Frequently asked questions about expansion joints in heating systems

We are still preparing products.

But you can look at other categories.

Common Questions About Compression Unions in Heating Systems – Expert Answers from Practice

Compression unions may seem like simple installation components, but they are the source of an incredible number of questions in practice. Customers, and sometimes even experienced installers, deal with issues such as selecting the correct size, sealing, compatibility, or unexpected leaks after installation. In this article, we have gathered all the essential questions that actually repeat in practice – and we answer them with technical depth, not just superficially.

If you are looking for a specific product or solution, you will also find direct links to proven elements from the Hepworth range that are commonly used in Slovakia. If you want to approach the topic more systematically, our Knowledge Centre also includes a separate article How to Choose the Right Compression Union for Heating and also The Difference Between Compression Unions and Straight Adapters.

What is a compression union and where is it used in heating systems?

A compression union (in English compression fitting or union) is a fitting that connects two pipes or connects a pipe to a valve without the need for soldering, crimping or welding. The entire connection works on a mechanical principle: when tightening the nut, the sealing (usually a brass or copper olive) is compressed around the pipe and creates a watertight connection. Since no fire or special tools are needed during installation, these unions are popular not only among professional installers, but also among skilled DIY enthusiasts.

In heating systems, compression unions appear in many places. Typically, they are used to connect radiators (for example, through corner or straight valves), boiler, expansion tank, pump, thermostatic valves and other valves. They are also irreplaceable where you need a connection that can be disassembled later – which is simply not possible with a soldered connection without cutting.

For copper piping in heating systems, the most commonly used sizes are 15 mm and 22 mm on the pipe side, while the threaded part is usually 1/2" or 3/4" (BSP – British Standard Pipe). Therefore, in practice, we often encounter combinations such as compression union 1/2" x 15 (the most common size for radiator connections) or compression union 3/4" x 22 for larger pipe runs to the boiler or collector.

What is the difference between a compression union and a straight adapter?

This is one of the most common confusions. Both fittings look similar, have a threaded part and can be slipped onto the pipe, but they function a bit differently. A compression union has two nuts – one on each side – and is designed to be disassembled. Its center is a passage with a sealing, so in case of failure or configuration change, it is enough to unscrew the nuts.

A straight adapter is simpler in terms of mechanics: one side is a rigid threaded neck (internal or external thread) and the other side is a compression (screw) nut on the pipe. A straight adapter is therefore used to connect between a pipe and a device with a fixed thread – for example, to connect a pump, pressure gauge or drain valve. A typical example is straight adapter 1/2" x 15 with internal thread, which has a compression fitting for 15 mm copper pipe on one side and an internal 1/2" BSP thread on the other side for attaching a valve.

A more detailed analysis of this difference can be found in the article Difference Between Compression Union and Straight Adapter in the Knowledge Centre.

How do I determine the correct size of the compression fitting I need?

This is the most common source of errors in practice. Customers sometimes confuse the outer diameter of the pipe with its nominal diameter or thread size. For compression fittings on copper pipe, a simple rule applies: the number in millimeters indicates the outer diameter of the pipe. Therefore, for standard copper pipe with an outer diameter of 15 mm, you need a fitting marked "x 15", and for a pipe with an outer diameter of 22 mm, you are looking for "x 22".

The threaded part is given in inches (BSP) and refers to the standardized thread diameter: 1/2" has an outer thread diameter of approximately 20.9 mm and 3/4" is approximately 26.4 mm. These values are standardized, so a 1/2" from one manufacturer can be screwed into a 1/2" from another manufacturer. In practice, this means that the compression fitting 3/4" x 15 has a compression fitting for 15 mm pipe, but a 3/4" threaded part – which is, for example, a typical size for connecting some circulation pumps or larger valves.

How to measure the pipe diameter correctly? The most reliable method is using a sliding caliper (vernier caliper) placed on the outside. If you don't have a caliper, you can use a piece of paper: wrap it tightly around the pipe, mark the point where the paper overlaps, measure the length of the strip and divide it by the number π (3.14159) – you will get the outer diameter. A more detailed guide on measuring is available in the article Compression fitting dimensions: how to correctly measure the pipe diameter.

Can I use Hepworth compression fittings on materials other than copper?

Hepworth compression fittings are primarily designed for copper pipe, where the olive mechanically seats on the copper surface and creates a permanent seal. They work equally well on chrome-plated copper pipes. Problems arise with other materials.

On aluminum or plastic pipes (PEX, PERT, polypropylene), these fittings do not fit directly – you need either special fittings designed for the specific material, or insert a reinforcement sleeve (insert). Without the correct insert, the olive may cut unevenly into the softer material, and the joint may not seal, or the pipe may deform over time. More about this issue can be found in the article Compatibility of Hepworth fittings with other pipe systems and materials.

On steel, Hepworth brass fittings are used only in combination with rigid steel pipes and special steel olives. A standard brass olive on a steel pipe may be subject to galvanic corrosion over time, so it is necessary to consult the manufacturer or choose suitable transition fittings for this combination.

Why is my compression fitting leaking right after installation – where did I go wrong?

This is a situation that almost every plumber has experienced at least once. There may be several causes, each with its own characteristic symptoms:

Loose nut: The most common cause. The fitting seemed to seal when tightened by hand, but under pressure (0.5–2 bar in heating systems), it starts to leak. Solution: tighten it by an additional 1–1.5 turns with a wrench.
Damaged or incorrectly installed olive: If the olive is not aligned with the pipe or is deformed due to repeated disassembly, the seal will not be even. The olive must be replaced.
The pipe is not cut straight: A slanted cut causes the olive to not seat evenly around the entire circumference. The pipe must be cut precisely at 90° and free of burrs – ideally with a pipe cutter, not a file.
Contaminated surface: Dust, scratches from scraping, or an oxide layer on the copper pipe surface can prevent the olive from sealing properly. It is recommended to lightly sand the pipe surface with fine sandpaper before installation.
Excessive PTFE tape on the compression thread: If you applied PTFE tape on the compression thread (the olive nut thread), you did nothing good. PTFE tape should be applied only to BSP threads, not on compression joints.

A detailed diagnostic procedure is available in the article Why is my compression fitting leaking and how to fix it.

PTFE tape or hemp fiber – what to use for sealing threads?

It is important to distinguish which type of thread you are dealing with. For BSP threads (i.e., the inch-based part where you screw in the valve), PTFE tape or hemp fiber with sealing compound is used. Both methods work, but each has its own specifics:

PTFE (Teflon) tape is clean, non-corrosive, suitable for temperatures up to 260 °C, and chemically resistant. It is wound in the direction of the thread (to prevent it from unraveling during tightening), usually 3–5 layers. In heating systems, PTFE tape is standard because it is easily available, inexpensive, and reliably seals when applied correctly.

Hemp fiber with compound (e.g., Loctite 55 or Hemp + Fermit) is an older, yet proven method. The fiber is wound into the thread and the compound fills and impregnates it. An advantage is greater volume compensation – hemp slightly swells when in contact with water and sometimes seals uneven threads better. A disadvantage is the longer preparation time and the need to wait for the compound to harden.

On compression threads (i.e., the part where the olive seats on the pipe), neither of these materials is used for sealing. The sealing is ensured exclusively by the olive. If someone winds PTFE tape on the compression thread as well, it may prevent the nut from rotating sufficiently and the olive may not deform properly. More about this topic can be found in the article How to seal compression fittings in heating systems: PTFE vs. hemp fiber.

Can I disassemble and reuse a compression fitting?

Theoretically yes, but with an important condition: during the first tightening, the olive is permanently deformed and seats on the pipe. If you disassemble the fitting and want to reuse it at the same location on the pipe, the olive will seal again only if the pipe is not moved. Even a millimeter shift in the pipe will cause the olive to seat differently, which may result in a leak.

In practice, it looks like this: if you disassemble the coupling for service (e.g., to replace a radiator valve) and the pipe remains in the same place, you can retighten the joint and it will usually seal. However, if you move or shorten the pipe, it is better to replace the olive. Replacement olives are cheap and usually sold in packs – a good investment in certainty.

An important practical note: when retightening the joint, it is recommended to visually inspect the condition of the olive. If it is unevenly deformed, cracked, or corroded, it needs to be replaced. Checking the couplings during the annual heating service is a standard part of the inspection – more about this in the article Checking and retightening compression joints during annual heating service.

How does the correct installation of a compression coupling proceed step by step?

For those doing it for the first time or wanting to verify the procedure, here is a concise technical sequence:

Step 1 – Pipe preparation: Cut the pipe with a pipe cutter (not a file!). The cut must be perfectly perpendicular. Remove burrs from the cut end by spreading or using fine sandpaper. The outer surface of the pipe for about 2 cm from the end should be clean and free of scratches.
Step 2 – Fitting the nut and olive: First, slide the nut over the pipe (thread facing toward the olive), then the olive. The order is important – if you reverse it, you will not be able to assemble the joint.
Step 3 – Inserting into the coupling body: Insert the pipe fully into the coupling body or fitting until it reaches the stop. At this point, the cut end of the pipe should be resting against the internal stop of the fitting.
Step 4 – Manual tightening: Tighten the nut by hand as much as you can. The olive should now be in a preliminary position.
Step 5 – Tightening with a wrench: Tighten the nut with a wrench by an additional 1.5 to 2 turns beyond the manual tightening point. Hold the coupling body/fitting with a second wrench to prevent it from rotating. Do not over-tighten – excessive force can deform the olive so much that it loses its elasticity and cracks.
Step 6 – Pressure test: Fill the system and check the pressure. Monitor all new joints for at least 15 minutes.

The full procedure with photos and practical tips can be found in the article Installing a compression coupling on copper pipe step by step.

What are the maximum operating parameters for these couplings?

Compression couplings Hepworth are made of brass (or nickel-plated brass), while the olives are made of copper or brass. These materials have the following typical limiting parameters for heating system installations:

Maximum operating pressure: 28 bar (at room temperature). For heating systems with a normal operating pressure of 1.5–2.5 bar, this value provides a significant safety margin.
Maximum operating temperature: 110 °C (safe for systems with 90/70 °C parameters). At temperatures above 110 °C, it is necessary to use special industrial fittings.
Minimum temperature: -20 °C (relevant for installations in unheated areas during winter).
Medium: water, air, gases (depending on the product certification – always verify certification for gas media when used for gas).

These couplings are fully suitable for low-temperature floor heating (40/30 °C), radiator heating (70/50 °C), and condensing boilers (60/40 °C) in all cases. It is important to follow the correct installation procedure, as even a coupling with excellent parameters will not seal properly if it is installed incorrectly.

When is it better to choose a direct transition with an internal thread instead of a compression coupling?

This is a question that often determines the correct design of the piping layout in practice. A direct transition with an internal thread – for example, direct transition 3/4" x 22 with internal thread – is the correct choice when you have a pipe on one side and a fitting or device with an external BSP thread on the other side. Typical practical examples include:

Connecting a pressure gauge (usually with a 1/2" external BSP thread) to a 15 mm pipe
Connecting a drain valve to the piping system
Connecting a boiler (with an external thread) to a copper pipe
Installing an automatic air vent in the piping system

A compression coupling, on the other hand, is used to connect two pipe sections to each other – for example, when extending an existing piping system, repairing a damaged section, or connecting in series with a removable joint. In short: if you have pipe on both sides, use a coupling; if you have pipe on one side and a threaded fitting on the other, use a direct transition.

What are the most common mistakes when selecting the size of a coupling?

From practical experience, we know that customers most often make these mistakes:

1. Confusing the external and internal diameter of the pipe: A copper pipe DN 15 (nominal diameter 15 mm) has an exact external diameter of 15 mm, but an internal diameter of only about 13 mm. The coupling is fitted to the external diameter, so 15 mm is the correct choice. For DN 22, the external diameter is 22 mm – this is straightforward, as the nominal diameter equals the external diameter. However, be careful: this rule does not apply to steel pipes, where the diameter refers to the internal (bore) diameter!

2. Mixing imperial and metric dimensions: The 1/2" threaded part is not 12.7 mm (which would be half an inch) – it is 20.9 mm in the outer diameter of the thread. BSP is a British standard system and the values must be looked up in a table, not directly converted from inches.

3. Forgetting the type of thread: Internal and external threads are opposite. If you have a fitting with an external thread (male), you need a fitting with an internal thread (female). If you have a fitting with an internal thread (hole), you need an external thread. Always check in the catalog which is internal and which is external.

If you are unsure, the article Internal vs. External Thread: What type of transition do you need? provides a visual comparison with specific examples from heating systems.

Can I use these fittings for drinking water supply or only for heating?

Brass fittings from Hepworth are generally approved for drinking water supply, but it is necessary to verify the certification of the specific product (e.g. WRAS in the UK or national certifications in Slovakia). For heating, where the water is not potable and circulates continuously through the same pipes in a closed loop, this certification is not so critical. In practice, these fittings are used without problems in both applications and long-term market experience confirms this. If you plan to install for drinking water supply, check the technical documentation of the product for specific certification.

Frequently asked questions (FAQ)

Do I need to use sealing paste or is the olive alone sufficient during installation?

For compression fittings (where the olive sits on the pipe), no paste or teflon is needed – the olive seals mechanically by itself. Sealing paste is applied exclusively to BSP threaded connections (imperial threads on valves). Applying paste to a compression fitting can actually be harmful, as it changes the friction during tightening and you may over-tighten past the correct position of the olive.

How many turns are sufficient for sealing during installation with a wrench?

The rule of thumb is: tighten by hand until it stops, then add 1.5 to 2 full turns with a wrench. For smaller sizes (15 mm), sometimes 1.5 turns are enough, for larger sizes (22 mm), better 2. Never exceed this range by force – the olive can be deformed too much or the body of the fitting can crack in the case of cheaper copies. If the connection still leaks after 2 turns, the problem is elsewhere (cut, olive, dirt) – tightening further will not fix it.

What to do if the olive is stuck on the pipe and won't move down during disassembly?

An olive that has been sealed is firmly embedded in the pipe – this is normal and desired. If you need to remove the olive (e.g. due to moving the fitting), the simplest solution is to cut the pipe behind the olive and start with a new end. Trying to pull the olive off by force with pliers or a screwdriver usually damages the pipe. In extreme cases, there are special olive extractors, but for a standard heating installation, it is more economical to cut the pipe short.

Can I connect a copper pipe to a plastic one using these fittings?

Not directly. Compression olives are designed for solid metal pipes, where they can be properly compressed. Plastic pipes are too soft – the olive would dig in unevenly or the pipe could deform during tightening. For a transition between copper and plastic (PEX, PERT), special transition fittings are used, where the plastic side has a crimping or screw mechanism for plastic and the metal side has a BSP thread. An alternative is to use an insert (reinforcement sleeve) on the plastic side and then use a compression fitting, but only if the fitting manufacturer explicitly allows it.

How do I know that the fitting is from a quality manufacturer and not a cheap copy?

Hepworth is an established British brand with a long history and their products must meet the BS EN 1254 standard. On original fittings, you will find a clear stamp of the size, material (e.g. "CW617N" for brass) and possibly a batch code. A copy usually has a less precise casting surface, inaccurate threads and an olive made from a weaker alloy. A practical test: when tightening an original olive, you feel a uniform progressive resistance – a cheap olive may suddenly stop or skip. For heating systems, where a faulty connection can cause damage to your home, it always pays to invest in verified products.

How many years will a compression fitting last in a heating system without problems?

With proper installation and under normal operating conditions (temperature up to 90 °C, pressure up to 3 bar), a brass compression fitting should last 30–50 years without any intervention. In practice, the lifespan is shortened mainly by temperature shocks (frequent rapid switching between cold and hot water), aggressive water with low pH values, or insufficient sealing of the threaded part, where moisture and oxygen cause thread corrosion. Once a year during regular service, a visual inspection and possibly a gentle tightening is sufficient – not a re-tightening, just to check that the nut is not loose.

Conclusion: What to take away from this article

Compression fittings may seem like inconspicuous elements at first glance, but their correct selection, installation and maintenance determine whether your heating system will work trouble-free for decades or whether you will repeatedly return to the same spots with pliers and rags. The most important rules from practice are: measure the outer diameter of the pipe (not the inner, not the nominal), cut the pipe cleanly and perpendicularly, put the nut and olive on in the correct order, tighten with a wrench exactly 1.5–2 turns – no more, no less – and use teflon or hemp on the BSP thread, but nothing on the compression fitting.

If you still don't know which specific fitting is right for your case, start with the category compression fittings, where you will find an overview of the entire Hepworth range filtered by size. And if you need a direct connection to a valve with a thread, also check out direct transition 1/2" x 15 with internal thread or the larger variant direct transition 3/4" x 22 with internal thread, depending on the size of your distribution system.

Do you have a question about this topic?

Not sure or dealing with a specific situation in your home? Write to us – we are happy to help.