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How to choose the right compression fitting for heating

Compression fittings are one of those components in heating systems that are often overlooked during installation design – until the plumber starts physically assembling the piping and realizes that without them, not a single radiator, valve, or circulation pump can be connected. At first glance, they may seem like unassuming parts, but their correct selection determines whether the connection will be tight, strong, and long-term reliable, or whether you will be dealing with dampness under the boiler and lifting floorboards in a year.

In this article, we will take a comprehensive look at the topic: what a compression fitting actually is, what types exist, how to choose the diameter and thread, when to reach for a straight-through adapter instead of a classic fitting, what materials and pressure parameters to monitor, and how the entire selection looks in practice – from the boiler through the manifold to the last radiator. If you are interested in a deeper look at specific installation procedures or sealing compression fittings, you will find them in related articles in this Knowledge Center.

What is a compression fitting and where is it used

A compression fitting (also known as a compression coupling or nipple fitting) is a fitting that allows you to connect a heating pipe – most often copper, but also multi-layer aluminum-plastic – to a threaded outlet of a valve, boiler, pump, or other device. It consists of a body with a thread, a compression (olive) ring, and a nut that, when tightened, compresses the ring around the pipe and creates a mechanically tight and pressure-resistant connection.

In a standard heating system, you will encounter compression fittings almost everywhere: when connecting radiators (valve outlet), when connecting a floor heating manifold, when installing a circulation pump, expansion tank, safety valve, boiler, or gas burner. It is therefore a universal transition fitting between rigid pipe and threaded fittings of valves.

It is important to understand that a compression fitting is not the same as a crimp fitting or a solder coupling. Unlike crimp fittings, it does not require special crimping pliers, and unlike solder couplings, it does not require a flame. Compression fitting is sufficient to tighten with a wrench – which is why it is very popular during renovations, where you do not want to make a fire in a finished apartment, or when connecting devices that will later be disconnected for service.

Basic dimensions: how to navigate the numbers

Selecting the correct fitting starts with two parameters: pipe diameter and thread size. Both values must match exactly – not even by a millimeter.

Pipe diameter (tube)

Measuring copper (Cu) pipe is marked by the outer diameter in millimeters. The most common sizes in heating are:

15 mm – standard radiator branch, valve connection, small piping
18 mm – less common, some older piping or special valves
22 mm – main piping, boiler connection, manifold
28 mm – strong main branches, larger buildings
35 mm and more – industrial applications, large boiler rooms

In family homes and apartments, 90% of cases cover diameters of 15 mm and 22 mm. For these sizes, you will find in the Hepworth range, for example, a compression fitting 1/2" × 15 or a compression fitting 3/4" × 22.

Multi-layer (PEX/Al/PEX) pipe has a different labeling system – usually outer × inner diameter, e.g. 16×2 or 20×2. For such pipes, you need special adapters or compression fittings designed for the specific type of pipe. A standard Cu compression fitting will not fit a multi-layer pipe.

Thread size (inch system)

Threads on valves and boilers are mostly in the inch system BSP (British Standard Pipe), marked by the abbreviation G. Common sizes:

1/2" (G1/2) – radiator valve, smaller valves, safety valves
3/4" (G3/4) – boiler inlet/outlet, manifold, circulation pump
1" (G1) – larger pumps, collectors, TÚV tanks
1 1/4", 1 1/2", 2" – industrial and larger installations

Attention: the inch thread size does not directly correspond to the outer diameter of the pipe. For example, a 1/2" thread has an actual outer diameter of about 20.9 mm, although it is paired with a 15 mm pipe. This is a common source of confusion for people who are shopping for the first time.

Socket with soldering vs. straight transition: when to use which

This is a question that customers often struggle with. At first glance, both parts look similar – they have a thread on one side and a pipe connection on the other. The difference, however, is crucial and depends on what type of thread the valve you are connecting to has.

Socket with soldering has an external (male) thread on the threaded side. You screw it into the internal thread of the valve. Typical example: a radiator valve has an internal 1/2" thread, you screw in a socket with an external 1/2" thread and slide a 15 mm copper pipe onto the other side. This combination is by far the most common in practice.

Straight transition with internal thread works the opposite way – it has an internal (female) thread on the threaded side. You use it when the valve (or a piece of pipe) has an external thread. Typical scenario: a boiler outlet with an external 3/4" thread – you slide a straight transition 3/4" × 22 with internal thread onto it, and continue with a 22 mm copper pipe.

A more detailed analysis of this topic can be found in the article Difference between a socket with soldering and a straight transition and also in Internal vs. external thread: what type of transition do I need. Here we will limit ourselves to a practical rule: always check what type of thread (internal or external) the device connection you are connecting to has – and choose the opposite type of socket.

Material and pressure parameters – what to look for when buying

The Hepworth sockets you find in this category are made of brass (a copper-zinc alloy). Brass is practically standard in heating: it is corrosion-resistant when in contact with water, thermally stable, mechanically strong, and compatible with copper pipes without galvanic corrosion. Compared to ordinary steel, brass has a significantly longer lifespan in heating systems and does not require protection by galvanization.

The pressure parameters that a quality socket for heating should meet:

Maximum working pressure: at least 10 bar (standard heating systems operate at 1.5–3 bar, but a reserve is necessary)
Maximum medium temperature: at least 110 °C (heating systems typically operate at 70–90 °C, a boiler can briefly reach 95 °C)
Test pressure: usually 1.5 times the working pressure, i.e. at least 15 bar

Hepworth brass sockets meet these parameters with a reserve. Cheaper alternatives made of galvanized zinc or thin-walled steel can be problematic when exposed to hot water over a long period – they may crack, corrode or jam during future tightening.

How to correctly determine which socket you need – step-by-step procedure

The following procedure has proven effective in practice and significantly reduces the risk of ordering the wrong part:

Step 1 – Measure the outer diameter of the pipe. Use a caliper. Measure the outer diameter, not the inner one. If you don't have a caliper, you can use a tape measure or a piece of paper wrapped around the pipe and calculate the diameter from the circumference (d = circumference / π). A more detailed measuring procedure is described in the article Socket dimensions with soldering: how to correctly measure the pipe diameter.

Step 2 – Determine the type and size of the thread on the valve. Unscrew the valve cover or look directly at the outlet. If you see the thread surface from the outside (the thread pitch is visible on the sides), it is an external thread. If the thread is inside the hole, it is internal. The thread size in inches is usually stamped on the valve body, or you can measure it using a thread gauge.

Step 3 – Combine both dimensions and choose the correct type. Practical examples:

Radiator valve with internal 1/2" thread + Cu pipe 15 mm → socket with soldering 1/2" × 15
Boiler outlet with external 3/4" thread + Cu pipe 22 mm → straight transition 3/4" × 22 with internal thread
Distributor with internal 3/4" thread + Cu pipe 22 mm → socket with soldering 3/4" × 22
Distributor with internal 3/4" thread + Cu pipe 15 mm → socket with soldering 3/4" × 15 (reducing socket)

Reducing sockets: when the pipe and thread do not match in size

A special category consists of so-called reducing (transition) sockets, where the pipe diameter and thread size do not match the "standard" combination. For example, a socket 3/4" × 15 may seem counterintuitive at first glance – customers think that a 3/4" thread must be paired with a 22 mm pipe. That is not the case.

Reducing sockets appear in practice in several situations:

Connecting a thin branch to a thicker main line – the distributor has 3/4" outlets, but the pipes to the radiators are 15 mm. In this case, you need a 3/4" × 15 socket.
Connecting a lower-performance pump – the pump body has a 3/4" thread, but the supply pipe is only 15 mm.
Renovating an old piping system – the original system was built with different dimensions, and the new valves have a different thread.

Always read the full product designation carefully when placing an order – both numbers are equally important.

Sealing and tightening – the principles that matter

Even if you have selected the correct fitting, an improperly sealed or tightened one can leak. Sealing at fittings with compression connections occurs at two places:

Sealing between the olive and the pipe – the compression ring (olive) deforms around the tube when tightening the nut and creates a metal-to-metal seal. This location does not require any sealant or sealing tape – it works on its own. The condition is that the pipe is straight, clean, and not cut at an angle.
Thread sealing – the place where the body of the fitting is screwed into the valve or into it. This location is sealed using PTFE tape or hemp fiber with sealing paste. The choice between these two methods is explained in the article How to seal compression fittings on heating fittings: PTFE vs. hemp fiber.

Regarding tightening force: the nut of the fitting is tightened by hand and then an additional 1 to 1.5 turns with a wrench. Over-tightening is just as bad as under-tightening – when over-tightened, the olive deforms too much and the pipe can crack or the joint can become leaky due to thermal expansion. An under-tightened nut will leak from the beginning. More on this in the article Installation of compression fittings on copper heating pipes step by step.

Compatibility: Hepworth with other systems

One of the advantages of brass Hepworth fittings is their standardized BSP thread system, which means they are compatible with valves and fittings from other manufacturers, provided they follow the same standard. In practice, this means: a Hepworth fitting can be combined with a Danfoss valve, a Viessmann boiler, a Grundfos pump, or Rehau manifolds – provided both parts have matching thread sizes in the G (BSP) system.

Be careful with interchangeability with the American NPT thread system – it has a different pitch and thread angle and is not compatible with BSP, even with the same nominal size. This mistake occurs with some imported pumps or drain valves. A more detailed description of the issue can be found in the article Compatibility of Hepworth fittings with other systems and pipe materials.

Common mistakes when selecting and ordering fittings

Over the years, as plumbers and DIY homeowners work with heating systems, the same mistakes keep repeating. Here is a list of the most common ones – and how to avoid them:

Mixing up the external and internal diameter of the pipe

The customer measures the internal diameter of the pipe (e.g., 13 mm) and orders a "13 mm pipe" fitting – such a thing does not exist. Copper pipes are always labeled by their external diameter. Correct: 15 mm (external) for a standard heating pipe with a 1 mm wall thickness.

Mixing up the inch-based thread size with the pipe diameter

People think that a 1/2" fitting will fit a 1/2" pipe (approx. 12.7 mm). No – 1/2" refers only to the thread size, not the pipe diameter. A 1/2" × 15 fitting has a 15 mm pipe connection, even though the thread is 1/2".

Ordering a standard fitting instead of a straight transition

The customer needs to connect a pipe to an outlet of a boiler that has an external thread. They order a compression fitting – but it also has an external thread. Two external threads cannot be connected to each other. The correct choice is a straight transition with an internal thread.

Insufficient number of pieces

Each radiator has two branches (supply and return), and each branch usually requires at least one fitting. With 8 radiators and installation from scratch, you should plan for at least 16 pieces, plus extra for the boiler, pump, expansion tank, and possible errors. Always order 10–15% extra – returning the surplus is much easier than stopping the installation due to a missing piece.

Reusing a damaged or old olive

When removing an old fitting, the olive (compression ring) deforms. Some plumbers reuse it – which is a mistake, because once compressed, the olive may not seal properly during reinstallation. Always use a new fitting with a new olive.

Typical scenarios from practice

Scenario 1: Radiator replacement in a panel building. The old radiator is unscrewed, the valves are original and have an internal thread of 1/2". New Cu pipes are cut to size with a diameter of 15 mm. Solution: two couplings with compression 1/2" × 15 (one for the supply valve, one for the return valve).

Scenario 2: Installation of a new condensing boiler. The boiler has a heating pipe output with an external thread of 3/4", the return pipe input also has an external thread of 3/4". The main distribution runs in Cu pipes of 22 mm. Solution: two straight transitions 3/4" × 22 with internal thread – one for the output, one for the boiler input.

Scenario 3: Floor heating manifold. The manifold has outlets with an internal thread of 3/4", the radiators are connected via Cu pipes of 15 mm. Solution: one coupling with compression 3/4" × 15 for each circuit. With six circuits, this means 12 pieces (supply + return side).

Scenario 4: Installation of a circulation pump. The pump has an internal thread of 1" on both sides, the pipe is Cu 22 mm. In this case you need couplings 1" × 22 – a reducing combination, which you need to pay attention to when ordering, to avoid confusing it with 3/4" × 22.

Inspection after installation and service

After installation and filling the system with water, let the heating run for several hours at full operating temperature and pressure. Then visually inspect each connection – look for drops, moisture or a white coating (mineral deposits from a leaky spot).

Compression fittings may slightly loosen in the first few days of operation due to thermal expansion – therefore, after 2–3 weeks it is recommended to walk the entire distribution with a wrench and gently tighten. Detailed instructions can be found in the article Inspection and tightening of compression fittings during annual heating service.

If a fitting leaks despite correct selection and installation, the causes and solutions are described in the article Why a compression fitting is leaking and how to fix it. The most common causes are an incorrectly pushed-in olive, a cut pipe at an angle or insufficient thread sealing.

Most frequently asked questions (FAQ)

Can I use a compression fitting on plastic or multi-layer piping?

A classic compression fitting with an olive is mainly intended for copper piping. For multi-layer (PEX/Al/PEX) or purely plastic pipes, a special compression fitting with an internal insert is required, which prevents deformation of the soft pipe during tightening of the olive. Never use a standard Cu compression fitting on plastic or multi-layer piping – the connection may be leaky or the pipe may be damaged.

What kind of wrench do I need for installing a compression fitting?

The most suitable is an adjustable spanner (hose wrench) or a combination wrench with the corresponding jaw width. For a 1/2" × 15 fitting, it is usually a 22 mm wrench, for 3/4" × 22 a 27–32 mm wrench depending on the manufacturer. It is recommended to always tighten using one wrench for the nut and another to hold the body of the fitting, so as not to apply torque to the valve.

Can I disassemble and reassemble a compression fitting?

The body of the fitting and the nut can be, but the olive (compression ring) is permanently deformed after the first tightening and may not seal reliably after reassembly. Industry practice is to always replace the entire fitting with a new one during reassembly, or at least use a new olive (if available separately). This is especially true for pressure testing – if the piping has to be disassembled and reassembled, expect to replace the fittings.

How long do brass compression fittings last in a heating system?

Properly dimensioned and installed brass Hepworth compression fittings have a lifespan of several decades – in practice, it often happens that a fitting is replaced due to a piping renovation rather than due to material failure. A critical factor is the quality of the water in the system: aggressive soft water with low pH can attack the brass body over the long term. In the majority of standard installations with chemically treated boiler water, the lifespan is practically unlimited within the normal operating period of the building.

Is it necessary to add sealing also on the olive, not just on the thread?

No. The olive (compression ring) ensures sealing mechanically – metal to metal – and any sealing materials (Teflon, hemp fiber, paste) between the olive and the pipe are not appropriate. Sealing is applied exclusively to the threaded part of the fitting, where the body is screwed into the valve. Adding Teflon tape to the olive would actually worsen the sealing, as it would prevent proper metal contact.

What if I am unsure about the diameter of the existing piping?

The most reliable method is direct measurement with a caliper. If you cannot access the pipe without disassembly, you can estimate the diameter based on the age of the installation and visual comparison: in apartments built after 1990, the dominant diameter is 15 mm for radiator piping, 22 mm for the main piping. Older piping may be in steel threaded piping (Ø 1/2" or 3/4" external) – for these, different types of adapters are required. If you are unsure, it is always better to photograph the old fitting or a section of the pipe and consult rather than order blindly.

Conclusion: the right fitting is not a small detail

Selecting a compression fitting may seem like a routine task at first glance – but every experienced plumber knows that these seemingly trivial fittings are the cause of most minor leaks that trouble heating owners for years. Choosing the correct pipe size, the correct thread size and the correct type (coupling vs. straight transition) are three steps that require a moment of attention – but will save you hours of problem-solving later.

Brass compression fittings from the Hepworth range cover the common needs of heating installations in houses and apartments reliably and with a long lifespan. The full range, including reducing and standard combinations, can be found in the compression fittings category on atria.sk. If this topic raises further questions about installation, sealing or compatibility, visit other articles in our Knowledge Center, where each of these topics is discussed in depth.

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