How to choose the right elbow for HEPWORTH plastic pipe

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How to choose the right elbow for HEPWORTH plastic pipe – complete practical guide

Selecting an elbow for the HEPWORTH plastic pipe system may seem like a trivial matter at first glance. You go to the store, state the size, and leave with a bag of fittings. In practice, however, I know from years of work with central heating and water distribution systems that the most errors occur precisely with elbows. Either the wrong angle, the wrong type of thread, the wrong transition between diameters, or the pressure and temperature conditions in a specific pipeline are not even considered. The result? Leaky connections, unnecessary damage to plaster, and additional costs.

This article will help you choose the right elbow for your specific case – from basic understanding of the HEPWORTH system through types of elbows, dimensions, angular variants to practical recommendations from the field. We will also cover reduced elbows, combinations with threads, installation details and typical errors that can be avoided.

What is the HEPWORTH system and why it makes sense to stay within it

HEPWORTH is a system of plastic (more precisely polypropylene and PVDF) pipes and fittings primarily designed for central heating installations, but it is also used for hot and cold water distribution. The basic philosophy of the system is based on simple installation without welding or soldering – connections are made mechanically using compression (crimping) fittings, or with a sealing ring. This means that a skilled plumber does not need a blowtorch or special equipment for welding plastics.

Why stay entirely within the HEPWORTH system? Primarily due to compatibility. Each elbow, T-piece, bushing or cap in this system has a precisely defined geometry – diameter, insertion length of the pipe, sealing force of the rubber and sealing pressure. When you start mixing fittings from different manufacturers, you can never be sure whether the outer diameter of a pipe from one manufacturer fits exactly on the sealing ring of a fitting from another. Micron deviations in diameter make a huge difference in long-term sealing.

In practice, I have seen cases where a homeowner saved a few cents on fittings from another manufacturer. After three years, the connection behind the ceiling in the bathroom started to slowly leak. The damage to the plaster, tiling and wooden beams cost several times more than an original HEPWORTH fitting would have cost.

Types of elbows in the HEPWORTH system – overview and differences

Elbows in the HEPWORTH system are divided according to several criteria: according to the angle, whether they are reduced or equal diameters, and whether they contain threaded ends (sleeve). Each type has its typical use.

90° elbow (K-piece) – the most commonly used type

The 90° elbow – in HEPWORTH terminology also called a K-piece – is used to change the direction of the pipe by ninety degrees. It is by far the most used type of elbow in any pipeline. It is used for bends along walls, for connecting to radiators, for transitions through corner structures, or for routing pipes between the floor and the wall.

An example of a standard 90° elbow with the same cross-section is K-piece 15 x 15, where both ends have a diameter of 15 mm. There is also a larger variant K-piece 22 x 22 for distribution branches with higher flow. Both work on the same principle of a compression connection – the pipe is inserted into the fitting, the sealing ring and gasket ensure watertightness even under the working pressure of a typical heating system.

When choosing a 90° elbow, pay attention primarily to the outer diameter of the pipe you will be inserting into it. The HEPWORTH system works with the outer diameter, not the inner one – this is a key difference compared to metal pipelines, where the inner diameter (DN) is traditionally given.

45° elbow – when it is a better choice

The 45° elbow is less commonly used, but in many situations it is technically a better choice than a sharp 90° bend. The topic of angles is covered in a separate article (45° elbow or 90° elbow – when to use which type), here we will mention the basics.

Main advantages of the 45° elbow: lower pressure loss during flow, less turbulence in the connection, longer lifespan in systems with higher volumetric flow. If you have the possibility to route the pipe so that you can use a pair of 45° elbows instead of one 90°, the pipeline hydraulics will be better.

Specific products of this group: 45° elbow – 15 x 15 for smaller diameters and 45° elbow – 22 x 22 for larger ones. Both have the same diameters on both ends – so they are not reduced.

Elbow with a sleeve – when you need to connect plastic to metal

A special category is the elbow with a sleeve, where one end has a plastic compression connection (for HEPWORTH pipe) and the other end is threaded – either external (internationally marked as BSP external) or internal thread. These fittings are mainly used to connect plastic pipelines with metal components – valves, radiator connections, brass fittings or steel valves.

A typical representative is Elbow with a sleeve 1/2" x 15. Here, one end is adapted for a HEPWORTH pipe of 15 mm diameter and the other end has a 1/2" thread – the most common thread on radiator valves. Precisely this type of fitting solves the transition from a plastic pipeline to a radiator, valve or other metal component.

Important note: when using elbows with a thread, always check whether the opposite component (e.g. a valve on a radiator) has the same type of thread – external or internal. Interchangeability is not automatic here. This topic is discussed in detail in the article External thread vs. internal thread on an elbow – what difference does it make?

Reduced elbows – when to use them and when to avoid them

A reduced elbow combines two functions at once: changing the direction of the pipe and at the same time transitioning between two different diameters. For example, from 22 mm to 15 mm or from 28 mm to 22 mm. It is an elegant solution that saves space and reduces the number of fittings in the distribution system.

When is it worth reaching for a reduced elbow? Primarily when:

In one place in the distribution, you need to change direction and reduce the diameter at the same time (typically when branching off from the main branch to a side branch)
You have limited space – every additional fitting takes up space and every additional connection is a potential weak point
You want to minimize the number of pressure losses (each fitting is a resistance in the hydraulic circuit)
You are working with interrupted piping at radiators, where the transition is made from 22 mm supply pipe to 15 mm supply pipe to the radiator itself

On the other hand, a reduced elbow is not the right choice if you need to perform the reduction elsewhere than at the bend – in that case, it is better to use straight reducing sleeves and a standard elbow. More on deciding between combined and separate reduction can be found in the article "Reduced elbow or separate reduction – what is more cost-effective."

How to correctly determine the diameter – outer diameter, not bore

This is one of the most common causes of customer complaints and product returns. The HEPWORTH system (as well as most plastic pipe systems) labels pipes and fittings according to the outer diameter in millimeters. This is a fundamental difference compared to traditional heating practice, where people commonly refer to pipes as "half an inch", "three quarters" or DN 20, DN 25 – where these numbers indicate the nominal bore (inner diameter).

In practice, this means: if you have a HEPWORTH plastic pipe in your hand and want to choose the correct elbow, measure the outer diameter of the pipe with a caliper. Common values in the HEPWORTH system are 15 mm, 22 mm and 28 mm (outer diameter). An elbow labeled "15 x 15" will therefore accept a pipe with an outer diameter of exactly 15 mm.

Be careful with the confusion with copper pipes: copper pipes are also labeled by outer diameter and in European practice the same nominal diameters are common – 15 mm, 22 mm, 28 mm. The outer diameters are the same, but the wall thickness (and therefore the inner diameter) differs. HEPWORTH compression fittings are primarily dimensioned for plastic pipes – when using them with copper pipes, always verify the product's explicit compatibility.

If you are not sure what diameter your pipe has – I recommend reading the article "How to determine the correct diameter and elbow angle for your distribution system," where the measurement procedure is described step by step, even if you don't have a caliper.

Elbow angle in practice – how to choose the right one

The HEPWORTH system offers elbows in two basic angles: 90° and 45°. Which angle you need depends on the geometry of your distribution and the spatial conditions.

When to reach for 90° unambiguously

A 90° elbow is used everywhere you need a sharp angle – for example, when running pipe along a wall and then turning toward the radiator, when transitioning from a horizontal floor pipe to a vertical wall pipe, when connecting to a radiator valve (in combination with elbows with a bend), or in limited space where there is not enough room for a longer route with two 45° elbows.

When to consider 45°

A 45° elbow is suitable primarily for main distribution branches, where hydraulic balance is important. A 45° bend has a significantly lower local resistance coefficient (ζ) – it ranges around 0.35–0.5 compared to 0.9–1.5 for 90°. In practice, this means a lower required pump pressure, quieter operation and lower energy consumption of the circulation pump in larger systems.

Another advantage of 45° elbows is their lower tendency to trap air bubbles – at sharp 90° bends, air is more likely to get trapped, which causes noise and worse heat transfer. If your radiator ever "buzzed" or did not heat evenly, the cause could be an air pocket right behind a 90° bend.

In practice, both types are combined: main lines ideally with 45° elbows, and local branches and connections with 90°. This combination is the most hydraulically efficient and at the same time installable in real conditions.

Reduced elbows versus separate reduction – practical decision-making

When solving a transition from one diameter to another together with a change in direction, you have two options: a reduced elbow (in one piece) or a combination of a standard elbow + a reducing sleeve. Both solutions are functional, but each has its advantages and disadvantages.

Advantages of a reduced elbow in one piece

Less connection points = fewer potential leak points
Smaller installed volume – saves space in installation cores or underfloor ducts
Simpler and faster installation – one fitting instead of two
Lower overall hydraulic resistance compared to two separate components

When is a combination of elbow + separate reduction better

When you need the reduction at a different location than the bend (for example, a reduction in the middle of a straight section)
When you are looking for a specific reduced elbow and it is not in stock – two standard available parts are better than waiting for a special order
When the routing of the distribution changes – it is easier to replace one part than a special combined part in future modifications
When the diameters are combined in an atypical way (for example, 28 × 15) and a reduced elbow in such a combination does not exist

From practice: for standard installations in family homes, where the distribution is run behind plaster or in the floor, I recommend using reduced elbows wherever possible. Every fitting less in a poured floor is a win. On the other hand, for overhead distribution in technical rooms, where the distribution is visible and serviceable, separate reduction and elbow are perfectly fine.

Material and temperature parameters – what it can withstand and what it cannot

The plastic HEPWORTH pipe is designed for temperatures and pressures typical for central heating in family homes. The exact parameters vary according to the pipe material, but as a rough guide it holds:

Maximum operating temperature: 80–95 °C (depends on the specific type of pipe and wall thickness)
Maximum operating pressure at 20 °C: 10–16 bar
Maximum operating pressure at 80 °C: 6–10 bar (pressure decreases with temperature)
Recommended operating temperature for central heating: up to 70 °C with pressure up to 4 bar – this covers the vast majority of standard installations

A key parameter for elbows (and for all compression fittings) is also the sealing capability under cyclic loading. With each boiler on/off cycle, the piping thermally expands and contracts. Plastic pipes have a significantly higher thermal expansion coefficient than metal – up to 10 times higher than steel. This means that long sections of plastic piping will noticeably elongate when heated.

Elbows play an important structural role here – each bend in the piping system functions as a natural compensator for length expansion. A properly designed piping system with sufficient bends will "move" within these bends without creating pressure on the clamps or joints. A piping system that is too straight and lacks bends can cause pipe lifting or joint loosening after repeated heating cycles.

Elbow installation – most common mistakes and how to avoid them

HEPWORTH compression fittings are designed to be installed even by less experienced plumbers. Despite this, typical mistakes occur in practice that lead to leaks. A detailed step-by-step installation guide can be found in the article "Step-by-step installation of elbows on copper and plastic pipes," here we summarize the key points:

Cut pipe end: Always cut the pipe at a right angle, not at an angle. An angled cut causes uneven seating of the seal and will eventually lead to a leak. Use plastic cutters or a fine-toothed saw, not a router.
Unbeveled (unrounded) pipe edge: Always remove burrs and bevel the outer edge after cutting. A sharp edge when inserting the pipe can damage the rubber seal in the fitting, which is not visible but will cause a slow leak after years.
Insufficient insertion depth: The pipe must be inserted into the fitting to the full depth of the stop. Incomplete insertion is the most common cause of immediate leaks after the system is started. Before insertion, mark the correct depth on the pipe with a pencil.
Damaged seal: Always check the condition of the rubber seal before installation – whether it is cracked, extruded or deformed from transport. A faulty seal will not ensure tightness even with correct installation.
Over-tightening the thread: Do not tighten the thread with excessive force when installing elbows with a compression ring – the threaded connection between plastic and brass components has a precisely defined tightening torque. Excessive force can break the plastic thread or deform the seal.

I also recommend visually checking each joint after installation and the first pressurization of the system – and, if technically possible, keeping the system at an increased test pressure for the first few hours. This way, any sealing problems will be detected before the piping is embedded or covered by a false ceiling.

Sizing – how to choose the right elbow size for the entire piping system

Selecting the diameter of an elbow should not be based solely on the pipe that leads to it. Proper sizing is based on hydraulic calculations – from the volume flow rate, flow velocity, and allowable pressure losses. For standard single-family homes, however, there are well-established rules of thumb:

Main distribution pipe: Usually 22 mm or 28 mm outer diameter. For a system with multiple radiators and a boiler output up to 15 kW, 22 mm is sufficient, while 28 mm is recommended for larger systems.
Branches (connections to radiators): Standard 15 mm outer diameter. A 15 mm pipe at a typical flow rate of 0.5–1 m/s can transfer a power of about 2–4 kW, which covers most standard radiators.
Radiator connections: Always 15 mm, since radiator valves have 1/2" or 3/4" BSP threads, which correspond to HEPWORTH 15 mm or 22 mm pipe.

Reduced elbows are therefore most commonly used when transitioning from 22 mm to 15 mm – precisely at the point where a branch is taken from the main piping to a radiator. This is the most frequent situation of all. The topic of sizing calculations for various combinations of pipe diameters is covered in a separate article "Sizing elbows when transitioning between different pipe diameters."

Practical scenarios from real customer projects

Scenario 1: Complete reconstruction of piping in a single-family home

The customer had a house from the 1980s with steel pipes that had started to corrode and leak. He decided to completely replace them with HEPWORTH plastic pipes. The main branch in the boiler room and corridors was implemented in 22 mm pipe, while the connections to individual radiators were in 15 mm. At each point where a 15 mm radiator connection branched off from the 22 mm main pipe, a reduced T-elbow was used – and on the radiator connection itself, an elbow with a 1/2" × 15 compression ring was used, since the radiator valves had a 1/2" external thread. In total, 28 elbows with a 1/2" × 15 compression ring were used in the house (for the supply and return of each radiator).

Scenario 2: Heating Installation in an Extension

The customer had an existing boiler with a 22 mm copper piping system and needed to connect a new radiator in the extension through a party wall. The wall opening was tight – there was no space for two 90° elbows in a row. Solution: a combination of two 45° elbows, which elegantly saved 15–20 cm of required space in the wall. Used were 45° Elbow – 22 x 22 for a 22 mm pipe, and despite the fact that the customer originally had a copper system, the manufacturer confirmed compatibility of this fitting for both materials.

Scenario 3: Repair of a Leaking Joint after Insulation

The customer bought a house where the previous owner had embedded plastic pipe directly into the polyurethane foam insulation system. After one year, moisture appeared on the wall. Upon inspection, it turned out that the 90° elbow had been inserted only halfway – the pipe was not marked and the plumber had pushed it in "by eye". Replacing the elbow without damaging the plaster was impossible, as the elbow was entirely embedded. The repair costs were significantly higher than the cost of a ten-minute proper insertion of the pipe during installation.

Most Frequently Asked Questions about HEPWORTH Elbows (FAQ)

What is the difference between a K-piece and a 90° elbow in the HEPWORTH system?

Essentially none – the K-piece is just a commercial/technical designation for a 90° elbow. Both names are used synonymously. The term K-piece is more common in technical documentation and among plumbers, while "90° elbow" is more understandable for regular customers. For example, the product K-piece 15 x 15 is therefore a 90° elbow with the same 15 mm cross-section on both ends.

Can I use a HEPWORTH elbow for cold water as well, not just for heating?

Yes, HEPWORTH fittings are suitable for both cold and hot potable water systems, provided they are certified for contact with drinking water (which should be verified for each specific product). They are dimensioned for higher temperatures in heating systems, so for cold water at up to 20 °C, they are more than sufficient. It is important to use pipes and fittings from the same certified system.

What should I do if the elbow leaks after installation?

First, check whether the pipe is inserted to full depth and whether the rubber seal is not damaged. With most HEPWORTH compression fittings, the joint can be disassembled and reassembled correctly – the fitting is usually reusable as long as it is not visibly damaged. More details can be found in the article Why the elbow is leaking and how to fix it without replacing the entire joint.

Is sealing paste or Teflon tape required for HEPWORTH elbows?

With the compression (plastic) end of the fitting – where the pipe is inserted – sealing is ensured by a rubber O-ring built into the fitting. No paste or tape is used here and in fact, it is counterproductive. On the other hand, with a threaded end (if the fitting has a thread), sealing follows the standard practice for threaded connections – Teflon tape or hemp fiber with paste is used here, just like with forged threads.

What is the maximum temperature at which I can use HEPWORTH elbows?

For standard heating, the operating temperature is up to 80 °C with pressure up to 6 bar. Some models are certified for 95 °C as well, but always with a reduced maximum pressure rating. For standard central heating systems with supply temperatures around 55–70 °C and pressure of 1.5–2 bar, these elbows are absolutely safe and long-term reliable.

Can I combine 45° and 90° elbows in one piping system?

Not only can you – it is actually recommended. In practice, these types complement each other: 45° elbows on long runs to reduce hydraulic resistance, 90° elbows on local connections and in areas with limited space. A combination of both types gives the most hydraulically optimal and at the same time space-saving result. Before designing the piping system, also read the article Frequently Asked Questions about Reduced Elbows from HEPWORTH for more practical tips.

Conclusion – Summary of Choosing the Right Elbow

Selecting the right elbow for HEPWORTH plastic piping is not rocket science, but it does require a systematic approach. Always follow this order: first determine the outer diameter of the pipe (not the inner diameter, not inches), then decide on the required angle (90° or 45° depending on the route and hydraulics), decide whether you need a reduced elbow or the same diameter, and finally consider whether you need a combination with a thread in that location – for example, when connecting to a radiator valve.

Do not forget that every additional elbow is also an additional hydraulic resistance. Good system design minimizes the number of bends to the necessary minimum – every elbow you do not install is one that cannot leak. However, if a bend is needed, choose the right type, the right size, and install it according to the rules.

In the case of more complex piping systems – multi-storey buildings, long horizontal runs over 10 meters, combinations of different diameters in one circuit – I recommend consulting the hydraulic layout with a specialist before purchasing the fittings. This will save you not only money on incorrectly purchased materials, but also time and nerves in case of a potential repair.

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