Compatibility of Hepworth Fittings with Other Pipe Systems and Materials

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Compatibility of Hepworth Push-Fit Fittings with Other Systems and Pipe Materials

When dealing with heating system renovations or connecting a new heat source to an existing distribution system, you will almost always face the question: will this fitting work with the pipe I have available? Hepworth push-fit fittings are one of the most widely used products in this category on the Slovak market. However, their compatibility with various pipe materials, thread standards, and other systems is a topic that deserves a thorough and detailed answer. It is not just about dimensions – it is about pressure parameters, expansion, material compatibility, and long-term sealing reliability.

This article will examine the compatibility of Hepworth fittings systematically: from basic material combinations through thread standards to practical scenarios in renovations where multiple generations of pipe systems meet at once.

What is the Hepworth system and why compatibility matters

Hepworth is a British manufacturer with a century-old tradition, now integrated into the Aliaxis group. Their pipe fittings for heating – including a full range of push-fit fittings, straight transitions, and other fittings – are dimensioned according to European standards EN 1254 and respect the British standard BSP (British Standard Pipe), which is also the reason why they work so well on the Slovak and Czech markets, where most plumbing also uses BSP threads (G-threads according to ISO 228).

Hepworth fittings are mainly available in combinations where a compression or crimped part connects to a copper pipe and a threaded part connects to a valve, pump, valve body, or boiler. This dual function – mechanical connection to the pipe + threaded transition to the fitting – is the reason why when using them, compatibility issues arise from two directions: from the pipe side and from the thread side.

Copper: the primary and most compatible material

Hepworth push-fit fittings are primarily designed for copper pipe according to EN 1057. Copper pipes in heating systems are available in two basic diameter ranges – metric (e.g. 12×1, 15×1, 18×1, 22×1, 28×1.5 mm) and imperial (1/2", 3/4", 1"). Historically, metric dimensions have been used in Slovakia and the Czech Republic, with 15 mm and 22 mm being by far the most common.

Hepworth fittings for copper work on the principle of compression or crimped fittings. Compression types – such as the push-fit coupling 1/2" x 15 – compress a ferrule (ring) around the copper pipe when the nut is tightened, creating a mechanically stable and sealed connection. For copper, the ferrule slightly deforms and bites into the surface of the pipe when tightened, ensuring a seal without the need for soldering.

A crucial prerequisite for compatibility with copper: the outer diameter of the pipe must exactly match the nominal size of the fitting. A 15 mm OD pipe into a 15 mm fitting – no additional tolerance. If you were to insert a pipe with an OD of 14.8 mm (an older product or a different standard) into a compression fitting for 15 mm, the ferrule would not compress properly and the connection would be leaky or unstable. Therefore, measuring the diameter before purchase is absolutely essential – we also address this topic in the article Dimensions of Push-Fit Fittings: How to Measure the Pipe Diameter Correctly.

Steel pipe (black and galvanized steel)

The older residential buildings in Slovakia are full of welded steel distribution systems, either in black or galvanized finish. Hepworth fittings come into play here mainly through the threaded part – usually where an old steel distribution needs to be separated from a new copper or plastic branch.

Hepworth compression fittings are not directly intended for direct connection to a steel pipe, because the ferrule principle is not reliable with the hard and thick surface of a steel pipe. Therefore, when connecting to steel, we use Hepworth fittings via the threaded part – that is, the threaded side is screwed onto a steel valve or a welded threaded adapter, and the compression side continues with a copper or plastic pipe.

Critical point: threads on steel pipes in Slovakia are practically always BSP (Whitworth/G-thread, ISO 228). Hepworth fittings are standard in BSP, so the threaded connection works without adapters. However, be careful with old Soviet-era buildings – in some cases, Russian metric threads (MK – metric conical) appear, where G 1/2" and M16×1.5 look quite similar, but are not mutually compatible. Before tightening, measure the diameter and pitch of the thread with a caliper or thread gauge.

From the perspective of galvanic corrosion: copper and steel are sufficiently far apart in the electrochemical series that galvanic corrosion can occur when they are in contact in the presence of water. In a closed heating system with inhibitors, this risk is lower, but in practice, it is recommended to insert an intermediate element made of a neutral material (brass) or to use a dielectric fitting when connecting a copper pipe directly to steel. Hepworth threaded fittings have a brass body, which works as a suitable intermediate link in this regard.

Plastic pipe: PE-X, PP-R, PB and multilayer pipes

Modern heating systems increasingly combine different types of plastic piping. In this case, the situation with the Hepworth compression range becomes more complex and requires attention.

PE-X (cross-linked polyethylene)

PE-X pipe usually has the same outer diameter as copper (e.g. 16 mm, 20 mm), but the inner diameter is different and the wall is softer. Hepworth compression fittings for copper are not directly intended for PE-X without a special insert sleeve, because the softer wall of the pipe deforms unpredictably when the ferrule is compressed. Solutions include either special compression fittings for PE-X with a solid insert or crimping technology.

On the threaded side, however, there is no problem – direct transition 1/2" x 15 with internal thread or its equivalents are commonly used as transition elements between a PE-X system and a BSP thread of a valve.

PP-R (polypropylene)

PP-R is a welding system – pipes and fittings are joined using polyfusion (heat welding). Direct connection to Hepworth compression fittings is not possible because PP-R pipe is not compatible with the compression olive. For the transition between PP-R and heating fittings, PP-R fittings with internal or external brass threads are used – and the Hepworth coupling is normally tightened onto this thread (BSP).

Multilayer pipes

A very common type today in floor heating – a composite of PE-RT + aluminium + PE-RT or PE-X + Al + PE-X. The outer diameter is the same as with copper pipes (e.g. 16×2, 20×2), but for compression connection it is necessary to use fittings specially certified for multilayer, i.e. with an insert that prevents deformation of the soft plastic wall. Hepworth also has fittings for multilayer in its range, but the basic series intended for copper is not recommended for multilayer without verification from the manufacturer.

Thread standards and their compatibility: BSP, NPT and metric threads

This is one of the most common mistakes when selecting fittings – it can cause many sleepless nights on the construction site. Hepworth fittings use the BSP thread (British Standard Pipe, standard ISO 228 / EN ISO 228-1) as standard, i.e. the parallel G-thread. The pitch, profile angle (55°) and tolerances are precisely defined. In Slovakia and throughout the EU, BSP is the dominant standard for heating fittings.

Problems arise when encountering:

NPT (National Pipe Thread) – an American tapered thread, profile angle 60°. Visually similar to BSP, but they are not mutually compatible. The taper of NPT means that even if the thread is tightened several turns, the connection does not seal and there is a risk of the threaded fitting cracking under greater tightening. NPT appears in Slovakia in American or Asian HVAC equipment (some air conditioners, pumps, boilers from non-standard markets).
BSPT (British Standard Pipe Taper) – a British tapered thread, also different from the parallel BSP. Sometimes appears in British fittings. The tapered thread seals metal-to-metal by the cone itself, while the parallel BSP requires sealing tape or hemp.
Metric threads M – common angle 60°, pitch according to ISO. Occur in some automation components, actuators, temperature sensors. An M→G adapter can sometimes be used, but this solution must never be used under full system pressure without verifying the pressure parameters of the adapter.

How to distinguish BSP from NPT in practice? Buy a cheap thread gauge – a set of metal templates for both standards costs less than a tenth. Alternatively: BSP 1/2" has a pitch of 14 threads per inch (tpi), NPT 1/2" also has 14 tpi – but the tooth profile differs (55° vs. 60°). A more reliable verification: BSP has parallel thread sides, NPT has a slight taper (1° 47'). If you are unsure, bring a sample to the store.

Thread dimensions and nominal sizes: inches vs. millimeters

Hepworth compression fittings with soldering have a dual-dimensional marking, for example "1/2" × 15" or "3/4" × 22". The first number is the thread size (in inches BSP), the second is the outer diameter of the copper pipe in millimeters. This marking is not unnecessarily complicated – it is a logical consequence of the fact that the threaded side is marked in inch standards (fittings, valves, boilers...) and the pipe side in metric (copper EN 1057).

Common combinations in heating of family houses:

1/2" × 15 – very common, connection of heating units (radiators), valves, air vents, expansion tanks. See for example compression fitting with soldering 1/2" × 15.
3/4" × 15 – less common, appears in the connection of a manifold or collector, where the valve has a 3/4" thread and the pipe remains 15 mm. See compression fitting with soldering 3/4" × 15.
3/4" × 22 – typical for the main distribution, connection of pump, circuit distributor, boiler connection. Available as compression fitting with soldering 3/4" × 22.

Important practical note: the inch dimension of the thread (1/2", 3/4") does NOT coincide with the outer diameter of the thread in millimeters! BSP G1/2" has an outer thread diameter of approx. 20.9 mm, G3/4" has approx. 26.4 mm. This is a historical anomaly – the inch marking of threads comes from the era when the inner diameter of cast iron pipes was measured, not the outer diameter of the thread. This does not change technical compatibility, but when looking for a replacement part, it can confuse you if you do not know what you are looking for.

Hepworth and other brands of fittings: interchangeability

Alongside Hepworth, fittings from Conex Bänninger, Yorkshire (Aalberts), Giacomini, Viega and many Asian manufacturers appear on the Slovak market. The question of interchangeability is practically important – when you are buying additional parts for an unfinished job or during repairs.

The good news is that all branded compression fittings for copper in European dimensions (15 mm, 22 mm, etc.) according to EN 1254-2 are mutually interchangeable in terms of dimensions. The nut, olive and sealing principle are standardized, so a Hepworth nut can fit on a Conex body. In practice, however, I do not recommend mixing them in one joint – different manufacturers may have slightly different olive and sealing set tolerances, which can cause micro-leakage over long-term use.

For the threaded part it is clear: BSP thread is BSP thread. A Hepworth fitting with a G1/2" thread fits on any valve with a G1/2" thread regardless of the manufacturer – as long as the thread is made within the ISO 228 tolerance. If you encounter a problem with threading, look for the cause more in mechanical damage to the thread than in incompatibility of standards.

Pressure and temperature limits: when compatibility is not enough

Dimensional compatibility alone does not guarantee safe operation. Hepworth fittings are dimensioned for:

Maximum working pressure: usually 10 bar at 110 °C for brass compression fittings (specific values should always be verified in the manufacturer's technical sheet for the given product)
Maximum temperature for olive compression joints: 110–120 °C (depends on the material of the olive – commonly brass, sometimes with EPDM sealing)
For low-temperature floor heating (max. 45–55 °C), the conditions are naturally met with a significant reserve

Where a problem may arise: if you connect a Hepworth compression fitting to a copper pipe, but on the threaded side there is, for example, a new boiler with a maximum pressure of 3 bar – the limiting factor is not the fitting, but the boiler. Conversely, in industrial applications with high pressure and temperature (over 120 °C), compression fittings are not suitable and a soldered or crimped system should be used.

For standard heating of a family house (working pressure 1.5–2.5 bar, temperature 60–80 °C), Hepworth compression fittings operate well below their limits and their reliability is long-term when installed correctly.

Renovations: where generations of piping meet

The most interesting – and practically the most complex – situation arises during the renovation of an old house, where three generations of piping systems meet at once. A typical example from practice:

A house from 1975, original steel pipes DN20 and DN25, partially replaced in the 90s with copper 15 mm and 22 mm, a new condensing boiler from 2023 with 3/4" outputs. The customer wants to buy an additional radiator for the extension and connect it with a 15 mm copper pipe.

Practical solution: On the steel distribution, we find a suitable place, drill and weld or solder a branch with a G1/2" thread. On this thread we fit a straight transition 1/2" × 15 with internal thread, thus switching to a 15 mm copper pipe. From there we continue with the copper pipe all the way to the radiator, where we fit another fitting on the appropriate valve. The entire job is done without welding on the steel distribution, only with tightening the thread and compression nut.

Another common scenario: The customer replaces an old gas boiler with a heat pump. The boiler had a 1" flanged connection, the heat pump has a G3/4" internal thread. Here we solve the transition 1"→3/4" using a reducing brass fitting, behind which we fit a straight transition 3/4" × 22 with internal thread and continue with a new distribution from 22 mm copper.

Sealing materials and their compatibility with heating media

Compatibility of the fitting with the piping system does not end with dimensions – the sealing material plays a critical role. In Hepworth compression fittings, sealing is ensured by an olive (compression ring) and sometimes also by an elastomeric O-ring. In the threaded part, the installer ensures the seal using Teflon tape (PTFE) or hemp fiber with sealing paste.

The following recommendations apply to heating systems:

PTFE tape (Teflon) – compatible with water, steam, heating media, and inhibitors. Easy to apply, but if wound incorrectly (too few layers, wrong direction), it may leak. More details in the article How to seal the thread on heating fittings: Teflon vs. hemp fiber.
Hemp fiber + sealing paste – a traditional solution, durable and reliable when used correctly. Hemp swells and seals the thread perfectly. Compatible with all common heating media, including glycol mixtures.
Anaerobic sealants (Loctite 577, Unipak) – a modern solution, ideal for threads where you cannot use tape (e.g., in very small or damaged threads). Compatibility depends on the specific product – always check the packaging to confirm it is suitable for hot water up to the specified temperature.
EPDM O-rings – in some Hepworth fittings, the O-ring is part of the sealing system. EPDM is compatible with water and steam, but NOT with mineral oils. If you use oil as a heat transfer medium (some industrial applications), use FKM (Viton) O-rings.

Be careful with glycol mixtures in antifreeze heating systems (family homes with heat pumps, cottages): glycol reduces the surface tension of the liquid and can penetrate through leaks that water would not. Therefore, glycol systems require higher sealing quality – better to use a new PTFE tape with more layers or an anaerobic sealant certified for glycol mixtures.

Practical tips for selecting the correct Hepworth fitting in combined systems

From dozens of customer cases during renovations, the following practical conclusions emerge:

Always first determine the material and outer diameter of the existing pipe – before purchasing anything else. Without precise measurement, you are buying a problem, not a solution.
Determine the direction of the thread on the valve (internal or external) – this determines the type of fitting you choose. The difference between internal and external threads is explained in the article Internal vs. external thread: what type of transition do you need.
With steel piping, always check the condition of the thread before installing a new fitting. A rusty or mechanically damaged thread cannot be saved by any sealing tape.
Never mix fittings from different systems in one joint – for example, a compression body from one manufacturer with an olive and nut from another. Dimensions may be nominally the same, but tolerances differ.
For PE-X and multilayer systems, always use fittings explicitly certified for the specific material and diameter – do not buy a "universal" fitting for copper just because it "looks the same".
The difference between a coupling and a straight transition is functional, not just terminological – if you are unsure, read the article Difference between a threaded coupling and a straight transition.

Certification, standards and what to look for on the packaging

For responsible installation in permanent residential heating, it is important that the fittings used meet European standards. Hepworth fittings for copper are certified according to:

EN 1254-2: Compression fittings for copper and copper alloys
EN ISO 228-1: Pipe threads – BSP/G thread
DW 143 / WRAS (British approval for potable water) – not directly relevant for heating, but indicates overall product quality

The packaging of Hepworth fittings should include: the size marking (e.g. 1/2" × 15), the material of the body (brass), the material of the olive, pressure and temperature range, and the EN 1254 standard number. If these details are missing or unclear, it is a warning sign – especially with cheap Asian alternatives, where the marking "15 mm" may actually correspond to a different standard (e.g. 15.88 mm = 5/8").

Frequently asked questions (FAQ)

Can I use Hepworth compression fittings on PE-X pipe from underfloor heating?

Not directly. Standard Hepworth compression fittings are dimensioned for copper pipe with a rigid wall. For PE-X and multilayer pipes, it is necessary to use fittings with an insert or rigid sleeve that prevents deformation of the soft plastic wall when tightening the olive. Without the sleeve, leakage or pipe rupture is possible. On the threaded side of the Hepworth fitting, there is no problem – the BSP thread is compatible regardless of the type of pipe behind it.

I have an old steel pipe with a Russian thread – will it fit with a Hepworth fitting?

It depends on the specific thread. In the vast majority of cases, steel piping in Slovakia from the socialist era is equipped with BSP threads (G-thread), which are fully compatible with Hepworth fittings. Genuine Soviet metric threads (MK) are rare and usually appear in special industrial installations. If you are unsure, measure the outer diameter and pitch of the thread – or bring a sample to the store.

Why do I see "1/2" × 15" on the packaging of a Hepworth fitting – what does each number mean?

The number "1/2" (or 3/4") is the size of the BSP thread in inches – that is, the size of the threaded side of the fitting, which is screwed onto the valve, tap, or boiler. The number "15" (or 22) is the outer diameter of the copper pipe in millimeters, for which the compression side of the fitting is intended. These two numbers are not linked by any simple conversion – they are two different dimensions for two different parts of the same fitting.

Can I mix olives and nuts from different manufacturers in one compression joint?

Technically, they may fit, as the dimensions are standardized according to EN 1254-2. In practice, however, I do not recommend it – different manufacturers have slightly different tolerances, olive hardness, and seating surface geometry. The result may be a joint that holds under initial pressure, but after thermal cycles (expansion and contraction during heating), it may start to leak. For certainty, use components from one manufacturer in one joint.

What are the maximum parameters (pressure, temperature) that a Hepworth brass compression fitting can withstand?

For standard brass compression fittings, the maximum working pressure is 10 bar at temperatures up to 110 °C, and higher pressure at lower temperatures (e.g. 20 °C) according to the technical data sheet. For standard heating in a family home (pressure 1.5–2.5 bar, temperature 60–80 °C), you are operating well below these limits. For high-temperature or high-pressure applications (industry, steam), compression fittings are unsuitable – use soldered or crimped systems.

What to do if the thread on an old valve is not BSP or NPT, but something "in between"?

Damaged or non-standard threads are quite common in older installations. First step: verify the exact pitch and outer diameter with a thread gauge or caliper – it may be a BSP thread in a poor condition, not another standard. If the thread is truly damaged, the most reliable solution is to cut out the section with the damaged thread and insert a new coupling with the correct thread. Using improvised adapters under pressure in heating is not a safe solution.

Conclusion: compatibility is not a given – but it can be managed

Hepworth threaded fittings are among the most universal and compatible products on the market within European BSP/EN 1254 standards. They work reliably with copper piping, can be connected to steel piping via the threaded part, are interchangeable with most European valves, and handle the parameters of standard heating systems with a significant margin.

Problems arise only where different standards and generations of technology meet: plastic piping requires special fittings, American NPT threads are incompatible with BSP, and damaged or non-standard threads always require attention before installing a new fitting. For most real installations in Slovakia – where copper, brass, and BSP threads dominate – working with Hepworth fittings is straightforward, provided you know the dimensions and principles we have discussed here.

If you are planning a renovation or expansion of your heating system and are unsure about the correct choice of fitting, we also recommend reading the article How to choose the right union with a union nut for heating and for installation details also Installing a union with a union nut on copper pipe step by step – together they give you a complete picture from selection to a completed and tested installation.

Do you have a question on this topic?

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