What pipe diameter to choose for underfloor heating (16, 17, 18, 20, 25 mm)

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What pipe diameter to choose for floor heating: 16, 17, 18, 20 or 25 mm?

One of the first and at the same time most important decisions when designing floor heating is the selection of the correct pipe diameter. At first glance, it seems simple – after all, the difference between 16 and 20 mm is just a few millimeters. In practice, however, this seemingly small detail affects the hydraulic performance of the entire system, the volume of water in the circuit, heat losses, the speed of floor heating, material costs, and pumping energy. Over the years of working on projects, we have seen many cases where the investor or installer chose the diameter "by eye" – and the result was either a poorly heated house or an oversized system with a noisy circulation and unnecessarily high consumption. This article will give you concrete answers: when to reach for 16, 17, 18, 20 or 25 mm, what influences the choice and how to avoid the most common mistakes.

Basic overview – what the individual diameters mean in practice

Before we get into the details, it is important to clarify what dimensions actually exist on the market and what is hidden behind the numbers. The pipe size is given as the outer diameter × wall thickness, for example 16×2 or 20×2. The internal diameter (bore), which determines the hydraulic properties, is obtained by simple subtraction: 16 − 2 − 2 = 12 mm, or 20 − 2 − 2 = 16 mm. It is precisely the internal diameter that determines how much water can flow through the pipe at a given pressure and speed.

The most commonly used dimensions in European floor heating practice are:

16×2 mm – very common abroad (Germany, Austria), less common here
17×2 mm – de facto standard for Slovak and Czech markets for ordinary family houses
18×2 mm – less typical, appears in some multi-layer pipe systems
20×2 mm – standard for larger circuits, industrial halls, public buildings, or long loops
25×2,3 mm – distribution pipe, manifold, connecting branch; used as a heating loop under the floor only exceptionally

Hydraulics in short: why the diameter matters

Flow (amount of water flowing through per unit of time) increases with the fourth power of the internal radius of the pipe – this is the Hagen-Poiseuille law. Simplified: doubling the internal diameter results in a sixteen times higher flow at the same pressure drop. This is why increasing the diameter from 12 to 16 mm (i.e., from 16×2 to 20×2 mm) dramatically changes the hydraulics of the circuit.

For floor heating, the following parameters are key:

Water flow speed – the recommended range is 0.2 – 0.5 m/s; lower speed leads to poor venting and sludge, higher speed to noise and erosion
Pressure loss per meter of length – in a properly designed system, it should not exceed 150 – 200 Pa/m in the loop
Total pressure loss in the loop – the optimum for heating circuits is 15 – 30 kPa; at higher values, you need to use a more powerful circulation pump
Maximum loop length – for 17×2 mm, typically max. 80–100 m, for 20×2 mm up to 120 m

From the graph it is clear that at the same flow, the 17×2 pipe has significantly higher hydraulic resistance than the 20×2, especially at flows above 2 l/min. It is precisely here that problems arise in practice: if the loop is long and the diameter is small, the pump is not sufficient and some rooms remain cold.

16×2 mm pipe – where it has its place

The 16×2 mm diameter is common in German-speaking countries and in some dry-laid floor heating systems (system boards with grooves). In Slovakia, you will encounter it mainly in documentation for imported systems or when using special profiled profiles for renovation floor heating with low structural height.

Main advantage: the smaller outer diameter allows for a thinner embedding in anhydrite or concrete – sometimes only 4 cm screed layer is needed instead of 5–6 cm. This is relevant in renovations, where every centimeter of floor height matters. Disadvantage: higher hydraulic resistance compared to 17×2 mm (the seemingly small difference in inner diameter of 12 vs. 13 mm has a measurable impact on pressure loss in long loops in practice).

Practical scenario: Renovation of an apartment in a panel building, where the investor does not want to lose door height. Loops are short (30–50 m), circuits smaller – here 16×2 mm works without problems.

Pipe 17×2 mm – Slovak standard for single-family homes

This is by far the most popular size on the Slovak market for new single-family homes and apartments. Why exactly 17×2? The combination of a suitable inner diameter (13 mm), reasonable wall thickness and well-established PEX-a production makes it an ideal compromise between hydraulics, price and flexibility. Moreover, most distribution manifolds available on the Slovak market are dimensioned exactly for this size.

In practice: for an isolated single-family home with a room heat loss of 50–80 W/m² and a spacing of 15 cm, 17×2 mm is sufficient for loops up to 80 m in length. For a standard room of 15–20 m², this corresponds to one loop per room with flow rates of 1.5–2.5 l/min. These values are very well manageable by standard three-speed circulation pumps hydraulically.

On atria.sk you can find this pipe in several packages according to the size of the project: PEX Pipe 17×2 – 120 m for a smaller apartment or an additional room, PEX Pipe 17×2 – 240 m for a standard single-family home with 3–4 rooms, and PEX Pipe 17×2 – 600 m for larger projects, where it pays off to have a stock for the whole construction. Larger packaging is always more cost-effective per meter – if you can estimate the total consumption in advance, do not buy multiple smaller reels.

When is 17×2 mm not sufficient?

With a long loop (over 90–100 m) or with increased flow (e.g., due to higher thermal output required for a room), 17×2 mm starts to hit its limits. Pressure loss increases non-linearly, the pump operates at an unsuitable point, and the loop may be hydraulically unbalanced compared to shorter loops on the same manifold. In such cases, either shorten the loop (connect two shorter ones instead of one long one) or use 20×2 mm.

Pipe 18×2 mm – a speciality for some multi-layer systems

The size 18×2 mm is less common, but you will encounter it in some multi-layer (PEX-AL-PEX or PE-RT/AL/PE-RT) systems of specific manufacturers, where 18 mm outer diameter is the standard for their product range. Hydraulically, it is very close to 20×2 mm – the inner diameter is 14 mm. Connecting materials (fittings, clips) are typically system-bound, so they are not always compatible with 17×2 or 20×2. If you are considering this size, always check the availability of fittings and manifolds for that diameter.

Pipe 20×2 mm – for long loops, halls and demanding projects

Twenty is a natural choice when the project exceeds the capabilities of seventeen. An inner diameter of 16 mm means significantly lower hydraulic resistance – at the same flow rate, pressure loss is 40–60% lower. This is especially noticeable in these situations:

Long loops over 90–100 m (e.g., in large rooms or halls)
Industrial and commercial buildings with large areas (warehouses, showrooms, restaurants)
Low-temperature systems with heat pumps, where it is important to minimize losses
Systems with gravitational or limited pump pressure
Floor heating in garages, where larger spacing and longer loops are common

PEX Pipe 20×2 – 200 m is a suitable package for a larger project or several long loops. An important practical detail: 20×2 mm is harder to bend manually in narrow spacing of 10–15 cm – the minimum bend radius for PEX is typically 5× outer diameter, i.e., at least 100 mm. This can be a problem with a dense snake-like pattern – a pipe heater or special bending tool is needed. That is why most installers in apartments and smaller houses prefer 17×2 mm, where manual laying is much easier.

Pipe 25×2.3 mm – distribution, not loops

Twenty-five is a distribution pipe. In the heating system, it is used to connect manifolds to the boiler, for main distribution branches between floors, or for collector lines. It is almost never laid into the floor as a heating loop – it is too rigid, thick and difficult to bend into a dense pattern. You will find it, for example, as a supply and return line between the technical room and the manifold, installed in a protective sleeve or groove in the wall.

If your project requires long distribution lines (e.g., from the boiler in the basement to the manifold on the upper floor, more than 10–15 m), PEX Pipe 25×2.3 – 200 m provides sufficient capacity and minimal pressure loss on the supply line.

Comparison table: which diameter for which project?

Dimension	Inner diameter	Maximum loop length	Typical use	Note
16×2 mm	12 mm	60–70 m	Renovations, low structural height, dry-laid systems	Higher resistance, shorter loops
17×2 mm	13 mm	80–100 m	Single-family homes, apartments, new builds – universal standard for SK/CZ market	Best price/performance ratio
18×2 mm	14 mm	90–110 m	Some multi-layer systems of specific manufacturers	Check fitting compatibility
20×2 mm	16 mm	100–120 m	Halls, larger commercial spaces, long loops, garage heating	Harder manual laying
25×2.3 mm	19.4 mm	–	Distribution pipe, supply lines to manifolds	Not for heating loops

How Diameter Affects Floor Layer Thickness

The floor construction must cover the pipe with its load-bearing layer (self-leveling compound, anhydrite, concrete) to ensure minimum coverage. The recommended minimum coverage above the pipe surface is 30–45 mm (depends on the specific manufacturer and standard). The total minimum thickness of the load-bearing layer is therefore the outer diameter + coverage:

16 mm pipe: min. 46–61 mm load-bearing layer
17 mm pipe: min. 47–62 mm load-bearing layer
20 mm pipe: min. 50–65 mm load-bearing layer
25 mm pipe: min. 55–70 mm load-bearing layer (almost never used in loops)

The difference between 16 and 20 mm is only 4 mm in floor thickness – in new construction, it is practically negligible. In renovations, where you are fighting for every centimeter, it can be decisive, and here it makes sense to consider 16 mm instead of 17 mm.

Impact of Diameter on Price and Material Consumption

A larger diameter does not automatically mean a more expensive project – it depends on the total length and type of system. A kilogram of PEX is a kilogram of PEX regardless of the diameter, but with 20×2 mm you will consume more material per meter of length than with 17×2 mm (more plastic for the same length). For a project with a total loop length of 400 m, you can expect a price difference of 10–20 % between 17×2 and 20×2 mm for the pipe alone. In addition, you need to add the manifold and fittings, which must be dimensioned for the selected diameter – the differences can be greater here. Always calculate the system as a whole.

Another thing that investors often underestimate: a larger volume of water in a larger diameter pipe prolongs the heating time of the system after a cold start. For 17×2 mm and 100 m loop, the water volume is 13.3 liters, for 20×2 mm and 100 m it is 20.1 liters. This difference is not dramatic with modern boilers with continuous regulation, but with older systems with simple thermostatic control, it can affect the response time.

Pressure and temperature: PN6 and PN10 in the context of pipe diameter selection

All mentioned diameters are produced in various pressure classes. The designation PN6 means a nominal pressure of 6 bar at 90 °C, PN10 means 10 bar at 60 °C. For standard floor heating (operating temperature 35–55 °C, system pressure 1.5–3 bar), both pressure classes are more than sufficient with a large safety margin. Choosing between PN6 and PN10 is therefore usually not a reason to change the diameter – you choose the diameter according to hydraulics, not pressure. A more detailed discussion of this topic is covered in the article "Which pressure and temperature (PN6, PN10) suit my floor heating system?" in our Knowledge Center.

Choosing the diameter step by step: practical procedure

The procedure in practice looks like this: First, you prepare or have prepared a thermal calculation of the building – without it, you are working blindfolded. Then, for each room, you calculate the length of the loop (a detailed procedure can be found in the article "How to calculate the length of pipe needed for floor heating"). If the length comes out to be 80–90 m, go for 17×2 mm. If it is a larger space or a garage with a long loop over 100 m, use 20×2 mm. Dimension the distribution lines (from the boiler to the manifold) separately – there you usually use 25×2.3 mm or the same diameter as the loops, depending on the distance.

Common mistakes in choosing the diameter and how to avoid them

Mistake No. 1: One diameter for the whole house regardless of loop lengths. We have seen houses where someone laid 17×2 mm for a garage with one loop 110 m long. Result: permanently cold floor in the garage and an overloaded pump. Solution: either shorten the loop into two shorter ones, or use 20×2 mm for the garage.

Mistake No. 2: Too small a diameter due to "savings". Sometimes we come across cases where someone uses 16×2 mm where 17×2 mm should be used, with the argument that it is cheaper. The price difference in the pipe is minimal, but the difference in pressure losses and system performance is real – especially with long loops.

Mistake No. 3: Using 25×2.3 mm as a heating loop. It has happened. Someone had leftover distribution pipe and decided to lay it in the floor. Result: inflexible, impossible to make a dense pattern, the thermal performance of such a "snake" in the floor is unpredictable and the hydraulics are catastrophic.

Error No. 4: Inconsistent diameter in the same system without balancing. If you have 17×2 and 20×2 mm loops on one manifold, the hydraulic resistance of each loop will be different and without proper balancing using flow control valves or flow regulators, the 20×2 mm loops will take water at the expense of the 17×2 mm loops. The system must be hydraulically balanced.

To learn how to avoid failures and leaks caused by incorrect installation, read the article „Common failures and leaks in floor heating piping: causes and solutions" in the Knowledge Centre.

Pipe material and its relation to diameter

When choosing the diameter, you should also consider whether it is PEX or multilayer pipe. PEX (cross-linked polyethylene) is more flexible and easier to bend at the same outer diameter – this is an advantage when laying a dense pattern (spiral). Multilayer pipe (PEX-AL-PEX, PE-RT/AL/PE-RT) is stiffer due to the aluminum layer, but holds its shape better and has lower thermal expansion. For 17×2 mm, PEX can be bent with a radius of 7–8× the outer diameter without heating (i.e. approx. 120 mm), while 20×2 PEX requires a larger radius or the help of a heat gun for manual bending.

A detailed comparison of materials can be found in the articles „How to choose pipe for floor heating: PEX vs. multilayer" and „PEX pipe vs. multilayer pipe: differences, advantages and disadvantages" in the Knowledge Centre.

Real examples of projects: how we chose the diameter

Project A – Single-family house 140 m², new construction, 5 rooms: Heat loss approx. 50 W/m², spacing 15 cm. Loops 60–85 m. Choice: 17×2 mm for all rooms. 5-loop manifold, circulation pump Grundfos UPS 25-60. The system works without problems with flow rates of 1.8–2.4 l/min per loop.

Project B – Car showroom, 400 m², reinforced concrete slab: Heat loss approx. 40 W/m² (new facade insulation). Loops designed for max. 110–120 m (6 loops). Choice: 20×2 mm for loops, 25×2.3 mm for distribution piping from the boiler. Result: flow per loop 3.2 l/min, pressure loss per loop 22–26 kPa – within the norm.

Project C – Renovation of an apartment in a panel building, 65 m², 3 rooms: Construction height limited (old floor was milled, new max. +4 cm). Choice: 16×2 mm dry-laid into a system slab with milling. Loops 35–50 m. Result: works, floor heats to the desired temperature, height balance OK.

Project D – Single-family house with garage, 180 m² + 40 m² garage: House: 17×2 mm. Garage: one loop 95 m, the investor wanted to keep the same diameter. After calculating pressure loss (approx. 190 Pa/m at 2 l/min), it was clear that 17×2 mm was borderline. Decision: divide the garage into two loops of 48 m each or use 20×2 mm. The investor chose two shorter loops of 17×2 mm – a cheaper manifold was not worth it, he paid for a second loop on the manifold. In this case, a cleaner solution would have been 20×2 mm for the entire garage and one loop on the manifold.

Most frequently asked questions (FAQ)

Can I combine 17×2 mm and 20×2 mm on one manifold?

Technically yes, but you must ensure hydraulic balancing. Loops with 20×2 mm have significantly lower resistance and without flow control valves, they will take disproportionately more water. Each loop on the manifold must have an adjustable flow meter (flowmeter) or control valve to set the required flow according to the thermal output of each loop. Without this balancing, the system will be hydraulically unbalanced.

What is the maximum length of a loop for 17×2 mm pipe?

In practice, a safe limit is 80–100 m at normal flow rates of 1.5–2.5 l/min. At 100 m and 2 l/min, the pressure loss per loop is approx. 170–190 Pa/m, which is at the upper limit of the recommended range. If you have a larger area, it is better to divide into two shorter loops or switch to 20×2 mm. For precise calculations, consult a designer or use hydraulic calculators from system suppliers.

Is 17×2 mm the same as 16×2 mm – are the fittings interchangeable?

No, these are different dimensions and the fittings are not interchangeable. The outer diameter of 16 mm vs. 17 mm is a real physical difference. Fittings (compression, crimped, Euroconic) are manufactured for specific outer diameters. Never use fittings designed for 16 mm on 17 mm pipe and vice versa – the connection will be leaky or unreliable and a leak under the screed layer is likely.

Can I use 25×2.3 mm pipe as a heating loop in the floor?

We do not recommend it. 25×2.3 mm pipe is intended for distribution piping, not for heating loops. It is too stiff and the minimum bending radius is very large, so it is not possible to create a functional serpentine pattern at a reasonable spacing. In addition, the water volume in such a loop would be so large that the entire system's hydraulics would be difficult to control. Use it for supply and return piping, and always use 16, 17 or 20 mm for loops.

Will a change in diameter affect the choice of manifold or pump?

The diameter of the loop pipe itself does not directly determine the choice of manifold – most distribution manifolds have standard inlet sizes via adapters. The pump, however, does – with a larger diameter (20×2 mm) and long loops, the total pressure loss in the system is lower, so you can use a less powerful (= more economical) pump. Conversely, small diameters and long loops require a pump with a higher pressure range.

How can I determine how many meters of pipe I need in total?

Total length depends on the area of the rooms, the spacing of the loops, and the length of the supply and return lines to the manifold. A rough formula: (room area / spacing in meters) + 2× distance from the manifold + 10–15 % reserve. For example, a 20 m² room at 0.15 m spacing: 20 / 0.15 = 133 m + supply lines. A detailed calculation procedure can be found in the article „How to calculate the length of pipe needed for floor heating" in the Knowledge Centre.

Conclusion: the golden rule for choosing diameter

For the vast majority of new constructions in Slovakia – single-family houses and apartments – 17×2 mm is the correct and proven choice. It provides an excellent balance between hydraulic performance, ease of installation, availability of fittings, and price. For loops longer than 90–100 m, industrial halls or garages, go for 20×2 mm. For renovations with limited structural height, use 16×2 mm and dry-laid system slabs. Design distribution piping in 25×2.3 mm.

Never choose the diameter just by eye or based on what happens to be in stock. Always start from the loop lengths, room heat losses, and hydraulic calculations. If you are unsure, consult – a small mistake in the design is very difficult and expensive to fix under the concrete. The correct pipe diameter is the foundation on which the entire performance and comfort of floor heating rests for decades.

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