in the article How to properly install a press fitting on a steel pipe without leakage.
Why do I sometimes see a 15 mm diameter and sometimes Rp 1/2" on the same pipe – are these different things?
Yes, these are two different dimensions of the same connection and both must match. "15 mm" describes the outer diameter of the press pipe – it is crimped into the press sleeve of the fitting. "Rp 1/2"" describes the thread on the other side of the fitting – it is screwed into the valve. Connecting adapters therefore always have two numbers: the size of the press side and the size of the threaded side. Both must match the reality, otherwise the installation will not work.
Conclusion: what to take away
The dimensional system of steel pipes and fittings for heating is a product of historical development, in which metric and imperial systems overlap. Simplified: press pipes are marked by their outer diameter in millimeters (15, 18, 22 mm...), threaded pipes and valves use the imperial system (1/2", 3/4", 1"...), with nominal DN marking, and these two systems are not directly interchangeable.
Key rule for practice: always measure with a caliper, do not rely on estimation or similar appearance. Before purchasing fittings, note the outer diameter of the press pipe and the size of the thread on the valve. If you are unsure, verify it on site – a thirty-second measurement is cheaper than an incorrect delivery or complaint.
For a deeper look at the installation, selection and maintenance of the entire press system, we recommend further articles in our Knowledge Center: Press fittings vs. threaded fittings: when to use which solution, Corrosion of steel pipes and fittings: causes, prevention and solutions and Common faults of press fitting connections on steel pipes and how to fix them.
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