How to choose the right straight pipe coupling: diameter, material, and pressure parameters
How to choose the right straight coupling for the pipe: diameter, material and pressure parameters
A straight coupling is one of the simplest elements of a heating installation – and precisely for this reason, many people do not pay enough attention to its selection. The reality, however, is that an incorrectly chosen coupling can cause leakage, corrosion or even a failure of the entire piping system. I have seen dozens of cases where a few euros were saved on couplings during a heating system renovation, and the result was damp ceilings, damaged floors and expensive repairs. This article will help you avoid these mistakes and clearly explain what to pay attention to when choosing the diameter, material and pressure parameters.
What is a straight coupling and where is it used
A straight coupling is a fitting that allows you to extend or connect two parts of a pipe of the same (or different) diameter along one axis without changing the direction. Unlike an elbow, a T-piece or a reducer, it is used exclusively for linear connections. In practice, you will encounter it everywhere where it is necessary to:
- extend an existing pipe during a heating system renovation,
- replace a damaged or corroded section of the piping system,
- connect a new pipe to an existing one after shortening,
- connect two separate heating circuits of the same profile,
- complete the piping after installing equipment (boiler, storage tank, circulation pump).
In the heating category of Hepworth, these are primarily pressed copper fittings intended for heating and plumbing systems. Their main advantages are reliability, quick installation and compatibility with standard copper pipes according to the EN 1057 norm.
Diameter of the coupling: a key parameter that must not be just "roughly"
The first and most important step in choosing a straight coupling is determining the correct outer diameter of the pipe. This is where the most frequent mistake is made – installers and DIYers often confuse the outer diameter with the inner diameter or with the nominal diameter according to old German standards (DN). The result is a coupling that either cannot be mounted or fits loosely and is not sealed after soldering.
For copper pipes, the outer diameter is specified. The most common dimensions in apartments and family homes are 15 mm, 16 mm, 18 mm, 22 mm and 28 mm. Each of these dimensions has a precisely defined wall thickness, and thus also an inner diameter.
For a standard family home with a two-pipe heating system, a simplified rule applies: the main lines from the boiler have a diameter of 22 mm or 28 mm, while the branches to the radiators are 15 mm or 16 mm. If you are connecting underfloor heating, you will most often encounter a 16 × 2 pipe (outer diameter 16 mm, wall thickness 2 mm).
Specific products for each dimension can be found directly in the category – for example, Straight coupling 15 x 15 is intended for a pipe with an outer diameter of 15 mm, Straight coupling 22 x 22 for a profile of 22 mm and Straight coupling 28 x 28 for the stronger 28 mm profile, which is commonly used for main branches of boiler rooms or larger buildings. For underfloor heating lines or thinner lines to radiators, there is also Straight coupling 16 x 16.
How to correctly measure the diameter of an existing pipe
If you are renovating an old system and you do not know what diameter you have, do not guess. Use a caliper and measure the outer diameter of the pipe. Never measure the inner diameter – you do not have the necessary tools without cutting the pipe, and in any case it would not help, since couplings are dimensioned according to the outer diameter. If you do not have a caliper, you can use a tailor's tape measure and measure the circumference of the pipe, then divide by π (3.14159) – you will get the outer diameter. This method is less accurate, but sufficient for orientation.
More on the correct determination of the diameter can be found in the article What diameter of a straight coupling do I need for my heating system, where there are also tables for old steel pipes with nominal DN dimensions.
Coupling material: copper, brass, cast iron or plastic?
Hepworth couplings, which you can find in this category, are made of copper alloy and their body is dimensioned for copper or compatible pipes. However, there are several material variants of straight couplings on the market, and it is important to know when to use which one.
Copper and Brass Fittings
These are the gold standard for heating distribution systems. Copper has excellent mechanical properties, high resistance to corrosion in the environment of heating water, good thermal properties and a long service life (50 years or more with proper use). Brass fittings are more resistant to galvanic corrosion in mixed installations where copper comes into contact with steel. For the installation of compression fittings, it is essential to use certified compression pliers – this prevents damage to the seal and deformation of the pipe.
Zinc-coated Steel Fittings
These are used exclusively in threaded systems (DIN thread – external/internal), where the joint is sealed with Teflon tape or fiber paste. In residential heating distribution systems, you will encounter them less frequently, mainly in older systems from the 80s and 90s. Their disadvantage is lower resistance to internal corrosion when the water is not properly treated (high oxygen content, low pH). Threaded fittings also have higher hydraulic losses due to the smaller effective cross-section in the threaded part.
Plastic (PP, PPR, PE-X) Fittings
Plastic fittings are common in underfloor heating and cold water distribution systems. For classic radiator heating with a medium temperature of 70–90 °C, use them only if they are certified for such a temperature range (PPR-CT class). Cheap plastic fittings intended for cold water quickly degrade and become brittle at heating temperatures. In practical selection: if you have copper or compatible piping, choose copper Hepworth fittings – this is a safer option with better long-term reliability.
Stainless Steel Fittings
Highly resistant, but more expensive. They are used in industrial applications and systems with special requirements for media purity (solar collectors, heat pumps with antifreeze mixtures). For standard heating in a family home, they are overdimensioned.
Pressure parameters: what does PN10, PN16 mean and when does it matter
The pressure parameters of the fitting are defined by the value PN (Pressure Nominal – nominal pressure), which indicates the maximum working pressure in bars at a reference temperature (usually 20 °C). For heating systems in apartments and family homes, the typical operating pressure is 1.5 to 3 bars, in some larger systems up to 4–6 bars.
Copper compression fittings Hepworth typically meet the PN16 class, which means they are dimensioned for a working pressure up to 16 bars at 20 °C. At higher temperatures (70–90 °C, typical heating water temperature), the maximum allowable pressure decreases – manufacturers provide pressure-temperature diagrams in technical documentation. In practice: for a typical family house with a closed heating system and pressure of 1.5–2.5 bar, a PN16 fitting is more than sufficient.
When should you pay attention to pressure parameters?
- Heating systems with district heating (CZT/DZT): pressure can reach 6–8 bar, and even more in case of failure. It is important to use certified fittings for the given pressure class and not to install cheap plastic alternatives.
- Solar thermal systems: in case of pump failure, pressure in a closed loop can increase even during collector stagnation, temperature can exceed 130 °C. Standard rubber seals fail – special EPDM or silicone variants are required.
- Heat pumps with brine circuits: low temperatures, but the medium is aggressive (glycol mixture) – it is necessary to verify the compatibility of the seal material with the mixture.
Compression or threaded straight fitting? Key decision
This decision affects not only the cost, but also the labor intensity of the installation, the reliability of the joint and the possibility of repair. In the Hepworth category, compression fittings prevail – and for good reason.
A compression fitting is placed on the pipe, the position of the seal is checked and the entire joint is compressed with special compression pliers. The result is a permanent, mechanically strong and watertight joint that does not require any additional maintenance or tightening. The disadvantage is that the connection is irreversible – if you need to disconnect the pipe, you must cut off the fitting.
A threaded fitting (with G or Rp thread) is installed using a threaded seal (Teflon, fiber paste) and a wrench. It is detachable, but requires precise sealing and regular checks – threaded joints are more prone to leakage under vibrations or temperature cycles. More about the choice between these types can be found in the article Difference between compression and threaded straight fitting: when to use which.
Practical scenarios from everyday practice
Scenario 1: Replacement of a section of the distribution system after a pipe burst due to frost
A typical spring situation: in an uncovered cottage boiler room, a 15 mm copper pipe bursts from the frost at two places. The damaged section must be cut out and replaced. Solution: we cut off the damaged section with a slight reserve, prepare a new piece of pipe and install a Straight fitting 15 x 15 on both ends. It is important to observe the minimum insertion depth of the pipe into the fitting (usually 15–18 mm depending on the size of the fitting) and to clean the pipe from scale and burrs before insertion.
Scenario 2: Extension of the main pipe when adding an annex
The owner of a single-family house is expanding the boiler room and needs to extend the main pipe from the boiler by 2 meters. The main pipe has a diameter of 22 mm. In this case, a Straight coupling 22 x 22 is suitable. Before installation, the system must be drained, the water removed from the relevant section, and one must wait until the pipes cool down – crimping on hot pipe is dangerous and can damage the seal.
Scenario 3: Connecting the boiler during renovation
The customer is replacing an old cast iron boiler with a condensing gas boiler. The new boiler has G¾ or G1 thread connections. Between the threaded boiler connection and the existing copper pipe of 22 mm, a combination of a threaded flanged fitting and a crimped coupling is usually used. In such a case, one must also consider expansion – condensing boilers operate with larger temperature differences and the pipe expands and contracts significantly between cold and hot states.
Scenario 4: Repair of underfloor heating after damage during construction work
During the bathroom renovation, the builders punctured the underfloor heating pipe. The pipe has a profile of 16 × 2. For the repair, a Straight coupling 16 x 16 and careful preparation of the pipe surface are required – the cut must be perpendicular, the edge must be free of burrs, and the pipe must be inserted to the stop. After crimping, the entire loop must be pressure tested (minimum 6 bar for 30 minutes) and the tightness must be checked before covering the floor.
A detailed installation procedure including common mistakes and their causes can be found in the article Installation of a straight coupling on copper pipe step by step. If you are dealing with a leak at an existing coupling, I recommend reading Tightness of a straight coupling: why the coupling leaks and how to fix it first.
Galvanic corrosion: when materials are not compatible
One of the underestimated aspects of coupling selection is material compatibility. If you combine copper and steel without a protective element in one installation, a galvanic cell is formed – copper and steel have different electrochemical potentials, and in the presence of an electrolyte (heating water), corrosion occurs. Copper is a noble metal, so it is the steel that corrodes, not the copper. The result is rust deposits, leaks, and gradual weakening of the steel components.
Solution: always insert a dielectric (insulating) coupling between the copper coupling and the steel pipe – for example, a special transition fitting or a dielectric gasket. This is often overlooked in renovations where copper patches are installed into an old steel pipe system. The consequences only become apparent after years – corrosion at the connection points.
If you need to connect pipes of different diameters (not only materials), read the separate article Can I connect pipes of different diameters with a straight fitting, where the possibilities of reducing connections are analyzed in detail.
What to consider when renovating an old heating system
Renovating an older heating system brings specific challenges. Old steel pipes often have corrosion and scale deposits on their inner walls, which reduce the cross-section. When you clean such a pipe or replace it with copper, the flow in the entire system suddenly increases, which can cause problems with hydrodynamic balancing. Replacing only one section without a comprehensive analysis can be counterproductive.
Another practical issue is that old pipes have slightly different dimensions than today's standards – in the past, so-called gas dimensions ½", ¾", 1", etc., were common, which are nominal (not outer). If you measure an outer diameter of 26.9 mm, it is likely a ¾" steel pipe, not a copper 22 mm one. Interchangeability does not exist here – special transition fittings must be used.
Read about the most common mistakes when planning and purchasing fittings during renovation in the article Straight fittings during heating renovation: most common mistakes when ordering and purchasing.
How to compare Hepworth with other brands on the market
On the Slovak and Czech markets, in addition to Hepworth, brands such as Viega, Geberit Mapress, Conex Bänninger, Watts and others are common. Each has slightly different profiles of pressing grooves and non-interchangeable seals – this means that the pressing pliers are tied to a specific system. Hepworth fittings are compatible with EN 1057 pipes and their pressing profile is standardized, which you will appreciate during repairs, as future installers will not need to have exactly the same tools.
A detailed comparison in the article Hepworth straight fittings vs. other brands: what they differ in and what to choose will also help you navigate the price range – cheaper is not always worse, but for press fittings in floor layers or in the wall, you prefer proven quality.
Most frequently asked questions (FAQ)
What is the difference between a 15 × 15 and a 16 × 16 fitting? Can I use one instead of the other?
No, these two fittings are for different pipe diameters and are not interchangeable. The 15 × 15 fitting is intended for copper pipes with an outer diameter of 15 mm (common radiator piping), while the 16 × 16 is for pipes with an outer diameter of 16 mm (typically layered PEX/Al pipes or 16 mm copper pipes for floor heating). The difference is only 1 mm, but precision is key in a pressed joint – a poorly seated joint cannot compress the seal properly and the joint will be leaky.
Is it possible to disassemble a pressed fitting and reuse it?
No. A pressed fitting is one-time and permanent. After pressing the fitting, it is not possible to disassemble it without damage. When repairing or modifying the piping, it is necessary to cut the joint with a pipe cutter and replace it with a new one. That is why it is important to plan the entire piping system well before pressing – every mistake means the consumption of new material.
How do I know if the fitting is properly pressed?
After pressing, the pressing groove on the fitting should show a visible deformation of a characteristic shape (depending on the system – hexagon, W-profile, etc.). Modern pressing pliers have a safety lock that prevents the jaws from opening until the pressing is completed. The most reliable method of checking is a pressure test – the system is filled with water, pressurized to 1.5 times the operating pressure, and monitored for at least 30 minutes to see if the pressure drops.
What is the lifespan of a copper pressed Hepworth fitting?
In a properly designed and maintained heating system (treated water, correct pH 8–9, low oxygen content), the lifespan of a copper pressed fitting is practically the same as the lifespan of the pipe itself – 50 years or more. The EPDM rubber seal, which ensures tightness, has a minimum lifespan of 25 years under continuous operation, and in practice, significantly more. Problems can arise with unsuitable media – water that is too acidic, chlorides, or aggressive media (e.g., glycol without inhibitors).
Can I use a straight fitting for hot water supply (TÚV) piping?
Yes, copper pressed Hepworth fittings are certified for potable water and TÚV piping. It is important to check that the fitting carries the Wras (UK) or DVGW (Germany) marking or a certificate according to local standards for contact with potable water. For TÚV piping, it is important to ensure that the system temperature is within the correct range – hot water should be at least 60 °C at the tank to eliminate the risk of legionella, but the fittings can easily handle this temperature in short-term contact.
What to do if the fitting starts leaking after pressing?
Leakage after pressing is most often caused by the pipe not being inserted to the stop, the seal being damaged or degraded (e.g., by contact with oil, grease or UV radiation before installation), or incompatible pliers being used for pressing. In any case, a leaking pressed joint cannot be repaired – it must be cut off and replaced with a new one. Before repair, it is necessary to identify the cause, otherwise the problem will repeat itself. More in the article Sealing of a straight fitting: why the joint leaks and how to fix it.
Summary: what not to forget when choosing a straight fitting
Choosing a straight fitting is not just a matter of price. It is a technical decision that affects the safety, reliability and lifespan of the entire installation. Here is a quick summary of what to verify when choosing:
- Outer diameter: always measure the outer diameter of the pipe with a caliper, do not confuse it with the nominal (DN) or inner diameter.
- Material: for copper and compatible piping, choose copper Hepworth fittings – they are reliable, long-lasting and certified.
- Pressure class: for standard apartment and family houses, PN16 is sufficient; for industrial or special systems, verify the designer's requirements.
- Type of connection: pressed joint for permanent and reliable connections; threaded for connection points where demountability is required.
- Material compatibility: when combining different materials, use a dielectric connection to prevent galvanic corrosion.
- Installation: use the correct tools, observe the insertion depth and always perform a pressure test after installation.
A straight fitting is a small component with great responsibility. An investment in proven products and correct installation will pay off in the form of trouble-free heating operation for decades. An overview of available sizes and current prices can be found directly in the Straight Fittings category.
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.