Difference between a union with a shut-off valve and a straight-through connection
Union with compression and straight transition: what is the difference and when to use which
When a plumber or a skilled DIY enthusiast is about to connect a new radiator, replace a circulation pump, or extend a heating branch, they inevitably face a decision: to reach for a compression union or is a straight transition sufficient? Both fittings seem to perform a similar function at first glance – they connect pipe in a straight line. Despite this, they are structurally different components with distinct properties, areas of application, and installation requirements. Confusing one for the other is one of the most common mistakes I see in the field, and the result is often a leak, the need to disassemble the entire branch, or at the very least, an unnecessary loss of time on the job site.
This article will thoroughly examine both types of fittings – their construction, geometry, physical sealing principles, typical situations from installation practice, compatibility with various pipe materials, and rules for selecting the correct size. If you are familiar with the Hepworth product range or are currently browsing the category of compression unions, the following lines will save you a lot of guesswork.
What is a compression union and how it works
A compression union is a fitting that allows a solid, yet removable connection between metal pipe (copper, steel) or plastic pipe (PE-X, PE-RT, PERT/AL/PERT) and a threaded end of a valve, pump, or other installation component. It typically consists of three basic components: the fitting body (with a fixed threaded or crimpable part on one end), a compression or union nut, and a sealing element – most commonly an O-ring or compression ring.
The sealing principle is mechanical: after inserting the pipe into the body and tightening the external nut with a wrench, the sealing element is evenly compressed around the pipe, creating a hermetically sealed joint capable of withstanding the pressures typical in heating systems – usually 6 to 10 bar working pressure, with test pressures often reaching 16 bar. It is important to note that this connection can be unscrewed at any time, the pipe can be pulled out, and the fitting can be reused or the sealing element replaced.
In practice, this means a huge advantage for maintenance: when it is necessary to replace a circulation pump, a diaphragm expansion tank, or a thermostatic valve, it is sufficient to unscrew two union nuts and remove the entire component – without cutting the pipe, without soldering, without welding. That is why compression unions are always used wherever future service needs are expected or where the installation must be reversible.
A typical dimensional combination commonly encountered in domestic heating installations is Compression union 1/2" × 15 – that is, a 1/2 inch (BSP) thread on one side and a 15 mm outer diameter pipe on the other side. This combination covers most thermostatic valves on radiators and light circulation pumps. For larger valves, such as pump groups or distributors, you will more likely reach for Compression union 3/4" × 22 or for a transition version 3/4" × 15, when you need to transition between a thicker and a thinner pipe.
What is a straight transition and how it differs from a compression union
A straight transition is a fitting with a fixed connection on both sides – on one side it has a thread (internal or external) and on the other side a pipe connection, most commonly a crimp or compression type. The key difference from a compression union is that a straight transition does not have a removable union nut. After installation, the connection is permanent, or its disassembly requires mechanical interruption – cutting the pipe or using a decompression tool.
The dependence on the type of thread is crucial for a straight transition. If you are looking for a transition between a thread and a pipe, and the threaded end points into the body of the valve with an internal thread (which is common on most radiator valves), you need an external thread on the transition. Conversely, if the body of the valve has an external thread, then the transition must have an internal thread. Straight transitions with internal threads are less common in practice, but they have their irreplaceable place – for example, when connecting to a body with an external thread (a manifold, certain types of pumps). Products such as Straight transition 1/2" × 15, internal thread or Straight transition 3/4" × 22, internal thread are specifically designed for this purpose.
Direct transition is therefore used in places where repeated disassembly is not expected, where it involves permanent embedding into the wall or floor, or where the valve construction does not allow the use of a union nut due to the space geometry. In the heating system, these are typically: ends of distribution branches, wall penetrations, embedding into underfloor heating distribution rooms, or connection to a fixed collector.
Technical differences step by step
Construction and number of components
A union is a multi-component system. In addition to the body of the fitting, it includes at least one free union nut (sometimes called a "union nut" or simply a "connecting nut") and a sealing ring. The body can be crimped, compression or with an internal thread, depending on the type of pipe material being connected. The nut is placed on the pipe and the pipe is inserted into the body and tightened with a wrench – typically a 24 mm, 27 mm or 32 mm wrench depending on the size. The whole mechanism relies on the nut compressing the sealing ring axially between the pipe and the seat in the body of the fitting when tightened.
A direct transition has a simpler construction in comparison: a body with two ends, one of which is threaded (internal or external) and the other is compression or crimped. No free nut, no sealing ring that needs to be inserted during installation. The threaded side is sealed in a standard way – PTFE tape, hemp fiber or anaerobic sealant. The crimped side is connected using a tool. Or, in the case of a compression system, it is sufficient to tighten with a wrench, but the connection is not easily disassemblable.
Disassemblability and service accessibility
This is the most significant practical difference. A union is designed to be easily unscrewed while keeping the existing pipe route intact. The technician unscrews the nut, pulls out the pipe, and the fitting is disassembled. Then a new fitting can be inserted, the pipe can be pushed back in and tightened. The whole operation takes a few minutes and does not require any special tools other than a wrench.
With a direct transition, especially of the crimp type, it is different: once crimped, the connection cannot be disassembled easily. There are special decompression tools on the market for the Hepworth crimp system (and other systems), but their use is slower, requires experience and in addition, the crimp ring or sealing ring must be replaced after disassembly. In practice, this means that you should never connect a component that is likely to require replacement – such as a pump, expansion vessel, automatic air vent or safety valve – using a direct transition. Always install these components using union fittings.
Sealing and resistance to leakage
Both systems, if properly installed, achieve the same level of tightness under normal heating operating pressures (0.5–3 bar working pressure, test pressures up to 6 bar). The difference lies in the long-term stability of the seal. A union with an O-ring seal is more sensitive to thermal cycling – after repeated heating and cooling, the O-ring may deform over time and the connection may start to leak slightly. Therefore, it is recommended to inspect all unions after the first heating season and, if necessary, slightly tighten them (more about this procedure in the article Inspection and tightening of unions during annual heating service).
The threaded end of a direct transition, on the other hand, depends on the quality of the thread sealing. An incorrectly applied or insufficient PTFE tape, or a thread that was twisted during installation, can lead to leakage. More about this issue is discussed in the article How to seal unions on heating fittings: PTFE vs. hemp fiber.
Dimensions and Marking: What the Numbers on the Packaging Mean
When purchasing fittings and transitions, you often encounter a pair of numbers separated by a cross, for example, "1/2" × 15" or "3/4" × 22". The first number is always the thread size in inches (BSP – British Standard Pipe), the second is the outer diameter of the pipe in millimeters. It is important to understand this clearly, as the relationship between these two dimensions is not linear and confusion is very common when purchasing.
- 1/2" BSP – internal thread diameter approx. 15.8 mm – used with pipes DN 15 (outer diameter 15 mm for copper, or 15–16 mm for PE), so the combination is 1/2" × 15
- 3/4" BSP – internal thread diameter approx. 20.9 mm – used with pipes DN 20–22 mm, typical combination 3/4" × 22 or transition 3/4" × 15
- 1" BSP – internal thread diameter approx. 26.4 mm – used with pipes DN 28 mm, larger boilers and commercial installations
The type of pipe also matters. Copper pipe has a standardized outer diameter: Cu 15, Cu 18, Cu 22, Cu 28. Steel pipe is marked according to its nominal diameter (DN), where DN 15 has an outer diameter of 21.3 mm. For plastic pipes such as PE-X or PERT, the outer diameter varies by manufacturer, but the Hepworth series covers common dimensions. The article Dimensions of compression fittings: how to correctly measure the pipe diameter discusses in detail how to choose the correct size.
For transition pieces such as Compression fitting 3/4" × 15, it means the thread is 3/4" and the pipe has an outer diameter of 15 mm – which seems to be an unusual combination, but in practice you need it, for example, when connecting a thin copper circuit to a larger threaded valve, or when transitioning between older steel and newer copper piping.
When to Use a Compression Fitting – Practical Scenarios
From installation experience, I know that most customers who come with the question "what to buy" actually need to understand what is happening in a specific situation. Here are several typical scenarios:
Scenario 1: Replacing a circulation pump. The pump has threaded connections on both sides, typically 1" BSP or 1 1/4" BSP. The pipe from the boiler is copper 22 mm. Solution: two compression fittings are used on each side of the pump. When replacing the pump, two nuts are unscrewed, the pump is removed, the new one is inserted, and the nuts are tightened. It takes 20 minutes including insulation and draining. Without compression fittings, it would be necessary to cut the pipe and solder, which is significantly more work.
Scenario 2: Connecting a thermostatic valve to a radiator. The thermostatic valve has a bottom inlet with 1/2" or 3/4" internal thread. The pipe is copper Cu 15. You can use either a compression fitting or a direct transition – it depends on whether you expect the valve to be replaced in the future. If yes (and most thermostatic valves are replaced every 10–15 years), use a compression fitting. If not, a direct transition with an external thread is sufficient.
Scenario 3: A new floor heating circuit. PE-X pipe 16 × 2 mm is connected to a manifold. In this case, you typically use direct transitions or compression fittings specifically designed for PE-X, as the circuit is fixed in the floor and there will never be a need for service access to the pipe. Serviceable compression fittings are on the manifold itself, where the flow of each circuit is individually regulated.
Scenario 4: Pipe transition through a gypsum board partition. A direct transition with the correct built-in length is the clear choice here. The pipe is threaded through the hole, the transition is placed in the hole, secured with sealant or a sleeve, and both sides are connected by crimping or compression technique. A compression fitting would not help here – its nut would be embedded in the wall without access to a wrench.
Scenario 5: A service port (bypass) at an automatic air vent. An automatic air vent is mounted on an elbow or T-piece, but it must be removable – its float can get stuck or worn over time. Solution: installation through a compression fitting or compression ball valves. A direct transition would not be suitable here.
When to Use a Direct Transition – and What Types Are Available
We choose a direct transition whenever we want a permanent connection without future disassembly requirements. These are situations where the pipe runs through structures, is placed in a conduit or shaft, or where the geometry does not allow the use of a compression nut.
Products in the Hepworth series offer direct transitions with internal thread – for example, Direct transition 1/2" × 15 with internal thread is ideal where copper pipe is connected to a valve with an external thread of 1/2" BSP. Similarly, Direct transition 3/4" × 22 with internal thread is used for larger dimensions, typically for larger manifolds, valve groups or main supply connections to a manifold block.
In general, we distinguish the following subtypes of direct transitions according to the thread:
- Internal thread (IG) × compression pipe – the valve has an external thread, into which we screw the transition, the other side is connected to the pipe
- External thread (AG) × compression pipe – the valve has an internal thread, into which a transition with an external thread is inserted
- Crimp transition with internal thread – modern systems, where the pipe is crimped with a tool, the threaded side is sealed with teflon as usual
Materials and Compatibility
Unions and transitions of the Hepworth series are made of nickel-plated or tin-plated brass (alloy CW617N or CW602N), which ensures good corrosion resistance against common media in central heating systems – mixed water with inhibitors, propylene glycol, primary circuits with temperatures up to 110 °C. They are certified for potable water (WRAS certification in the UK, DIN DVGW in Germany) and are used without problems in combined heating/water systems.
Compatibility with other systems is good as long as the thread dimensions are observed. The BSP thread (British Standard Pipe) is the most widely used thread in heating and plumbing worldwide, so Hepworth unions can be directly combined with valves from other manufacturers – Giacomini, Honeywell, Danfoss, Oventrop and others. The condition is that both partners have the same dimension and type of thread. This topic is discussed in more detail in the article Compatibility of Hepworth Unions with Other Pipe Systems and Materials.
Be careful with galvanized steel pipe: Hepworth brass fittings are compatible with it only when the flow direction is maintained to prevent electrochemical corrosion (bimetallic corrosion). Experts recommend inserting an insulating piece or a plastic sleeve in case of direct contact between brass and galvanized steel in circuits where aggressive water with low pH circulates.
Most Common Mistakes in Selection and Installation
Over the years of practical experience, I have repeatedly seen the same mistakes that lead to leaks, unnecessary repeated visits to the site, or premature replacement of the entire component:
- Forgetting to put on the nut before inserting the pipe into the union body. Once the pipe is inserted, the nut is blocked and cannot be placed on the thread of the body without removing the pipe. This is a classic "beginner's" mistake, but even experienced plumbers make it under pressure at the job site.
- Mixing up internal and external threads when ordering. An internal thread (IG) means the thread is inside the fitting – we screw a valve with an external thread into it. An external thread (AG) is the opposite. A customer brings in a "transition 3/4 to 22" and doesn't know whether they need IG or AG – so it is always necessary to first check what type of thread the valve has that it is being connected to.
- Using a direct transition instead of a union for valves that require service. Safety valves, automatic air vents and pumps are permanently embedded. The first repair will cost extra for cutting and soldering.
- Insufficient tightening of the nut. Hand tightening is not enough. The nut must be tightened with a wrench, about 1/4 turn after hand tightening. Insufficient tightening leads to leakage during pressure tests or during the first heating of the system.
- Using Teflon tape on the compression seal of the union. The compression O-ring seals on its own without the need for Teflon. Teflon is applied only to threaded connections, not to compression ends.
Dimensional Table: Overview of Typical Combinations
| BSP Thread | Pipe Diameter (mm) | Typical Use | Recommended Type |
|---|---|---|---|
| 1/2" | 15 mm (Cu) / 16 mm (PE) | Thermostatic radiator valve, small pumps, shut-off valves DN 15 | Union with compression 1/2" × 15 |
| 3/4" | 15 mm (transition) | Transition from thin copper circuit to a thicker valve | Union with compression 3/4" × 15 |
| 3/4" | 22 mm (Cu) | Pump groups, larger shut-off valves, collector block | Union with compression 3/4" × 22 or direct transition 3/4" × 22 |
| 1/2" | 15 mm (permanent installation) | Fixed ends in the wall, distribution sections without the need for disassembly | Direct transition 1/2" × 15, internal thread |
| 3/4" | 22 mm (permanent installation) | Installation in shaft, fixed collector connections | Direct transition 3/4" × 22, internal thread |
Pressure Tests and Pressure Classes
Hepworth systems are dimensioned for PN16 – that is, a maximum working pressure of 16 bar at a temperature of 20 °C. At higher temperatures, the maximum pressure decreases according to the material characteristics. At 90 °C (a typical operating temperature of older boilers), the recommended working pressure is about 10 bar, which is still significantly higher than the usual operating conditions of domestic heating systems (typically 1.5–2.5 bar static pressure + 0.5 bar pump operating pressure).
Pressure tests after installation are recommended to be carried out with cold water, not air (air is compressible and has a higher destructive potential in case of a rupture). The test pressure for domestic heating is usually 1.5 times the maximum working pressure, typically 6 bar maintained for at least 30 minutes. Hepworth unions with compression and direct transitions pass this test without problems, provided they are properly installed and tightened.
Price and Economic Decision
Unions with compression are generally 20–40 % more expensive than direct transitions of the same dimension, as they contain more components (nut, sealing, body machining). This price premium is, however, economically justified wherever service interventions are expected: the cost of an installer's visit and work to replace a pump without compression unions is several times higher than the savings on a cheaper direct transition during installation. In practice, a simple rule applies: an investment in a compression union for every valve that is replaced once every 5–15 years pays for itself at the first replacement.
On the contrary, for fixed piping, wall and floor installations where access to the piping is excluded, it is economically correct to use the cheaper direct transition without paying extra for a function that you will never use.
Frequently Asked Questions (FAQ)
Can I use a union with compression instead of a direct transition if I want to have a removable connection?
Yes, in principle always. A union with compression can withstand the same pressure and temperature conditions as a direct transition, and it adds the possibility of disassembly. The only limitation may be space – the compression nut needs several centimeters of space around it for turning the wrench. If the space is tight, you must consider whether the wrench will even fit there. In such cases, a direct transition is sometimes used for space reasons.
What wrench do I need to tighten a union with compression 1/2" × 15?
For the 1/2" × 15 Hepworth union nut, a 24 mm wrench is required by standard. For the larger 3/4" × 22, it is usually a 30 or 32 mm wrench. It is recommended to have two wrenches – one holds the body of the fitting or the valve, the other tightens the nut. Tightening with only one wrench without holding the other end can cause unintended stress on the valve or the threaded connection.
Is there a difference between O-ring and compression sealing in threaded unions?
Yes. An O-ring seal is a softer elastomeric ring (EPDM or NBR) that seals the surface around the pipe. It is ideal for copper and plastic pipes. A compression ring (olive) is a metal ring that plastically deforms and is pressed into the surface of the pipe when the nut is tightened – more suitable for metal pipes. Most modern Hepworth unions use an EPDM O-ring, which is compatible with temperatures up to 120 °C and resistant to heating inhibitors and propylene glycol.
Can I use a straight coupling instead of soldering on a copper pipe?
Yes, and in home practice, this is very common. A compression straight coupling (without soldering) is faster, does not require a torch, a gas cylinder, or soldering experience. It is a fully valid alternative in terms of pressure. However, be careful that compression fittings for copper (olives) are generally more sensitive to inaccurate pipe cutting – the pipe must be cut straight and faced (free of burrs). If the cut is angled, the olive does not press symmetrically and the joint may leak after some time.
What if the threaded union leaks after the first heating of the system?
This is a common occurrence, not a reason for panic. After the first heating, the piping and fittings thermally expand and the O-ring settles
Do you have a question on this topic?
Having trouble deciding or dealing with a specific situation in your home? Write to us – we are happy to help.