Direct connections during heating renovation: most common mistakes in measurement and purchase

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Direct fittings during heating system renovation: most common mistakes in measurement and purchase

Renovation of a heating system is a complex project, and even experienced homeowners and cottage owners often encounter seemingly trivial problems – for example, with the selection of the correct direct fittings. I have completed dozens of such projects and can say that mistakes in measuring and purchasing direct fittings are surprisingly common, costing people unnecessary money and adding hours of work. This article will examine each of these mistakes in detail so you can avoid them.

A direct fitting may seem at first glance to be a simple part – two ends, one passage. Despite this, problems often arise precisely with this "simple" part, which can then lead to leaks, repairs, or even the replacement of an entire section of pipe. Let's look at this systematically.

Why direct fittings are more problematic during heating system renovation than they appear

During a completely new installation, the technician determines the pipe diameters and designs the system from the beginning. During a renovation, however, you enter an existing system that was installed years ago, perhaps even decades ago. You may encounter a mix of several generations of materials, different standards, and various diameter norms.

In older buildings from the 80s and 90s, we commonly encounter steel pipes with threads, copper pipes, and combinations of different diameters on the same route. In newer buildings and after renovations, plastic systems, "multilayer" pipe systems, or precisely copper pipes with systematic use of Hepworth compression fittings are common.

When you intervene in such a system – replacing a boiler, adding a radiator, or relocating a pipe – you must know exactly what you are dealing with. It is precisely here that the first mistakes occur.

Mistake No. 1: Confusing the external and internal diameter of the pipe

This is the absolute most common mistake and I encounter it almost every time on a job where the customer has purchased the materials themselves. Direct fittings are labeled according to the external diameter of the pipe for which they are intended. For example, Straight fitting 15 x 15 is intended for a pipe with an external diameter of 15 mm.

The problem arises when the customer measures the internal diameter of the pipe. A copper pipe with an external diameter of 15 mm has an internal diameter of only 13 mm (with a standard wall thickness of 1 mm). If someone measures the internal diameter of 13 mm and searches for a "13 mm fitting," they will find nothing relevant – or worse, they will buy something else that does not fit.

Correct procedure: always measure the external diameter of the pipe using a caliper. If you do not have a caliper, you can measure the circumference of the pipe with a string and divide the result by the number π (3.14). From practical experience, however, I recommend investing in a cheap caliper – it is a tool that pays for itself many times over in any kind of plumbing work.

Typical external diameters of copper pipes for home heating and the corresponding direct fittings:

External Ø 15 mm → Straight fitting 15 x 15
External Ø 22 mm → Straight fitting 22 x 22
External Ø 28 mm → Straight fitting 28 x 28
External Ø 16 mm (PE-X, multilayer) → Straight fitting 16 x 16

Note that 15 mm and 16 mm are different diameters for different types of piping – copper pipes come in the range 15-22-28 mm, while plastic and multilayer pipes use the range 16-20-25 mm. This is the source of another common mistake.

Error No. 2: Confusing 15 mm copper pipe with 16 mm plastic pipe

This is a tricky mistake because the difference of one millimeter is easily overlooked at a quick glance. In practice, however, it means that the fitting does not fit – either it does not hold (if it is one millimeter larger, the gasket cannot compensate for the gap), or the pipe cannot be pushed in at all (if it is one millimeter smaller).

A situation I have encountered several times: the customer had original 15 mm copper pipes and wanted to extend the branch to the bathroom radiator using cheaper 16 mm multilayer pipe during renovation. They bought "15x15" couplings and wanted to use them for the 16 mm pipe. The result? The pipe could not be pushed in, even with greater force.

When connecting copper pipe with plastic pipe, you need a transition piece (reducer) or a special fitting for different types of pipe. A direct coupling 15 x 15 is intended exclusively for two copper pipes with an outer diameter of 15 mm on both sides. For information on connecting different diameters, see the article Can I connect pipes of different diameters with a direct coupling in our Knowledge Center.

Error No. 3: Underestimating the number of pieces during estimation

Customers usually calculate how many couplings they need for new pipe sections, but they often forget about:

Couplings for repairing damaged sections – in older systems, additional pipe sections may be damaged during insulation removal or demolition work and need to be repaired. Always keep 2–3 extra pieces.
Couplings for shortened pipes – if you measure the length incorrectly and cut the pipe too short, you will need an extra coupling to extend it.
Couplings for unexpected detours – behind the wall you find another pipe, cable or reinforcement, and the pipe route must be changed, which will require additional joints.
Lost or damaged pieces – the gasket of a coupling that has been lying on the construction site for a long time may crack or become unusable due to dirt. A customer coupling with a damaged gasket goes into the trash.

I recommend buying at least 15–20 % more than the calculated number as a reserve. For small orders (up to 10 pieces), buy at least 2 extra pieces. Returning unused goods is much easier than an urgent replenishment package that delays the completion of the installation.

Error No. 4: Incorrect insertion depth of the pipe into the coupling

The Hepworth compression coupling has a precisely defined insertion depth for the pipe. This value is not random – it is the result of engineering calculations that ensure that after compression, the gasket properly surrounds the pipe and the joint will be watertight even under system pressure.

What happens if the pipe is inserted more shallowly than the prescribed depth? The gasket is partially in the air, after compression it creates an incomplete contact and the joint will leak – at first only slightly, but under pressure and with temperature changes, the leak will worsen. For more information on the causes and solutions for leaking joints, see the article Tightness of a direct coupling: why the joint leaks and how to fix it.

Typical insertion depths for common copper pipe diameters with Hepworth compression couplings:

Diameter 15 mm: insertion depth approx. 18–20 mm
Diameter 22 mm: insertion depth approx. 22–24 mm
Diameter 28 mm: insertion depth approx. 26–28 mm

Before compression, always mark the insertion depth directly on the pipe with a marker or scratch line. It is a ten-second job that can save you hours of repairs.

Error No. 5: Buying the wrong type of coupling – compression vs. threaded vs. compression screw

Direct Hepworth couplings from the category direct couplings are of the compression type. This means that for their installation, you need special compression pliers. Without this tool, you cannot properly install the coupling – and if you try to do it another way (for example, just pushing it on without compression), the joint is not secured and will release at the first pressure or temperature increase.

Customers who are not familiar with the types of couplings sometimes buy compression couplings and only at home realize they do not have compression pliers. These pliers are commonly available at tool rentals, but you need to plan for them in advance – do not leave it to the day of installation.

Comparison of coupling types and their use in renovation:

Compression coupling (Hepworth press-fit) – fast installation, reliable, requires compression pliers, suitable for copper, excellent for systems under the floor or in the wall where you do not want to return
Threaded coupling – does not require special tools, but the thread must be well sealed (Teflon + sealant), suitable for steel pipe systems, larger diameter, boiler rooms
Compression (screwed) coupling – installation with a wrench, suitable for places where later disassembly is needed, does not require special tools, but is more prone to loosening if tightened incorrectly

More on this topic can be found in the article Difference between compression and threaded direct coupling: when to use which in our Knowledge Center.

Error No. 6: Ignoring the condition of the pipe end before installation

The Hepworth compression coupling works on the principle of creating a firm and watertight contact between the gasket in the coupling body and the outer surface of the pipe. Any defect on the pipe surface at the sealing point can lead to a leak.

The most common problems that customers underestimate:

Uneven pipe cut – a pipe cut with an angle grinder or file without subsequent leveling has an irregular end that cannot be properly inserted to the required depth and the gasket is unevenly loaded
Sharp edges and burrs after cutting – can cut the gasket during insertion, and the damage to the gasket may not be immediately visible
Dirt, oxidation layer and grease – on the surface of a copper pipe that has been lying in storage for a long time, oxidation occurs; before installation, it is necessary to lightly sand the end of the pipe with fine sandpaper
Oval (deformed) pipe end – due to incorrect handling or storage, the end of the pipe becomes deformed; before installation, it is necessary to use a calibration tool (re-rounder) to return the end to a circular cross-section

These are mistakes that during installation cost only a few minutes to fix, but if you neglect them, you can lose hours searching for a water leak location in a completed installation.

Mistake No. 7: Pressure and temperature parameters – insufficient attention

The Hepworth compression fitting is certified for specific maximum working pressures and temperatures. In a typical domestic heating system, these parameters are usually: working pressure up to 6 bar, working temperature up to 95 °C. If you are reconstructing a system with a condensing boiler or a heat pump, verify the actual operating conditions.

Problems arise in industrial or semi-industrial applications (boiler rooms of larger apartment buildings, heat exchanger stations), where pressures and temperatures can be higher. Other types of fittings are intended for such applications, and it is necessary to consult the manufacturer's technical representative.

However, standard Hepworth compression fittings in the sizes mentioned above are generally suitable for standard single-family homes and apartments. More about technical parameters and selection can be found in the article How to choose the right direct fitting for pipe: diameter, material and pressure parameters.

Mistake No. 8: Staged supply – why it is more expensive and complicates work

A very common scenario: the customer buys only as many fittings as they think they need, the technician comes for the installation and realizes that 5 more pieces are missing. The customer has to urgently order or run to the store. The technician waits, the customer pays for the downtime. Or – even worse – the technician does not finish the job, leaves and returns in a week. The entire system is out of operation in the meantime.

For heating system reconstruction, I recommend doing the supply in advance, with a reserve, based on a detailed measurement. If you are unsure about the measurement, call us – we can help you estimate the material consumption based on the system description.

Practical tip for measurement: go through the entire piping and mark each place where a new connection will be made. Do not forget that each new pipe section has at least two new connections (one at each end), plus additional connections if the route goes through an elbow or a branch. Finally, add a 20% buffer.

Mistake No. 9: Negative compatibility with existing fittings from other manufacturers

Reconstruction rarely means replacing everything from scratch. Most often, parts of the existing piping are preserved and new branches are added or damaged sections are replaced. Here comes the question: can I combine Hepworth fittings with fittings from another manufacturer if they have the same diameter?

In principle: if both fittings are for the same outer pipe diameter and both are compression type, they should technically work. But an important detail: compression pliers are designed for specific jaw profiles that correspond to the specific sealing profile in the fitting. Some systems (for example, Hepworth U-profile vs. TH-profile from other manufacturers) are not mutually compatible in terms of compression jaws.

A simple tip: if you are not sure about the manufacturer of the existing system, consult an experienced plumber or ask us. Incompatibility of jaws during compression is a less visible mistake – the connection looks correct from the outside, but it is not sufficiently compressed. More about system comparisons can be found in the article Hepworth direct fittings vs. other brands: what they differ in and what to choose.

Error No. 10: Improper storage and handling of fittings before installation

This may sound trivial, but it has a real impact on the quality of the installation. The Hepworth compression fitting has an EPDM rubber gasket inside. EPDM is an excellent material, resistant to hot water and heating media, but it must be handled correctly.

Concrete problems from practical experience:

Frost during storage – fittings stored at temperatures below –10 °C may have temporarily hardened gaskets. Before installation, they need to be acclimatized to room temperature (at least 2 hours).
UV radiation – prolonged exposure to direct sunlight degrades the EPDM gasket. Do not store fittings in direct sunlight, especially during the summer period.
Contact with oils and solvents – on construction sites, there are many chemicals (paints, varnishes, oils). Any contact of the gasket with these substances can damage it.
Mechanical damage – fittings thrown loosely into a bag with other hard parts (screws, clamps) may have surface-damaged gaskets.

The cheapest protection: keep the fittings in their original packaging until the moment of installation. This may seem obvious, but in practice, it often happens that the entire packaging is dumped into a common container with other materials, and the fittings remain there for the entire week of construction, exposed to everything possible.

Error No. 11: Installation without a pressure test after completion

This is not directly an error in the ordering or purchasing process, but it is an error in the completion of the work, which is closely related to all the previous steps. Every new installation or renovation should be concluded with a pressure test before closing into the walls or before embedding into the floor.

A water pressure test is carried out by filling the system with water and pressurizing it to 1.5 times the working pressure (typically 6 bar for a system with a working pressure of 4 bar) and leaving it for at least 30–60 minutes. If the pressure does not drop and there is no visible moisture or drops at any joint, the installation is in order.

Many customers skip the pressure test "because it looks fine". The result: after sealing the pipes in the wall, one joint starts to leak after some time. You have to break it open, repair it, and re-seal it. The cost of additional repairs is significantly higher than the cost of an extra hour of work during the original installation.

Practical scenario: family house, replacement of an old boiler and renovation of piping

For illustration: a customer in a house built in 1985 was replacing an old steel boiler with a condensing one. The original piping was partially copper (15 mm and 22 mm), partially steel with threaded connections. During the installation of the new boiler, the customer wanted to extend the branch to the bathroom and replace a short section of damaged 22 mm copper pipe in the boiler room.

The customer purchased:

5 pieces of Straight fitting 22 x 22 (for repair and extension of the branch in the boiler room)
3 pieces of Straight fitting 15 x 15 (for the branch to the bathroom)

The first problem: the customer forgot the transition fitting between the 22 mm copper pipe and the steel threaded pipe at the boiler inlet. This is not a straight fitting, but a transition with an external thread. This mistake extended the job by a day and added stress.

The second problem: one of the five 22 x 22 fittings had a surface-damaged gasket (likely due to improper storage at the seller's warehouse). This became visible only during a thorough inspection before installation – a slight bulge on the rubber-like ring. It was replaced. Good practice: the customer had bought 5 pieces instead of the minimum 4, so there was a spare.

Result: the system withstood a pressure test of 6 bar for 60 minutes without pressure drop. The installation was successful.

Practical scenario: apartment renovation, saving one radiator

Another customer – an apartment in a panel building – wanted to replace one old panel radiator with a design radiator. The heating system was a 16 mm copper pipe (multilayer). The customer bought fittings "15 x 15" because "16 and 15 are the same, a centimeter up or down".

Unfortunately, they are not the same. A 16 mm multilayer pipe has an exact outer diameter of 16 mm, not 15 mm. A 15 x 15 fitting is designed for an outer diameter of 15 mm and you simply cannot fit a 16 mm pipe into it. The customer had to return the goods, order the correct Straight fittings 16 x 16, and wait another day. In an apartment building, where neighbors were without heating, it was an additional stressful day.

How to approach the calculation correctly: step-by-step checklist

Based on practical experience, I recommend the following procedure for each renovation:

Material identification – determine what material is in the existing system (copper, steel, multilayer, PE-X). Copper is golden, over time brown/green, steel is gray/rusty, multilayer is white or gray with a smooth surface.
Measuring the outer diameter – measure the outer diameter at several points using a caliper and record it. If the diameter is 15 mm, you buy a 15 x 15 fitting. If it is 22 mm, you buy a 22 x 22 fitting. Etc.
Mapping the route and counting joints – draw a simple layout of the piping and mark each new joint location. Each new straight section = at least 2 fittings at the ends. Each repair = at least 2 fittings (the cut section is replaced with a new one, joints are on both ends).
Adding a reserve – add 20 % to the calculated number, at least 2 pieces as a reserve.
Checking tool availability – confirm that you have or can borrow crimping pliers compatible with the Hepworth profile, the correct size of pipe cutter, a calibration tool, and a marker for marking insertion depth.
Planning a pressure test – schedule time for a pressure test before closing the piping.

Most frequently asked questions (FAQ)

How do I find out if I need a 15 x 15 or 16 x 16 fitting if the pipe looks the same?

The only reliable method is measuring the outer diameter with a caliper. Visually, 15 mm and 16 mm pipes are easily confused, the difference is only 1 mm. If you don't have a caliper, you can identify the material: copper pipes for heating come in the range 15–22–28 mm, plastic and multilayer pipes come in the range 16–20–25 mm. If the pipe is golden (copper), it is likely 15 mm. If it is white or gray with a matte surface (plastic/multilayer), it is likely 16 mm.

Can I compress a Hepworth compression fitting with regular pliers or a vise if I don't have special crimping pliers?

No, absolutely not. Crimping pliers have precisely shaped jaws that deform the fitting body evenly around the entire circumference and to a precisely defined depth. Regular pliers or vises apply uneven force only at a few points, which leads to incomplete crimping. The joint may look compressed from the outside, but the gasket is not properly activated inside. When the system is filled with water, such a joint will start to leak. Crimping pliers are available for rent at tool rental services, and renting them for a day is financially acceptable.

I bought straight fittings 22 x 22 and need to use them only on one end (the other end is a larger diameter), what should I do?

A straight fitting is always for the same diameter on both sides. If you need to connect pipes of different diameters, you need a reducing fitting (reduction) – it has different diameters on each side. In the article Can I connect pipes of different diameters with a straight fitting, you will find a detailed overview of options, including when to use reducing fittings and what to watch out for.

Can I use Hepworth compression fittings for cold water as well, or only for heating?

Hepworth compression fittings are certified for potable water supply as long as the correct material variants are used (depending on the gasket material). For potable water, it is important that the gasket meets hygiene requirements. We recommend checking the technical data sheet of the specific product or consulting our team. For heating systems, Hepworth compression fittings are fully suitable.

What happens if I leave a joint in a system that has been filled with water but not yet crimped (an uncrimped joint)?

This is a safety feature of the Hepworth system – an uncrimped joint will leak water during operation. This is an intentional design that ensures an uncrimped joint is detected during the first filling of the system, before it is handed over to the customer. However, this should not be mistaken for a fault – once properly crimped, the joint will seal. If the joint continues to leak after crimping, the problem lies elsewhere (incorrect insertion depth, damaged sealing ring, or incorrect pipe diameter).

How long do Hepworth direct joints last in a standard heating system?

With proper installation and under normal operating conditions (pressure up to 6 bar, temperature up to 95 °C, medium pH between 6.5–8.5), Hepworth crimp joints are designed to last for decades – in practice, they can last 50 years or more. An important factor is the quality of the heating medium: water with an unsuitable pH or aggressive additives can shorten the lifespan of the sealing ring. For more information on care and inspection of piping, read the article Maintenance and inspection of direct joints in home heating.

Conclusion: Measuring and purchasing direct joints – an investment in peace of mind

A direct joint is a small and relatively inexpensive part. Despite this, as we have seen, errors in its selection, measurement, or purchase can lead to consequences that are costly in both time and money. Spending five extra minutes measuring the outer diameter of the pipe, checking the type of pipe, calculating the number of joints with a reserve, and verifying the availability of the correct tools – these are the steps that distinguish a smooth renovation from a time-consuming problem-solving process.

If you are unsure about any of the situations described above, check the relevant article in the Knowledge Centre or ask us directly. It is better to ask once before placing an order than to deal with incompatible parts on site. And remember: when it comes to Hepworth joints, the rule is – correct diameter, correct insertion depth, correct crimping, and a pressure test at the end. No less, no more.

Do you have a question about this topic?

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