Frequently asked questions about direct reduced couplings for heating
Common questions about direct reduced couplings for heating – a comprehensive overview
Direct reduced couplings are among the most commonly used fittings in heating pipe systems, and despite that, many uncertainties still surround them. Customers ask about dimensions, pressure parameters, the difference between crimped and threaded couplings, when to use a reduced coupling and when to prefer another type of fitting. This article answers real questions from practical experience – not theoretical definitions, but specific situations you encounter in a standard heating system.
If you are unsure whether a direct reduced coupling is the right choice for your case, also read our article When to use a reduced coupling and when to use another type of reducing fitting, where you will find a comparison with reducers, transition pieces and other alternatives. Here we focus primarily on common questions and practical answers.
What exactly is a direct reduced coupling and how does it differ from a standard coupling?
A standard (equal) coupling connects two pipes of the same diameter – for example 22 mm to 22 mm. A direct reduced coupling does the same, but each end has a different diameter. The shape is still straight (not an elbow, not a T-piece), only one end has a larger and the other a smaller opening. This allows you to transition from one pipe diameter to another in one piece without any intermediate part.
In practice this means: if you have a main pipe DN 22 and you want to branch off to a larger circuit DN 28, or conversely – reduce the pipe before a manifold from 28 mm to 22 mm – you will use a direct reduced coupling. The only alternative would be another type of fitting with an inserted reducer, which means more connections, more space and potentially more leak points.
Technically, the conical transition in the body of the coupling is key – it ensures a smooth change in cross-section without significant turbulent flow. In crimped couplings (e.g. Hepworth), this transition is shaped directly in the matrix, so the result is geometrically accurate and repeatable.
What dimensions are most common in practice and how to read them correctly?
The notation „22 x 15" or „28 x 22" looks simple, but in practice it happens that the customer confuses the order or incorrectly measures an existing pipe. It is always read as: larger diameter × smaller diameter. So for the direct reduced coupling 22 x 15, one end is intended for a pipe with an outer diameter of 22 mm and the other for a pipe of 15 mm.
The most common combinations in domestic heating systems:
- 22 × 15 mm – the most common reduction in central heating systems; transition from the main pipe to radiator connections
- 28 × 22 mm – typical combination when transitioning from a boiler outlet or manifold to circuit pipes
- 35 × 28 mm – less common, but occurs in larger installations or commercial buildings
- 28 × 15 mm – a jump of two sizes, usable, but you should check whether the flow parameters are acceptable
Important note: the outer diameter (OD – outer diameter) differs from the nominal bore (DN) in copper and many multi-layer pipes. If you have a pipe marked DN15, its outer diameter is indeed 15 mm – this applies to copper and classic heating systems. With PE-RT or PEX pipes, the marking may be different. Always measure physically with a caliper, not just estimate based on a label on an old installation. More about dimensions can be found in the article Dimensions of direct reduced couplings: what size do I need.
Why it is sometimes not enough to just "guess" – material and pressure parameters
One of the most common mistakes: the customer buys a coupling by eye or based on how it "looked in the warehouse". However, in central heating systems you work with operating pressures of 1.5 to 3 bar and medium temperatures of 60–90 °C (lower in condensing boilers, but that is not the rule). Direct reduced Hepworth couplings in the crimped version for copper are commonly certified for operating pressure up to 10 bar at temperatures up to 110 °C – thus with a significant safety margin against normal conditions.
However, if you use an inexpensive, unqualified fitting from an unknown source with the same dimensions instead of a certified crimped coupling, the certification may be missing, the wall thickness may be below tolerance and you may have a problem during the first hydraulic test. More on this topic is covered in the article Material and pressure parameters of direct reduced couplings for central heating.
For practical orientation: the copper direct reduced fitting Hepworth is compatible with copper tubes according to EN 1057, works with crimping tools for profile TH (thermo) or M (press), and the resulting joint is permanently tight without the need for further inspection during operation. This is a crucial difference compared to threaded or soldered connections, where gradual loosening or oxidation can occur.
Most common situation in practice: transition from boiler to manifold
This type of job looks like this in practice: a condensing boiler has a G1" (threaded) outlet, followed by a transition piece to a 28 mm pipe, and five 15 mm loops come out from the manifold. Between the boiler outlet and the manifold, we measure a temperature of around 70 °C and a pressure of about 1.8 bar. At the inlet of each loop, we need to reduce from 28 mm (manifold outlet) back to 15 mm (floor heating loop). Here comes into play a combination of two fittings – first the direct reduced fitting 28 x 22 for the transition from the main pipe to the secondary distribution, and then the direct reduced fitting 22 x 15 before the inlet of each branch.
Would it be possible to use a 28 × 15 fitting (a jump of two sizes) directly? Technically yes, if it exists, but the flow conditions in the system must allow it – too abrupt a reduction can lead to undesirable increases in flow velocity and pressure losses. The heating system designer should verify during system sizing whether such a drastic narrowing is acceptable without hydraulic balancing.
Installation questions: what goes wrong most often in practice?
The installation of a crimped reduced fitting is quick – an experienced plumber can do it in two minutes – but if something goes wrong, it is usually one of these three reasons:
1. Pipe is not inserted far enough
Each crimp fitting has a stop in the body – the pipe must be inserted all the way to this stop, otherwise the crimp profile will not create the correct sealing point. In practice, it can happen that the pipe hits a thick dirt or burr and the plumber thinks it is inserted correctly. Solution: always clean the end of the pipe before inserting, check the inside of the fitting, and firmly push the pipe into the stop by rotating it. Some fittings have an indicator window – check if the pipe is visible.
2. Incorrect crimping – wrong pliers or wrong profile
Hepworth fittings use a specific crimp profile. If you use pliers for a different profile (e.g. V instead of TH), the crimped joint will be geometrically incorrect and will not seal. Profiles TH and M are not interchangeable. Always verify which profile the fitting requires before crimping. More on tool compatibility is covered in the article Installation of a direct reduced fitting step by step.
3. Mechanical damage to the sealing
The rubber seal (EPDM) in the fitting is sensitive to sharp edges and dirt. If the pipe has an unrounded edge or metal chips on it, the seal will be damaged during insertion and the joint will leak later. This usually becomes apparent only during pressure testing or after the first system start-up – therefore, a hydraulic pressure test before sealing the installation is absolutely essential.
Further practical tips and preventive measures can be found in the article Sealing of fittings: common problems and how to avoid them.
Can I use a reduced fitting in the opposite flow direction?
Yes. A direct reduced fitting has no preferred flow direction – the medium can flow from a larger to a smaller diameter and vice versa. The fitting is symmetrical in terms of pressure stability. In practice, however, you need to consider the system hydraulics: flow from a smaller to a larger diameter (expansion) causes a drop in velocity and a slight increase in static pressure, while flow from a larger to a smaller diameter (narrowing) increases velocity and local pressure losses.
From an installation point of view: if technically possible, we aim to install the fitting so that the conical transition points in the direction of flow (from larger to smaller), which is hydraulically advantageous for most heating circuits. However, in reverse circuits (e.g. return lines), this is not always possible and from the perspective of the fitting's lifespan, it is not critical.
Questions about compatibility – can I mix brands of fittings?
This is one of the most controversial topics in plumbing practice. Technically: crimp fittings from different manufacturers may have the same external dimensions and profiles, but internal tolerances, wall thicknesses, and types of seals can differ. For example, the EPDM seal of one manufacturer has a different Shore A hardness than another, which affects performance at higher temperatures.
The expert opinion is clear: within one system (one building, one circuit), connectors from one manufacturer and one certified product range should be used. Mixing different brands is only acceptable if both sides of the certificate meet the same standards (e.g. EN 1254-7 for copper crimp connectors) and if the plumber can prove this in the event of a complaint. A more detailed comparison can be found in the article Hepworth connectors vs. other brands: what to watch out for when choosing.
What is the difference between a crimped and a compression (screwed) reduced connector?
In the category of direct reduced connectors for heating, we mainly encounter two types of connections:
- Crimp connection – the pipe is inserted into the fitting and a profiled connection is crimped with special pliers; the sealing is permanent, no service is required; ideal for concealed runs (under plaster, in the floor)
- Compression (screwed) connection – the nut is tightened manually or with a wrench; the connection is removable; more suitable where access is needed for service or where crimping pliers are not available
For central heating and hot water, the crimped connection is preferred due to its long-term reliability. Compression fittings are more commonly used in cold water distribution, at connections to valves (where future replacement is expected), or in temporary installations. At temperatures above 80 °C and under pressure load, the crimped connection is significantly more reliable, as it does not depend on the tension of the screw, which can decrease over time due to cyclic thermal expansion.
How long will a direct reduced connector last in a heating system?
With correct installation, a properly dimensioned system and standard operating conditions (pressure up to 6 bar, temperature up to 110 °C), the lifespan of Hepworth copper crimp connectors is practically unlimited within the normal lifespan of the building – that is, 50 or more years. Copper as a material is inert to most heating media, oxidation forms a protective film and does not require protection with inhibitors, unlike steel pipes.
The EPDM seal inside the connector is dimensioned for long-term operation at high temperatures and has certified resistance to water, including softened or chemically treated water in closed systems. In an open system (without an expansion tank), long-term oxygen ingress into the system can accelerate corrosion of metal components, but copper itself and the EPDM seal are not primarily at risk.
Practical tip from customer experience: if the system has been out of operation for a long time (summer house, holiday home), we recommend a visual inspection of all visible connectors and a pressure test before restarting – not because the connectors degrade, but because after a longer period of inactivity, sediment or corrosion may appear in other parts of the system, causing increased pressure or contamination. Also see the article Maintenance and inspection of connectors in heating systems.
Can I use a direct reduced connector with other media than water?
Copper crimp connectors Hepworth are primarily certified for:
- hot and cold potable water
- heating water in closed systems
- compressed air (with limitations – check the specific certificate)
- cooling water (in HVAC applications)
They are not suitable for: natural gas (other certifications and fittings), aggressive chemicals, acids or concentrated salt solutions. When using a non-freezing mixture (e.g. glycol in floor heating), it is necessary to verify whether the specific connector range is compatible with the given concentrate – most glycol mixtures up to 50% concentration are compatible with copper, but always read the safety data sheet of the non-freezing mixture.
FAQ – most frequently asked questions by customers
Do I have to do a full hydraulic test again when replacing an old connector?
When replacing a single connector in a functioning system, a full hydraulic test is usually not required – a pressure test of the repaired section after closing the section valves is sufficient. If you are replacing multiple connectors at once, or if it is a reconstruction of a larger unit, a hydraulic test of the entire system is recommended and should always be documented when handing over the work to the customer.
Is there a connector that reduces from 22 to 15 and at the same time allows disassembly without tools?
A standard crimp connector (e.g. direct reduced connector 22 x 15) is permanent after crimping – you cannot disassemble it without cutting the pipe. If you need a detachable point (e.g. for a sensor, flushing or future expansion), use a combination with a ball valve or an installation connector with a screw-on boss at that joint. This is a typical situation before entering a manifold or before a circulation pump.
What if I have an old soldered installation and want to connect a crimped reduced connector?
This is common during renovations. The condition is that the existing pipe is copper EN 1057, in good condition (without corrosion and without possible internal tin plating from previous soldering), and that the outer diameter matches the size of the connector. Cut off the old soldered joint with a metal saw (not a disc grinder, which deforms the pipe), round off the edge, deburr – and the crimped connector can be installed normally. In practice, such a hybrid installation is done regularly and does not cause problems, as long as the pipe is properly prepared geometrically.
Why is the system calculator suggesting a larger fitting than I would expect?
Design software calculates pipe diameter according to flow rate and pressure loss – not according to what "looks reasonable". If you have a long loop with multiple radiators, the system will suggest a larger diameter to ensure pressure losses do not exceed the pump capacity. This then means a larger diameter at the loop inlet and sometimes a surprisingly large reduction at the end of the branch. Always follow the design documentation or consult with the designer – saving on thinner pipe can cost more on the pump or uneven heating.
I have a reduced fitting 28 × 22 – can I use it for both return and supply?
Yes, the straight reduced fitting 28 x 22 is structurally the same regardless of flow direction. The distinction between supply and return is important for the overall system design, but not for the selection of a specific fitting. Both directions (supply and return) are usually in the same diameters, so the fittings are the same.
What if the fitting leaks after installation – is it broken, or did I make a mistake?
In the vast majority of cases, it is an installation error – the pipe was not inserted far enough, the edge was not properly rounded, or it was pressed with the wrong profile. New fittings from a reputable manufacturer have a statistical failure rate of thousandths of a percent. Procedure: depressurize the system, inspect the leak location, cut it out and replace the fitting. Do not reinstall a used (tested, but not pressed) fitting – the sealing may have been damaged during the first incorrect attempt. A more detailed guide on solving leaks can be found in the article How to correctly connect pipes of different diameters without water leakage.
Conclusion: what to take away for practical use
Straight reduced fittings may seem like a trivial component, but the correct selection and installation determine whether your system will work trouble-free for decades or you will return for repairs after the first heating season. Key principles from practical experience are simple: measure the outer diameter with a caliper, do not combine different brands in one system without verifying certificates, always deburr and round the pipe edges before installation, and perform a pressure test after each installation before closing the system into the wall or floor.
If you are unsure about the selection – whether you need a straight reduced fitting or another type of fitting – check out other articles in the Knowledge Center: How to choose the right straight reduced fitting for heating will help with basic orientation, and When to use a reduced fitting and when another type of reducing fitting addresses situations where an alternative is better.
Do you have a question about this topic?
Struggling to decide or dealing with a specific situation in your home? Write to us – we are happy to help.