How to choose the right press fittings for steel pipes in heating

We are still preparing products.

But you can look at other categories.

How to choose the right press fittings for steel pipe in heating

Press fittings for steel pipes are now part of the basic equipment of every installer who specializes in professional heating. Despite this, I still encounter situations on jobs where someone uses the wrong size, mixes incompatible systems from different manufacturers, or underestimates the importance of choosing the right sealing for a given type of medium. The result? Leaks shortly after handing over the work, the need to remove part of the installation, complaints. This article is therefore not just a dry product overview – it is a practical guide on how to choose the right press fitting the first time, without unnecessary mistakes.

What is a press fitting and why use it on steel pipe

A press fitting (crimp fitting) is a fitting that is attached to a steel pipe by mechanical crimping using special pliers or a crimping head. Inside the fitting, a sealing ring (usually made of EPDM or FKM rubber) is placed, which deforms during crimping and creates a hermetic connection. The whole process takes only a few seconds, does not require fire, soldering paste, or threading – which, compared to threaded fittings, saves a huge amount of time and also eliminates the risk of fire that can occur with traditional soldering.

For steel pipes in heating, press fittings are particularly advantageous where they are installed in larger quantities – boiler rooms, apartment buildings, industrial buildings, school buildings. In single-family homes, they are increasingly being replaced by threaded fittings, because the customer wants visible fittings that they can tighten themselves if needed. But with crimping, the speed of assembly is unmatched.

Overview of press fitting types for steel pipe

Before we move on to the actual selection, it is important to clarify what types of fittings exist and what they are used for. Steel press fittings are usually made of carbon steel (with surface treatment – hot-dip galvanized or organic coating) or stainless steel. In standard heating (low-temperature and medium-temperature systems), we most commonly encounter the following types:

Straight couplings (spigots) – connect two pipes of the same diameter in line, without changing direction
Reducers – connect two pipes of different diameters; used when transitioning from a larger branch to a smaller one
Elbows 90° and 45° – change the direction of the pipe; 90° for sharp turns, 45° for milder, aerodynamically more advantageous ones
T-pieces – branch; three-way division of flow, same or reduced diameter
Connecting nipples with external or internal thread – transitional element between press pipe and fittings, boilers, pumps
Caps – closing the end of the pipe or temporary protection before connecting another section
Clamps and fastening elements – technically not fittings, but part of the system; ensure the stable position of the pipe in the structure

For heating, from a practical point of view, the most important are connecting nipples with internal thread – precisely at the connection to distributors, thermostatic valves, ball valves, and circulation pumps, they form a critical point of the entire system. An incorrect combination of pipe diameter and fitting thread is one of the most common causes of leaks.

Steel pipe diameters and corresponding fitting sizes

The first thing you need to know before ordering any fitting is the actual outer diameter of the pipe. This is where the first misunderstandings begin – when the customer says "fifteen", they usually mean a pipe with an outer diameter of 15 mm, but older installers might understand the nominal bore DN 15, which is a pipe with an outer diameter of 21.3 mm. These two pipes are completely different and a fitting from one will not fit on the other.

For press systems on steel pipes, the following are the most common dimensions (outer diameter of the pipe × wall thickness):

Outer diameter (mm)	Common wall thickness (mm)	Nominal bore DN	Typical use in heating
15 mm	1.2 – 1.5 mm	DN 10 / DN 12	Radiators connections, control valves
18 mm	1.2 – 1.5 mm	DN 15	Radiators distribution, larger connections
22 mm	1.5 – 2.0 mm	DN 20	Horizontal distribution, distributors
28 mm	1.5 – 2.0 mm	DN 25	Main branches, boiler room distribution
35 mm	1.5 – 2.5 mm	DN 32	Machine rooms, larger apartment buildings
42 mm	1.5 – 2.5 mm	DN 40	Boiler rooms, industrial distribution
54 mm	2.0 – 3.0 mm	DN 50	Primary circuits, CZT

Learn more about how to correctly determine the diameter for your system in the topic Steel pipe and fitting diameters: which size do I need for my system in our Knowledge Center.

Types of press fitting seals and their selection for heating

One of the most important, yet often underestimated aspects when choosing press fittings is the type of seal. Not every seal is suitable for every medium and every temperature range. In practice, we encounter two basic types:

EPDM seal (black color)

EPDM (ethylene-propylene-diene rubber) is a standard seal for heating systems with water. It can withstand temperatures from -20 °C to +120 °C, covering typical low- and medium-temperature systems. Not suitable for oily media and fuels – EPDM swells and loses its sealing properties when in contact with mineral oils. For heating with water, EPDM is the first and correct choice.

FKM / Viton seal (yellow or green color)

FKM (fluororubber) is a seal for more aggressive media – gases, oils, chemically treated water. It can withstand higher temperatures (up to 150 °C and more). Not necessary for standard heating, but can be beneficial in combined systems (e.g., biomass boiler with high-temperature peak, or solar circuit). FKM seals are more expensive.

Practical tip from practice: If you are installing a standard central heating system with a gas boiler and water temperature up to 90 °C, press fittings with EPDM seals are sufficient. There is no need to push the customer into more expensive FKM variants. However, if you are dealing with a system with a heat pump and corrosion inhibitors or a solar circuit with glycol, read the technical data sheet of the seal first – not every glycol solution is compatible with EPDM.

Selection of press fittings according to pressure and temperature parameters of the system

Press fittings for steel pipe in heating must meet the minimum pressure and temperature of your system. Typical values for single-family homes and smaller apartment buildings:

Operating pressure: 1.5 – 3 bar (maximum pressure during expansion 3 – 4 bar)
Operating temperature: 60 – 90 °C (with condensing boilers also 45 – 55 °C)
Test pressure: usually 1.5× operating pressure, i.e., 4.5 – 6 bar

High-quality steel press fittings from certified manufacturers are typically dimensioned for PN 16 (16 bar operating pressure) at 20 °C, while at 120 °C the maximum operating pressure drops to PN 10 or less (depending on the manufacturer). For standard heating, this parameter is usually comfortably met.

For larger projects – industrial halls, boiler rooms for apartment buildings, district heating – it is essential to check the technical data sheet of the specific manufacturer of the press fittings and confirm that the product meets the required PN at the operating temperature. Neglecting this step can have fatal consequences.

Connecting fittings with internal thread: a critical point in the system

From my experience, connecting fittings are precisely where the most mistakes are made. The problem is that here two different dimensioning logics meet: the outer diameter of the pipe (in mm) and the thread size of the valve (in inches, according to the BSP/Whitworth standard). These two numbers do not have a direct mathematical relationship – you need to know them or have a table at hand.

The most common combinations you encounter on jobs in Slovakia:

15 mm pipe + Rp 1/2" – typically a radiator valve connection, thermostatic head, small ball valve. This is by far the most common combination in radiator circuits. Take a look at the connecting fitting 15mm-Rp1/2" – a standard fitting for these purposes.
15 mm pipe + Rp 3/4" – less common, used for connecting to valves with a larger flow rate while keeping a thin pipe. For example, some special valves or distributors with 3/4" input. Available as connecting fitting 15mm-Rp3/4".
18 mm pipe + Rp 1/2" – feeding from an 18mm branch to a 1/2" valve; common when connecting a horizontal distribution to a radiator valve without pipe reduction. See connecting fitting 18mm-Rp1/2".
18 mm pipe + Rp 3/4" – standard connection of an 18mm pipe to a 3/4" threaded valve: larger flow ball valves, three-way valves, pump inlets. Available as connecting fitting 18mm-Rp3/4".
22 mm pipe + R 1" – connecting the main 22mm branch to 1" valves; boiler connections, circulation pumps, collectors. This is connecting fitting 22mm-R1".

If you are unsure which combination is correct for your specific case, I recommend reading the topic How to choose the correct combination of pipe diameter and fitting thread (e.g. 15mm-Rp1/2" vs. 18mm-Rp3/4") in the Knowledge Center – there you will find a detailed table with all combinations and practical examples.

Compatibility of systems from different manufacturers: what to pay attention to

This is a topic that is not discussed loudly enough. There are several press fitting systems for steel pipe on the market – Viega Megapress, Victaulic, Geberit Mapress, Flamco and other manufacturers. Each of these systems has its defined geometry of the crimping profile, groove shape and sealing geometry. Even if the fitting appears to fit on the pipe, crimping with a tool from another manufacturer can lead to incorrect compression of the sealing ring and hidden leakage, which will only become apparent after some time.

Therefore, the rule is strict: the fitting, crimping tool and head must be from an approved combination according to the manufacturer's instructions. Some manufacturers must be mutually compatible (e.g. some profiled press systems are compatible with Ridgid or Klauke tools), but this must always be verified in the technical documentation, not assumed.

From the perspective of the designer or installer, it is most sensible to stick to one system for the entire project and not switch to another manufacturer unnecessarily. If this is unavoidable, it is necessary to use a transition fitting or threaded connection as an intermediate link.

Conditions for correct crimping: tools, calibration, procedure

Correct selection of the fitting is only the first step. Equally important is correct crimping. The most common errors when installing press fittings on steel pipe that I have seen on projects:

Insufficient insertion of the pipe – the pipe is not inserted all the way to the stop in the fitting; after crimping, part of the sealing ring remains uncontacted with the surface of the pipe. The fitting holds, but during pressure testing or after some time, it will start to leak.
Damaged or dirty pipe – corrosion, scratches, irregular cut; the sealing ring cannot adapt hermetically to such surfaces. Always cut the pipe with a straight cut, deburr and degrease it.
Incorrect crimping head – using a head of a different size or profile (TH, M, U...) than required by the fitting.
Uncalibrated crimper – hydraulic and electric crimpers require regular checks of the crimping pressure. If the crimper under-crimps, the joint will not be hermetic; if it over-crimps, it can damage the fitting or the pipe.
Crimping on a wet pipe – the pipe is still full of water before crimping; this in itself is not an error, but during inspection it is necessary to distinguish condensation from real leakage.

A detailed step-by-step installation procedure can be found in the topic How to correctly install a press fitting on a steel pipe without leakage in the Knowledge Center.

Deciding between press fittings and threaded fittings

The question of when to reach for a press fitting and when for a threaded solution has no one-size-fits-all answer – it depends on several factors. Here is a practical overview:

Parameter	Press fittings	Threaded fittings
Installation speed	Very fast, seconds per joint	Slower, threading required
Initial tool costs	High (crimper)	Lower (threading tool)
Fitting costs	Higher	Lower
Repairability	Special tools required	Easy disassembly with a wrench
Suitability for SDK and precast walls	Excellent	Possible, but risk of protrusion
Risk of human error	Minimal (visual inspection)	Higher (undertightened thread)

A detailed comparison of both technologies with real-life scenarios can be found in the topic Press fittings vs. threaded fittings: when to use which solution.

Corrosion of steel fittings: surface treatments and preventive measures

Steel press fittings are more susceptible to corrosion than their copper or stainless steel alternatives. On the market, we encounter several types of surface treatments:

Hot-dip galvanized (ZN) – a layer of zinc applied by dipping into molten metal; thicker and more durable than electrolytic galvanizing, suitable for humid environments and for fittings in contact with soil or concrete.
Electrolytic galvanized – a thinner layer, less resistant to mechanical damage; suitable for interior installations without aggressive conditions.
Epoxy or polyester coating – an organic protective layer; good resistance to moisture and chemicals, but sensitive to mechanical damage.
Stainless steel (AISI 316 or 304) – the most durable solution, but significantly more expensive; recommended for solar circuits with glycol, boilers with high humidity, and the food industry.

For standard heating in apartment and single-family homes, hot-dip galvanized or epoxy coating is sufficient. If you are installing in the floor (e.g., piping for underfloor heating) or in wet rooms, always prefer fittings with a thicker protective layer or use stainless steel variants. More about corrosion of steel pipe and fittings can be found in the topic Corrosion of steel pipe and fittings: causes, prevention and solutions.

Pressure tests after installation: what you need to know

Every press fitting system must be subjected to a pressure test after installation. Regardless of how high-quality fittings and tools you have used, a pressure test is the only objective verification of the joint quality before backfilling, embedding, or covering with a drywall construction.

Procedure for pressure testing steel press pipe in heating:

Fill the system with water and completely purge the air – air bubbles distort the measurement results.
Increase the pressure to the test pressure (usually 1.5 times the maximum working pressure, minimum 4 bar for standard heating).
Leave the system under pressure for at least 30 minutes without any pressure drop; ideally for a full hour or longer for larger systems.
Check the pressure gauges – the pressure must not drop by more than the allowed tolerance for material expansion.
Visually inspect each joint – look for drops, wet areas, or leakage.

Important: Unpressed press fittings (those where you forgot to crimp or where the crimper did not complete the full cycle) will immediately show up during a pressure test. This is a huge advantage of the system compared to threaded fittings, where a leak can slowly develop and remain hidden behind insulation for a long time.

Practical scenarios from practice: when to use which fitting

Scenario 1: Reconstruction of an apartment core, job in a panel building

The old piping is made of welded steel pipes with threaded fittings, the customer wants to keep the steel (condition set by the building manager). New piping will be run in a recessed wall behind the drywall. In such a case, a press system is ideal – quick assembly, no welding in the panel, possibility to prefabricate parts outside the space. For radiator connections I choose 15mm pipe with a fitting 15mm-Rp1/2" for the valve input. Horizontal piping is run in 18mm, ending towards the riser via a thread/press transition.

Scenario 2: Boiler room for an apartment building, primary circuit

This involves large diameters – 42 mm, 54 mm. Working pressure 4 bar, temperature 80/60 °C. A press system is suitable here, but it is necessary to use press fittings with certification for the given PN and temperature class. Valves are mostly 1" or 1 1/4" threads – connecting adapter 22mm-R1" (available as 22mm-R1") is used to transition from 22mm pipe to 1" valves in the manifold assembly. Larger diameters are solved directly via transitions or flanged joints.

Scenario 3: Single-family house, floor heating + radiators

This situation is more complex: floor heating runs at low temperature (35–45 °C) through a manifold, radiators are supplied directly from the boiler at 70–75 °C. The manifold has inputs in 3/4" thread – connecting adapter 18mm-Rp3/4" is exactly what is needed for connecting an 18mm branch. For radiator supply from horizontal branches in 15mm I choose standard elbows and couplings, at the end of the connection a 15mm-Rp1/2" adapter.

Most common mistakes when choosing press fittings and how to avoid them

Based on experience from job practice, I have compiled a list of the most common mistakes I see in installed systems:

Mixing up the outer diameter with the nominal DN – always measure the actual outer diameter with a caliper, do not rely on assumptions.
Mixing systems from different manufacturers without verifying compatibility – never crimp a fitting from one manufacturer with a tool from another without written confirmation of compatibility.
Using EPDM sealing for glycol systems – for circuits with concentrated glycol always check what type of sealing the manufacturer recommends.
Not maintaining the minimum insertion depth of the pipe into the fitting – each manufacturer specifies a minimum insertion depth for each diameter; this dimension must be followed.
Installation without pressure testing before covering – always, without exception, pressure test before backfilling or covering the installation.
Incorrect combination of pipe diameter and fitting thread – e.g. installing a 15mm-Rp3/4" adapter where the valve has a 1/2" input; the connection may fit, but it will not seal properly or may not fit at all.

If you are troubled by existing problems, read the topic Common faults in press fitting joints on steel pipe and how to fix them – you will find diagnostics and solutions for common faults there.

Long-term operation and maintenance of press fittings in steel heating

Press fitting joints on steel pipe are designed to be permanent – they are not intended for repeated disassembly and reassembly. Each crimp is one-time and final. This is their strength, but also a limitation. When planning the installation, always think about future service points: every section where disassembly may be needed (e.g. for replacing a filter, pump, or valve) must be terminated with a threaded joint or adapter, not directly with a crimped coupling.

A regular inspection of steel pipe with press fittings should include: visual inspection of fittings for signs of corrosion or moisture, checking the position of the identification band (if the manufacturer uses it to confirm the crimped state), pressure measurement in the system and comparison with the reference value. A more detailed procedure can be found in the topic Maintenance and inspection of steel pipe with press fittings in heating.

Frequently asked questions (FAQ)

Can I use press fittings for copper pipe on steel pipe as well?

No. Press fittings are specifically designed for a given pipe material. Press fittings for copper pipe have a different internal diameter, different groove geometry, and different sealing than fittings for steel pipe. Never mix these systems – the fitting may fit on the pipe, but after crimping the joint will be leaky or mechanically unstable. If you need to connect copper and steel, use a special transition fitting designed for this purpose, or a threaded adapter.

How do I know if a fitting is correctly crimped?

Most modern press fittings have a visual indicator – a colored identification band or a viewing window through which the pipe insertion can be seen. After crimping, the groove of the fitting must be uniformly deformed around the entire circumference (in the shape of a characteristic hexagonal or round groove according to the profile). An uncrimped fitting can also be recognized by hand – when rotated, it freely turns on the pipe. The most reliable check is a pressure test immediately after installation.

What is the difference between Rp and R thread markings on fittings?

Rp denotes a cylindrical internal (female) thread according to ISO 7-1 (British BSP). R denotes a conical external (male) thread. For press fittings with internal thread on the connecting adapters, Rp is used – that is, the fitting has a cylindrical internal thread and the valve you screw it onto has R (conical) or G (cylindrical) external thread. During installation, it is always necessary to use a thread sealing (hemp rope with paste, teflon tape or liquid sealant) – the cylindrical Rp thread does not seal on its own, a sealing element is required. More about the correct connection of valves and fittings can be found in the topic Connecting adapters with internal thread: how to correctly connect pipe with a valve.

Can I use press fittings on pipe where I will change the medium in the future (e.g. transition from water to glycol)?

Yes, but with a condition: when changing the medium, always check whether the sealing of the crimped fittings is compatible with the new medium. EPDM seals are excellent for water, but some glycol mixtures can cause their degradation – it depends on the concentration and type of inhibitor. Before switching to a glycol solution, consult the technical sheet of the sealing with the fitting manufacturer and the technical sheet of the inhibitor with its supplier. In case of doubt, it is safer to replace the fittings with FKM sealing before filling the system with glycol.

How long do press fittings on steel pipe last in heating systems?

With proper installation, compatible sealing, and water adjusted according to standards (pH 7.5–9.0, low oxygen content, appropriate hardness), the lifespan of press fitting connections on steel pipe is comparable to the lifespan of the pipe itself – practically for the entire lifetime of the building, i.e., 30–50 years or more. A critical factor is not the connection itself, but the corrosion of the steel pipe due to aggressive water, oxygen, or stray currents. The EPDM sealing has a standard lifespan of 50 years under proper operating conditions.

Is any special project approval required for press fittings on steel pipe?

For standard single-family homes and apartment buildings, no separate approval is required for press fittings – they fall into the category of standard plumbing accessories. However, installation must be carried out by a person with the appropriate professional qualifications according to current Slovak legislation (Decree No. 508/2009 Coll. for systems where required). A pressure test and inspection of the installation are mandatory for systems exceeding the specified pressure limit. For industrial and municipal systems, stricter rules apply – always consult with the designer and inspection technician.

Conclusion: Choosing the right press fittings pays off

Selecting the right press fittings for steel pipe in heating systems is not a matter of chance or just one number on the packaging. You need to know the outer diameter of the pipe, the type of medium and its temperature, the required combination of pipe diameter and valve thread, the type of sealing, the surface properties of the fitting, and system compatibility with your crimping tools. Each of these parameters determines whether the installation will last for decades without problems, or whether you will get a call from the customer in a year with a flooded boiler room.

If you are unsure about any step, it is always better to verify the information rather than improvise on the job. The knowledge center atria.sk is exactly for such situations – you will find answers to specific technical questions, dimension tables, installation procedures, and fault diagnosis here. And if you need specific products, the category steel pipe and press fittings contains certified fittings for professional heating systems.

Do you have a question about this topic?

Having trouble deciding or dealing with a specific situation in your home? Write to us – we are happy to help.