How to Choose the Right Compression Fitting for Pipes – Diameter, Thread and Material
Why choosing the right compression fitting really matters
A compression fitting is one of the most commonly used fittings in the entire heating and plumbing installation. It is an inconspicuous part that most people overlook – until it starts dripping. From experience, I know that a large portion of all leaks in cold and hot water distribution occur precisely at compression joints, and not because the joint was poorly tightened. The reason is much more mundane: an incorrectly chosen fitting for the given pipe, a wrong combination of diameter and thread, or a material that isn't suitable for the given medium or temperature.
In this guide, we will go through the whole topic systematically – from a basic understanding of what a compression fitting actually is and what it does in the distribution system, through dimensions and threads, to materials and typical situations from installation practice. If you're also interested in related topics such as sealing materials or step-by-step installation, you'll find them in the Knowledge Center in the articles Sealing Materials for Compression Fittings – Teflon, Hemp Fiber or Gasket and Installing a Compression Fitting on a Pipe Step by Step – What You Need and How to Proceed.
What is a compression fitting and where is it used
A compression fitting (also called a pressure or clamping fitting) is a fitting element that allows a mechanical, detachable connection of a pipe with another installation element – for example, a valve, manifold, boiler, direct branch, or a wall plate for water taps. Unlike press-fit or welded joints, it can be disassembled, cleaned, or replaced if needed without damaging the pipe.
In practice, it is most commonly encountered when:
- connecting plastic pipes (PEX, PE-RT, multilayer) to brass or steel fittings,
- supplying sink and shower faucets via wall plates,
- connecting boilers, water heaters and storage tanks,
- underfloor heating distribution (connecting the pipe to the manifold),
- connections to thermostatic valves, ball valves and filters.
The basic construction of a compression fitting consists of the fitting body (rigid part with a thread), a nut (which tightens the whole joint), and an insert or ring that deforms when tightened, creating a tight seal with the pipe. Some types use a rubber O-ring seal instead of a deformation ring.
Pipe diameter – the basis from which everything else is derived
The first thing you need to know before you reach for any fitting is the outer diameter of the pipe you will be installing it on. This is where most mix-ups happen, because two different labeling systems meet in installations – metric (in millimeters, typical for plastic pipes) and imperial/inch (typical for fitting threads and metal distribution systems).
The most common diameters of plastic pipes in residential and family home installations are 16 mm and 20 mm outer diameter. 16 mm pipes are used almost exclusively for underfloor heating and for supplying water to sink faucets. 20 mm pipes are common for main hot and cold water distribution, for supplying kitchen faucets, and for connecting boilers or water heaters.
A much larger dimensional group is 25 mm and 32 mm pipes, but these appear rather rarely in common residential installations – usually in collective distribution systems or industrial applications. In the compression fitting catalog at atria.sk/skrutkove-spoje/ you will find fittings for the most popular sizes, 16 and 20 mm.
How to correctly measure the pipe diameter? Always measure the outer diameter with a caliper. Take readings at two points (the pipe may not be perfectly round) and use the larger value. Never measure the inner diameter – it varies depending on wall thickness, and even a conversion table won't help you if you don't also know the pipe material. You'll find a more detailed measuring procedure in the Knowledge Center in the article Compression Fitting Dimensions – How to Correctly Measure and Match 1/2" and 3/4" to 16 and 20 mm Pipes.
Thread – inches, profile and the most common mix-ups
Once you're clear on the pipe diameter, the second key parameter is the fitting's thread. Here two different logics meet: on the pipe side (plastic or multilayer), it's a mechanical grip without a thread, while on the side of the fitting or the connected element, it's an inch thread – most commonly 1/2" (one half inch) or 3/4" (three quarters of an inch).
The relationship between the pipe diameter and thread size is almost always the same in practice:
- Pipe ∅ 16 mm → 1/2" thread – this is the combination you'll encounter with underfloor heating and sink faucet connections.
- Pipe ∅ 20 mm → 3/4" thread – this appears in main distribution branches, boiler and tank connections, and in places with higher consumption.
However, be careful – other combinations also exist. For example, a 1/2" fitting on a 20 mm pipe (with a reduction) or a 3/4" fitting on a 16 mm pipe occur, but are less common and require great care when selecting. Always check the full product designation, where both dimensions are stated.
External vs. internal thread – which one you get and why
Compression fittings are supplied with either an external thread ("male thread") or an internal thread ("female thread"). The external thread screws into the fitting – this is the typical case when connecting a faucet, valve, or tap. The internal thread receives the fitting's threaded part – typically when connecting to boiler nozzles or the external threads of manifolds.
In practice, connections for bathroom faucets via a wall plate almost always use an external 1/2" thread, because the wall plate and faucet itself are designed to receive an external thread. This topic is covered in more detail in the Knowledge Center article: External vs. Internal Thread on Compression Fittings – What's the Difference and When to Use Which.
In the atria.sk range you will find, for example, the Compression Fitting 1/2x16 with External Thread – a typical choice for connecting a 16 mm plastic pipe to fittings with a G1/2" thread. Similarly, for 20 mm pipes with a faucet or 1/2" valve, the Compression Fitting 1/2x20 with External Thread is intended. And for larger distribution systems with a 20 mm pipe and a 3/4" fitting, the Compression Fitting 3/4x20 with External Thread is used.
Thread profile – G (BSP) vs. NPT and why it matters
Inch threads are not all the same. In European installations – and therefore also in Slovakia – the standard is the G thread (BSP – British Standard Pipe), which is parallel (cylindrical). It is designated G1/2" or G3/4". This thread is sealed exclusively with a sealing material (Teflon, hemp, gasket) or a shaped seat, not by the thread geometry itself.
In the American system there is NPT thread (National Pipe Tapered), which is conical and, when tightened correctly, theoretically seals itself. In practice, NPT reaches the Slovak market mostly with imported equipment – for example, with American boilers or some industrial fittings. Mixing up G and NPT is fatal: the threads can be screwed together for a moment, but they won't seal, and during a pressure test or after some time they will start leaking.
The rule is simple: if you buy fittings or fixtures in a Slovak or Czech store, you will most likely get a G (BSP) thread. If you buy equipment from abroad or from manufacturers outside the EU, always check the technical data sheet.
Fitting material – brass, zinc, plastic or stainless steel
The material a compression fitting is made of is not just a matter of price. It affects resistance to corrosion, temperature, pressure and the chemical composition of the water. Let's look at the most commonly used materials.
Brass (CW617N, DZR brass)
Brass is the gold standard for compression fittings in residential installations. It has excellent mechanical properties, is resistant to corrosion from ordinary water, and handles temperatures up to 90–95 °C well (the temperature of a common heating system) well. The designation CW617N is the most common brass alloy for fittings, but in areas with more aggressive water (high chlorine content, low pH), DZR brass (dezincification resistant) is recommended, which resists zinc leaching from the crystal lattice.
Brass fittings are heavier than plastic ones, but provide a reliable thread seal and long service life. For most residential and family home installations, brass is the right choice.
Zinc alloy (Zamak, ZnAl)
A cheaper alternative to brass. Zinc fittings are commonly sold in hardware stores and used for "quick" plumbing jobs. They have acceptable mechanical properties at room temperature and low pressure, but at temperatures above 60 °C and at higher pressures they perform worse than brass. In aggressive or so-called soft water (low hardness, low pH), they corrode quickly, manifesting as white deposits and gradual release of material. It's best to avoid zinc fittings in heating distribution systems.
Plastic (POM, PP, PA)
Plastic compression fittings are lightweight, chemically inert and cheap. They are used in cold water distribution, garden fittings, and installations where metal would corrode (for example, in a chemically aggressive environment). Disadvantages: lower mechanical strength, plastic threads can easily be stripped when over-tightened, and at temperatures above 60 °C (some types above 40 °C) the material loses rigidity. Plastic fittings are therefore largely unsuitable for heating.
Stainless steel (AISI 316, AISI 304)
Stainless steel fittings are the most expensive, but also the most durable. Stainless steel is ideal for solar systems, boiler rooms with acidic condensation, food industry applications, or distribution systems where the water is chemically treated (for example, with corrosion inhibitors). AISI 316 stainless steel (with added molybdenum) has better resistance to chlorides than AISI 304. In ordinary residential practice, stainless steel fittings are unnecessarily expensive – brass is sufficient here.
Material overview table
| Material | Max. temperature | Corrosion resistance | Suitable for | Unsuitable for |
|---|---|---|---|---|
| Brass CW617N | 90–95 °C | Good (ordinary water) | Heating, DHW, cold water | Aggressive soft water |
| DZR brass | 90–95 °C | Excellent | Aggressive water, chlorinated areas | – |
| Zinc alloy | max. 60 °C | Poor (soft water) | Cold water (temporary use) | Heating, DHW, permanent distribution |
| Plastic (POM/PP) | 40–70 °C | Excellent | Cold water, garden, chemicals | Heating, higher temperatures |
| Stainless steel AISI 316 | up to 200 °C | Excellent (even chlorides) | Solar, food industry, condensation | Common residential systems (cost) |
Compression fittings and wall plates for water fixtures
A special and, in practice, very frequent group of compression joints are connections for sink and bathtub faucets via so-called wall plates (or wall plate holders). A wall plate is a wall element into which a compression fitting is built, and the faucet is then attached directly to it. The advantage is a clean solution without visible piping and easy replacement of the faucet without demolition work.
There are two basic types on the market: a single and a double holder. The Universal Single Wall Plate Holder for Water is used to hold one supply pipe, while the Universal Double Wall Plate Holder for Water is designed for a two-pipe connection – hot and cold water simultaneously in one body. The double variant is standard for sink faucets, where you have two supply pipes (DHW + CW) running in parallel in the wall.
For choosing the right compression fitting for a wall plate, a simple rule applies: if the wall plate is designed for a G1/2" thread (which is the vast majority of bathroom faucets), you use a fitting with an external 1/2" thread. For 16 mm pipes, choose the 1/2x16 fitting with external thread. Further information comparing single and double holders can be found in the Knowledge Center in the article Universal Wall Plate Holder for Water – Single vs. Double and When to Use Which, and about their correct mounting on the wall in the article How to Correctly Mount a Wall Plate Holder on a Wall – Distances, Anchors and Position.
Pressure, temperature and certification – what to read in the technical data sheet
Every compression fitting has an operating pressure (PN – nominal pressure in bar) and operating temperature stated in its technical data sheet. For common residential heating installations, the maximum system operating pressure is usually 3–6 bar, and the maximum temperature is 70–80 °C for boiler heating and 95 °C for low-temperature systems. Most brass compression fittings are rated for PN10 (10 bar) or PN16 (16 bar), which covers common residential conditions with a large margin.
Certification for drinking water is also an important parameter. If you are installing a fitting into a drinking water system (not just heating), the fittings should be certified according to EN 15768 or have hygienic approval corresponding to the legal requirements of the given country. On the Slovak market, brass fittings certified by DVGW (German standard) or WRAS (British standard) are commonly sold, and both are accepted in the EU.
Typical practical situations and how to solve them
Let's go through several specific scenarios that appear regularly in practice – and where choosing the wrong fitting costs the customer time, money and stress.
Situation 1: Replacing a bathroom faucet – old wall plate, new pipes
A customer has an old bathroom faucet and wants to replace it with a new model. The original distribution was made of 15 mm copper pipes, the new distribution is in a 16 mm plastic pipe. The old plumber used copper elbows directly attached to the wall. The new faucet also has G1/2" supply threads. Solution: a wall plate with an adjusted position and a 1/2x16 compression fitting with external thread for the new plastic pipe. Reason: 15 mm copper and 16 mm plastic have the same 1/2" thread, only the pipe diameter and mounting method differ.
Situation 2: Connecting a boiler with 3/4" nozzle connections
Modern condensing boilers most often have G3/4" connections for the heating water inlet and outlet. The distribution in the apartment is made of a 20 mm multilayer pipe. Here, the correct choice is a 3/4x20 fitting with external thread, if the boiler has an internal thread. Watch out for one detail: in condensing boilers, the condensate is acidic (pH 3–5), which can aggressively affect ordinary brass. For condensate connections, use plastic or stainless steel fittings.
Situation 3: Underfloor heating – manifold and 16 mm pipes
16 mm plastic pipes are connected to a brass manifold. The manifold has an external G3/4" thread on each outlet, onto which the compression nut is screwed. Here you don't need a fitting with an external thread, but a fitting adapted to screw onto the manifold's external thread – that is, a fitting with an internal 3/4" thread and an opening for a 16 mm pipe. Mixing up external and internal threads is the most common beginner's mistake here.
Situation 4: Garden connection from the interior
Cold water is led to the garden from inside through a wall, using a 20 mm plastic pipe, with a G3/4" garden tap on the outside. Solution: a 3/4x20 fitting with external thread on the inside, and a ball valve or garden valve with an internal G3/4" thread on the outside. Material: standard brass is sufficient, as the cold water temperature won't exceed 30 °C even in summer.
Most common mistakes when choosing a compression fitting
Over years of practice, we keep encountering the same mistakes. Let's summarize them so you can avoid them:
- Mistaking inner diameter for outer diameter. A customer measures the inner diameter of the pipe and chooses a fitting based on that. Result: the fitting is too small (for a pipe with a 12 mm inner diameter, you need a fitting for a 16 mm outer diameter).
- Confusing 1/2" and 3/4" threads. Both threads "catch" on each other for the first two or three turns, which misleads the installer. When tightened, the thread gets stripped, or if left loose, it will leak.
- Using a zinc or cheap plastic fitting in heating. After one season, white deposits, corrosion, or a cracked fitting appear. Saving money on fittings simply isn't worth it.
- Not using a sealant. No thread seals on its own. Without Teflon tape, hemp fiber, or sealing compound, the joint will leak. You can find more about sealing materials in the Knowledge Center.
- Over-tightening. Brass bodies withstand reasonable force, but excessive torque with a wrench can strip the fitting's thread, crack the fitting body, or damage the sealing thread on a thermostatic head. Tighten firmly, not with excessive force.
- Ignoring flow direction. With some fittings (e.g. with an internal insert), the flow direction is determined by the design. Reversed installation can cause reduced flow or leaks.
If a leak or joint problem does occur, you'll find a detailed breakdown of causes and repair procedures in the Knowledge Center article Causes of Water Leaks at Compression Fittings – Most Common Mistakes and Their Repair.
Inspection and maintenance of compression joints
A compression joint isn't a "set it and forget it" solution. We recommend visually inspecting all accessible joints once a year (for example, at the start of the heating season). Watch for:
- White or rust-colored dust around the joint – a sign of a water leak that is evaporating.
- Moisture or water droplets under the nut or on the fitting body.
- Rust discoloration (even on brass – this may indicate corrosion of an adjacent steel component).
- Cracked seals at the joint with a faucet or valve (rubber O-rings age over time).
You'll find a detailed guide to regular inspection and maintenance in the Knowledge Center article Maintenance and Inspection of Compression Joints – When to Tighten, Replace or Service a Joint.
Frequently Asked Questions (FAQ)
Can I use a 1/2" fitting on a 20 mm pipe?
Normally not – a fitting labeled "1/2x20" would need a special insert for a 20 mm pipe combined with a 1/2" thread. Standard 1/2" fittings are designed for a 16 mm pipe. If you need a transitional combination, look for a reducing fitting or a direct reduced fitting with the appropriate matching size. An incorrect combination usually leads to leaks or pipe damage during tightening.
What is the difference between a compression fitting and a press fitting?
A compression fitting is detachable – you tighten it with a nut and loosen it with a nut. A press fitting is installed with a special pressing tool, and the joint is permanent – you can't remove it without damage. Compression fittings are cheaper to purchase (they don't require a pressing tool), but if incorrectly installed or if the seal is damaged, they can leak. Pressing offers a higher degree of reliability but assumes correct pressing technique and the correct tool caliber.
Do I need to drain the whole system when replacing a fitting?
Yes, usually you do. When replacing a compression fitting on a closed circuit (for example, heating), you need to close the supply valves and partially or fully drain the relevant branch. The exception is if you have ball valves on each section, allowing you to isolate a small section. You can find more about the installation procedure in the Knowledge Center article Installing a Compression Fitting on a Pipe Step by Step.
Should I wrap the thread with Teflon or hemp fiber?
It depends on the application. Teflon tape is easier to apply, chemically inert, and suitable for most joints, including drinking water. Hemp fiber (with sealing paste) is more traditional, and for larger diameters (3/4" and above) it provides a more reliable sealing bed, as it fills in even smaller thread irregularities. For brass G-threads, Teflon is just as reliable as hemp – it's more a matter of habit and availability. You'll find a detailed comparison in the Knowledge Center article Sealing Materials for Compression Fittings – Teflon, Hemp Fiber or Gasket.
How can I tell if a fitting is brass and not zinc?
Brass has a characteristic golden-yellow color. Zinc alloys (Zamak) tend to be duller, more grayish, sometimes with a chrome or nickel-plated surface finish. The most reliable test is the price and the description in the technical data sheet – brass CW617N or DZR will always be explicitly stated. If a product is described only as "alloy" without further specification, it is probably a zinc alloy. A significantly lower price compared to comparable brass fittings is also a warning sign.
What wrench do I need to tighten a compression fitting?
For home installations, an adjustable wrench (pipe wrench) or a fixed open-end wrench of the appropriate size is sufficient. The nut of a 1/2" fitting is usually 22–24 mm wide, and the nut of a 3/4" fitting is usually 27–30 mm wide. Always use two wrenches – one holds the body of the fitting or pipe, the other tightens the fitting's nut. This prevents the whole joint from twisting during tightening and disturbing the seal.
Conclusion – how to proceed when choosing
Choosing the right compression fitting isn't complicated if you proceed systematically. Always start with the pipe – measure the outer diameter with a caliper. Based on the diameter, determine the thread size (16 mm → 1/2", 20 mm → 3/4"). Then determine whether you need an external or internal thread, depending on what's on the other side of the joint. Check the system's temperature and pressure, and choose the material accordingly – for heating and DHW, always reach for brass, not zinc alloy or plastic fittings unsuitable for heat.
Buying fittings to save a few cents per piece will show up after a few years in leaks, corrosion, and unnecessary repairs. Investing in proven brass fittings of the correct size is minimal compared to the cost of repairs. You'll find an overview of the available compression fittings and wall plate holders in the full range at atria.sk/skrutkove-spoje/, where you can compare dimensions, materials, and suitable combinations for your installation.
If you're unsure about the choice for a specific type of installation, or you have an unusual combination of pipes and fittings, we also recommend reading the article Frequently Asked Questions About Compression Fittings and Wall Plate Holders – Threads, Dimensions, Installation in the Knowledge Center, where you'll find answers to more specific practical cases.
Do you have a question on this topic?
Can't decide or are dealing with a specific situation in your home? Write to us - we're happy to help.