Frequently Asked Questions about Heating Pipes: Diameters, Fittings, Temperatures and Installation

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Frequently Asked Questions about Heating Pipes: Diameters, Fittings, Temperatures and Installation

When someone starts a heating system renovation or designs a new heating system, they usually run into a number of questions that can't be answered just by looking at a catalog. What does PN10 mean? Why are there different diameters and when should you choose which one? Can I use plastic pipes where the water flows at 90 °C? How does a compression fitting work and can it be disassembled? These are real questions we receive from customers every day – from DIY homeowners to experienced installers dealing with a specific problem on a particular job.

That's why this article is put together as an extensive FAQ with deeper technical context – not dry questions and one-sentence answers, but real explanations with numbers, practical examples, and links to related topics that will help you understand what's behind each answer. If you're looking for an overview without details, check out our article How to Choose the Right Heating Pipe: Copper, Multilayer, or Plastic? If you're interested in a specific step-by-step installation, we also have an article Installing Multilayer Pipe Step by Step: Tools, Fittings, and Compression Fittings. Here, however, we'll focus on the common uncertainties – the things people most often get wrong or don't understand about the technical terminology.

1. What do pipe dimensions mean – outer diameter, inner diameter, wall thickness?

This is the basis where a huge portion of customer questions begin. Pipes on the market are labeled either by outer diameter (d × s, where d is the outer diameter and s is the wall thickness), or by nominal diameter (DN – Diamètre Nominal), which is a historically derived value approximating the inner diameter, but it's not the same thing.

In practice, this means, for example: multilayer pipe 16×2 has an outer diameter of 16 mm and a wall thickness of 2 mm. The inner diameter therefore comes out to 12 mm. When buying fittings or compression fittings, always go by the outer diameter – that's the decisive factor for the connection. A similar logic applies to copper pipes: copper pipes are also labeled by outer diameter, e.g. Cu 15×1 means an outer diameter of 15 mm and a wall thickness of 1 mm.

Important note: when ordering fittings or compression fittings, always state the outer diameter, not the inner diameter and not DN. Sellers are used to this, but customers most often get confused here – they come with a note saying "I need fittings for 12" meaning the inner diameter, but in the catalog those fittings are listed as "for pipe 16×2". The difference is fundamental, and purchased goods can't be returned.

2. What pipe diameter do I need – 16, 18, or 20 mm?

This is one of the most frequently asked questions, and the answer depends on several factors: the heating system's output, the length of the piping, the number of circuits, and the heat source used. There's a more detailed calculation guide in our article What Pipe Diameter Do I Need for My Heating System?, but here we'll give a practical overview based on common scenarios.

16×2 mm – the most common diameter for underfloor heating. Circuits up to 80–100 m in length, output of a single circuit up to approx. 1,500–2,000 W. Ideal for standard rooms in a house with standard insulation.
18×2 mm – an intermediate diameter. Usable for underfloor heating with longer circuits (100–120 m), or for higher output per circuit. It's also found in radiator piping in houses where a 16 mm pipe wouldn't provide sufficient flow.
20×2 mm – suitable for distribution piping, connecting manifolds, or for larger systems (apartment buildings, larger houses over 200 m²). Usually not used directly in underfloor loops, but in the "backbone" of the system.

From practice: we see designers who plan the entire house uniformly with 16 mm pipes to save on material and labor. This mostly works if the circuits are properly designed (up to 80 m, balanced). Problems occur when someone designs a 120-meter circuit in 16 mm pipe – the pressure losses are too high, the circulation pump can't keep up, and the room isn't heated evenly. In that case, switching to 18×2 mm or splitting the circuit into two shorter ones helps.

3. What is PN10 and how does it relate to the temperature and pressure of the medium?

The designation PN (from French Pression Nominale) expresses the nominal pressure in bars at which the pipe or fittings can safely operate long-term. PN10 therefore means 10 bar. In typical family house heating systems, the working pressure is 1.5–3 bar, so PN10 provides a sufficient reserve.

But watch out – pressure and temperature are related. Plastic and multilayer pipes state their nominal parameters at a reference temperature (usually 20 °C or 70 °C). At higher temperatures, the material's load-bearing capacity decreases. That's why manufacturers state a pair of values, for example, as with the multilayer pipe 20×2: PN10 at T=70 °C and PN10 at T=95 °C. This means the pipe can handle 10 bar at 70 °C, but also at 95 °C – thanks to the aluminum layer in the composite, which maintains the pipe's geometry even at higher temperatures.

Pure polyethylene or polypropylene softens significantly at higher temperatures and loses its pressure resistance – this is a fundamental difference compared to multilayer (composite) pipe with an aluminum layer. That's why, for example, in low-temperature underfloor heating (max. 45–55 °C), cheaper plastic pipes can also be used, but in radiator systems with outlet temperatures of 75–90 °C, multilayer pipe or copper is a necessity. You can find more on this topic in the article Pipe Temperature and Pressure: What Do PN10, T=70°C, and T=95°C Mean in Practice?

4. What is a compression fitting and when should it be used instead of press fitting or soldering?

A compression fitting is a type of mechanical fitting that seals without the use of adhesive, solder, or press tools. It works on the principle of deforming a ring (ferrule) around the pipe when the nut is tightened – the ferrule presses into the outer surface of the pipe and creates a hermetic joint.

When compression fittings are useful in practice:

Repairs and renovations – on existing copper pipes where you don't want or can't solder (nearby flammable structures, tight space).
Temporary joints or service points – if you want the option to disassemble the joint later.
Smaller installation scopes – where renting a press tool isn't worthwhile.
Combining materials – for example, transitioning from copper pipe to multilayer pipe.

For 15 mm diameter copper pipe, for example, the compression fitting 15×1-EK is suitable, designed specifically for copper pipes with an outer diameter of 15 mm and a wall thickness of 1 mm. For multilayer or PB pipe of the same diameter, a different type is used – for example, the compression fitting 15×1 EK for PB and copper pipe from Hepworth, where the ferrule and seat geometry is adapted to the properties of softer or stiffer material.

Important: a compression fitting is NOT suitable for concealed installations in walls or floors without an inspection opening. Every mechanical joint must remain accessible. This is also a legal requirement under STN EN 806. A press joint, on the other hand, is certified for concealed piping.

5. What are the differences between pipe types – PE-RT, PEX, multilayer, copper?

This is a long topic that we cover in detail in the article Multilayer vs. Copper Pipe: Comparison of Properties, Price, and Lifespan. Here we'll give a practical overview based on the most common customer questions.

PE-RT (Polyethylene of Raised Temperature resistance) – plastic pipe without cross-linking. It's flexible, suitable for underfloor heating at lower temperatures (up to 60 °C typically, up to 90 °C for special types). No shape memory, so it partially springs back after bending – anchoring elements are needed. Low price.

PEX (Cross-linked Polyethylene) – stronger, more resistant to higher temperatures and pressures. Shape memory – the pipe "remembers" its original shape, which makes bending more difficult, but this is an advantage for press fitting. Long lifespan, also suitable for radiator piping.

Multilayer (MLCP – Multi Layer Composite Pipe) – a combination of PE or PEX layers with an aluminum strip welded butt-to-butt or overlapped. The aluminum layer gives the pipe shape stability (bend it and it stays bent), prevents oxygen diffusion, and significantly increases pressure resistance at higher temperatures. Suitable for both underfloor and radiator heating, for low-energy systems as well as older boilers with higher outlet temperatures. Joined by press fitting or compression fittings.

Copper – a classic material with a long lifespan (50+ years), excellent thermal conductivity, resistance to high temperatures and pressures. Joined by soldering (hard or soft solder) or press fitting. Higher material and labor cost. Not suitable in systems with aggressive water (low pH); in mixed installations, there's a risk of corrosion when in contact with galvanically incompatible metals.

6. How are piping systems sized – main circuit, branches, underfloor loops?

A typical family house has a hierarchical piping structure. Understanding this hierarchy is key to selecting the right diameter for each section.

Typical sizing for a house with underfloor heating:

Boiler → manifold: copper or multilayer 22–28 mm, or possibly steel. The total boiler output flows here – if the boiler is 15 kW and the temperature difference is 10 K, you need a flow rate of about 1,300 l/h, which dictates a minimum diameter of 22 mm.
Manifold → room (so-called tertiary circuit): commonly 18–20 mm, if that branch's output is above 3–4 kW or the distance is over 10–15 m.
Underfloor loop: 16×2 mm, length ideally up to 80–90 m (max 100–120 m for 18×2). Longer circuits mean high pressure losses and problems balancing the flow.

For calculating the length of pipe needed for individual circuits, we recommend reading the article How Many Meters of Pipe Do I Need for a Heating Circuit: Calculation and Planning.

7. Can I combine different materials in one system?

Yes, but with caution. In practice, combining materials is common – for example, copper from the boiler to the manifold and multilayer pipe in the underfloor loops. There are a few principles to follow:

Galvanic corrosion: Copper and steel in direct contact in an oxygen-rich environment create a galvanic cell that corrodes the steel. That's why a transition piece made of a neutral material (bronze, brass) is inserted between copper and steel, or the system is degassed and treated with inhibitors. Aluminum is even more sensitive to copper – direct contact is unacceptable (e.g., aluminum panel radiators in a copper system without protective measures can crack within a few years).

Pressure and temperature limits: Fittings and compression fittings must be rated for the maximum parameters of the entire system, not just the section where they're installed. If your system operates at 80 °C, every component – including fittings, seals, sleeves – must be able to handle this temperature.

Oxygen diffusion: Plastic pipe without an oxygen barrier (EVOH layer or aluminum layer) lets oxygen pass into the water, which accelerates corrosion of steel and cast iron components (pumps, boilers, radiators). If your system has such components, use only pipes with an oxygen barrier – multilayer IVAR Turatec pipe has this barrier thanks to its aluminum layer.

8. How to properly bend multilayer pipe and what's the minimum bending radius?

Multilayer pipe is bent by hand or with a bending spring, or with a mechanical bender. The aluminum layer gives it shape stability – the shape stays after bending. This is an advantage (fewer fittings, a smoother route) but also a disadvantage (a pipe once kinked can't be fully straightened).

Minimum bending radii (approximate values for IVAR Turatec or similar composites):

16×2 mm: minimum bending radius approx. 80–100 mm (curve diameter ~160–200 mm)
18×2 mm: minimum bending radius approx. 90–110 mm
20×2 mm: minimum bending radius approx. 100–130 mm

If you don't maintain the bending radius, the aluminum layer can become oval or crack. The pipe may appear undamaged, but the inner aluminum layer has microcracks, which become a source of corrosion and long-term weakening. Therefore, in hard-to-reach places (e.g., behind a bathtub, in corners), it's better to use fittings rather than try to make a sharp bend.

9. How is a compression fitting installed – step by step?

A detailed procedure is in the article Installing Multilayer Pipe Step by Step: Tools, Fittings, and Compression Fittings and also in How to Properly Connect Copper Pipe Using a Compression Fitting without a Water Leak? Here we'll give brief steps focused on the most common mistakes:

Cutting the pipe: Always use a pipe cutter (the wheel type), not a saw. The cut must be perpendicular and free of burrs. A slanted cut or burrs will cause leaks.
Sliding on the nut and ferrule: Before inserting the pipe into the fitting, first slide on the nut (backwards – thread facing the fitting), then the ferrule. The order is critical – once the pipe is inserted, you can't change it.
Inserting into the fitting: Insert the pipe fully into the fitting's seat until it stops. If it doesn't seat fully, the joint will leak or the ferrule will deform incorrectly.
Hand-tightening: First tighten the nut by hand until it starts to resist.
Tightening with a wrench: Tighten with a wrench 1.5 to 2 turns beyond hand-tight. Don't overtighten – the ferrule metal will deform, causing leaks or cracking.
Pressure test: Always perform a pressure test before covering (1.5 times the working pressure, for at least 30 minutes).

The most common mistake in practice: the customer doesn't insert the ferrule before inserting into the fitting, or inserts it backwards. The second most common mistake: the pipe isn't fully inserted – this isn't visible after tightening, but the joint leaks from day one.

10. What are the signs and causes of heating pipe leaks?

Leaks manifest either as visibly dripping water (if the joint is accessible) or as indirect signs: system pressure drops, the expansion tank loses pre-charge quickly, the boiler needs refilling more and more often. With concealed piping in the floor, it can show up as moisture in flooring, laminate boards lifting, or mold in an adjacent wall.

Common causes:

Improper tightening of the compression fitting (too little or too much)
Damaged or missing seal on press fittings
Corrosion from galvanic contact between incompatible materials
Material fatigue from exceeding temperature limits
Mechanical damage during construction work (e.g., drilling into the floor)
Water freezing when the system is shut down in winter without draining or using glycol

You can find more on diagnostics and repairs in the article Common Faults and Leaks in Heating Pipes: Causes and Repairs. Regular inspection is discussed in the article Maintenance and Inspection of Heating Pipe Systems: What to Watch for Each Season?

Frequently Asked Customer Questions (FAQ)

Can I use 16×2 multilayer pipe for radiator heating too (not just underfloor)?

Yes, multilayer pipe 16×2 is certified for temperature parameters T=70 °C and T=95 °C at PN10, so technically it's also suitable for radiator piping. In practice, it depends on sizing – if the radiator's output and the distance from the manifold are such that the flow in a 16 mm pipe would cause too high a pressure loss or a flow velocity above 1 m/s, it's better to use 18×2 or 20×2 mm. Multilayer pipe IVAR Turatec 16×2 is actually used for radiator connections in apartment renovations, running from the underfloor manifold to individual radiators over distances up to 10–15 m.

How many years does multilayer pipe last in the floor?

Manufacturers guarantee a lifespan of 50 years when operating parameters are maintained (PN10, T max 95 °C). In practice, the first generations of multilayer pipes installed in the 1990s are still functioning, which corresponds to 30+ years without problems. The condition is proper installation, maintaining the bending radius, absence of mechanical damage, and good water quality or regular chemical treatment of the system.

Do I need to use Teflon tape or sealant when installing a compression fitting?

No. A compression fitting seals via the ferrule – mechanical pressure on the pipe wall. Teflon tape or sealant on the thread is an incorrect procedure and can even interfere with the proper function of the ferrule (preventing it from fully seating). Teflon is used only on threaded joints (e.g., a ball valve into a threaded socket), where the seal is made on the thread, not on the ferrule.

Which is better – a press joint or a compression joint for multilayer pipe?

A press joint is faster, suitable for concealed piping, requires no tightening, but requires special press tools (starting from €200 for a basic set). A compression joint is slower, requires more care during installation, must not be concealed without an inspection opening, but requires no special tools. For professional installation on a larger scale, press fitting is more cost-effective. For a small number of joints (e.g., connecting 3–4 radiators during a renovation), a compression fitting is the faster and cheaper choice.

Can multilayer pipe make noise during operation?

Yes, and it's one of the most common complaints without an actual product defect. Multilayer pipe has greater thermal expansion than copper – it moves as it heats and cools. If it isn't routed freely (e.g., a sleeve fits too tightly on the pipe, or an anchor is too rigid), it will make cracking or clicking noises during temperature changes. Solution: protect wall penetrations with a sleeve of a diameter larger than the pipe, leave anchor clips with slight clearance (not rigid clamps without a sliding insert), and add expansion loops on long straight sections over 5–6 meters.

How long can a single underfloor loop be with 16×2 pipe?

The recommended maximum length of a single loop with 16×2 pipe is 80–90 meters (with a standard circulation pump and manifold setup). Some projects go up to 100–120 m, but then the pressure loss increases significantly, and it's difficult to hydraulically balance the system against shorter circuits. For longer loops, switch to 18×2 mm, where pressure losses at the same flow rate are significantly lower thanks to the larger inner diameter.

Conclusion: Choosing the Right Pipe and Fittings – What Not to Forget

Heating pipe isn't a commodity where "one is as good as another" applies. Every parameter – diameter, wall thickness, material, temperature class – has its own logic and a real impact on the functionality and lifespan of the entire system. From experience with dozens of projects, we see that the most common problems aren't caused by bad material, but by an unsuitable choice of parameters or installation mistakes that could easily be avoided.

If you're unsure about your choice, we recommend proceeding as follows: first determine the maximum operating temperature of the system, then the output and lengths of the circuits, and only then choose the diameter and material. During installation, make sure to use the right tools, follow the correct order of steps, and always perform a pressure test before covering the piping. If you're working with an installer, provide them with design documentation – any guesswork "on site" is more expensive than an hour of a designer's work in advance.

For a complete overview, we recommend browsing the other articles in this section of the Knowledge Center – each one covers one specific part of the topic in depth, so together they form a comprehensive manual for anyone undertaking a heating installation or renovation.

Have a question about this topic?

Can't decide, or are you dealing with a specific situation in your home? Write to us - we'll be happy to help.