What pipe diameter and spacing do I need for underfloor heating?

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What pipe diameter and spacing do I need for floor heating?

This question is among the most common we address when designing floor heating systems. And worse yet – it is also among those where the most mistakes are made in practice. Designers, installers, and DIY builders often think that choosing the pipe diameter and determining the spacing are just some "recommendations" that can be improvised and adjusted according to the situation. They are not. The diameter and spacing of the pipe directly determine how much heat your floor can deliver, what the surface temperature of the floor will be, what the pressure in the system will be, and ultimately – whether you will have heat or just an expensive concrete surface through which warm water flows.

In this article, we will look at the entire issue in depth: from basic physical principles through specific calculation methods to practical scenarios we encounter in real customer projects.

Why the pipe diameter is a key parameter

Floor heating works on the principle of radiant heat – warm water flows through pipes embedded in anhydrite or concrete, the floor slab is heated evenly, and heat radiates into the room. The entire system is designed so that the surface temperature of the floor does not exceed certain limits: in living rooms it is 29 °C, in bathrooms and hygienic areas 33 °C, and in edge zones (along the room perimeter) up to 35 °C. These values are not just comfort recommendations – they are hygiene standards established in the European standard EN 1264.

The pipe diameter directly affects:

Hydraulic resistance of the circuit – the thinner the pipe, the higher the resistance, the more powerful the pump you need
Maximum circuit length – a diameter of 16 mm allows longer circuits than a diameter of 14 mm at the same hydraulic resistance
Thermal output – the internal diameter of the pipe determines the water flow, which is directly related to the amount of heat transferred
Flexibility during installation – thinner pipe is easier to bend, but has lower mechanical resistance

Standard pipe diameters – overview and use

On the Slovak and European markets, the following outer diameters are most commonly used for floor heating: 14 mm, 16 mm, 17 mm, 20 mm, and 25 mm. In terms of material, PE-Xb (crosslinked polyethylene), PE-Xa, PE-RT, and PEX/AL/PEX (multilayer) dominate. Each diameter has its optimal application.

Pipe 14 mm (14×2 mm)

An outer diameter of 14 mm with a wall thickness of 2 mm (internal diameter thus 10 mm) is intended for smaller areas with higher required output. Typical use: bathrooms, toilets, entrance halls, where the area is small and the required output per m² is relatively high. The advantage is a much smaller bending radius – usually 5× the outer diameter, i.e. 70 mm. This allows tight loops without the need for auxiliary bends. The disadvantage is higher hydraulic resistance: the maximum recommended circuit length is 60–80 m.

Pipe 16 mm (16×2 mm) – the golden standard

This is by far the most used size in residential construction. Outer diameter 16 mm, wall thickness 2 mm, internal diameter 12 mm. The maximum recommended circuit length ranges from 80 to 120 m, with the optimum being around 80–100 m at standard spacing. 16 mm pipe is flexible enough for a spacing of 100 mm without the need for auxiliary fittings, while at the same time having acceptable hydraulic resistance. For most apartments and single-family homes, this is the number one choice.

Pipe 17 mm (17×2 mm)

A less common size that you will mainly find in some foreign-made system slabs. Hydraulically it behaves similarly to 16 mm, the installation is identical. If you come across it, there is no need to be afraid – just check the compatibility with the manifold.

Pipe 20 mm (20×2 mm)

Designed for larger areas and industrial applications. In a single-family home, you will see it exceptionally – for example, if the investor wants fewer circuits on a larger area (or in low-energy construction with low temperature differences, where a larger diameter compensates for a longer circuit). The maximum circuit length can reach 120–150 m. Disadvantage: a larger diameter requires a thicker anhydrite layer to cover it, at least 45 mm above the top surface of the pipe.

Pipe 25 mm and larger

This already belongs to the category of industrial floors – production halls, large greenhouses, parking garages with heating to prevent freezing. Irrelevant for standard construction.

Pipe spacing – the basis of thermal performance

Pipe spacing (also known as the axis distance or raster) is the distance between the centers of two adjacent pipe loops in the floor. It is a parameter that has a direct and fundamental impact on the thermal performance of the floor per m². Common spacing values are: 75 mm, 100 mm, 125 mm, 150 mm, 175 mm, 200 mm, 250 mm and 300 mm.

The basic rule is simple: the smaller the spacing, the higher the thermal performance per m², but at the same time higher pipe consumption and higher floor surface temperature at the same temperature drop of the heating water.

In practice, we most often encounter these spacings and their typical applications:

75 mm – edge zone near external walls and French windows, where it is necessary to compensate for higher heat losses; it is also used in very well insulated bathrooms with tiles
100 mm – standard spacing for bathrooms, corridors, and entrance areas with higher performance; also popular in houses with poor insulation of the building envelope
150 mm – the golden mean for most living rooms in standard new buildings; it provides good performance without the risk of surface overheating
200 mm – suitable for low-energy and passive houses, where heat losses of the rooms are low and a smaller floor-specific performance is sufficient
250–300 mm – used in passive houses with an excellent thermal envelope, or as a backup system in combination with another heat source

How to calculate the correct spacing for a specific room

The calculation is not rocket science, but it requires knowledge of several input values. Let's go through it step by step using a real example.

Input values you need to know

Heat loss of the room Q [W] – calculated by an energy assessment or designer; in practice, it ranges from 30 to 100 W/m² for standard buildings
Required specific thermal performance of the floor q [W/m²] – q = Q / room area
Temperature drop of the heating water – typically 35/28 °C or 40/32 °C for heat pumps, 45/35 °C for condensing boilers
Type of floor covering and its thermal resistance Rλ,B [m²K/W] – tiles have Rλ,B ≈ 0.01–0.02, carpet 0.10–0.15, solid floor 0.07–0.10

Practical example – living room in a new building

Let’s take a living room with an area of 25 m² in a new building with medium insulation. The heat loss of the room is 1,250 W, so the specific performance q = 1,250 / 25 = 50 W/m². Temperature drop 40/32 °C (average water temperature 36 °C). Floor covering: ceramic tiles with Rλ,B = 0.01 m²K/W.

From the nomograms of the EN 1264 standard (or from the manufacturer's software) under these conditions, a spacing of 150 mm with 16 mm pipe is ideal – the floor will provide approximately 50–55 W/m² with a surface temperature of around 26–27 °C. Comfortable, efficient, and safe.

If we choose a spacing of 200 mm, the performance would drop to approximately 38–42 W/m² – insufficient for this room. If we choose 100 mm, the performance would rise to 65–70 W/m² – unnecessarily overheating the floor and consuming more pipe.

Practical example – tiled bathroom

Bathroom 6 m², heat loss 360 W, specific performance 60 W/m². Floor covering: tiles. A logical choice here is a spacing of 100 mm with 14 mm or 16 mm pipe. Short circuit (~30–40 m), sufficient performance, tiles are comfortably heated to 28–30 °C. It makes sense to consider 14 mm pipe in bathrooms, as the small area requires a minimum bending radius at a spacing of 100 mm.

Edge zone – why it has a different spacing than the rest of the room

This is a topic that rarely appears in lay literature, but it is crucial in practice. Near external walls, French windows, and entrance doors, the heat loss is significantly higher than in the center of the room – cold air falls along the glazing or cold wall, creating drafts and discomfort. Therefore, the edge zone is designed with half the spacing of the main zone, typically 75 mm instead of 150 mm.

The width of the edge zone ranges from 0.5 to 1.0 m from the external wall. In this part, the pipe is laid in a denser pattern and a maximum surface temperature of 35 °C is exceptionally tolerated here (a walking surface, not a sitting furniture area). If the design does not include an edge zone, but the windows are large or the wall is poorly insulated, it is wise to design it even without the design – in practice, this means the first 2–3 loops from the window with a spacing of 75 mm and the rest of the room at 150 mm.

Effect of floor covering on the choice of spacing

This is one of the parameters that people most often underestimate. The floor covering is for the heat flow from the pipe to the room what a thick blanket is for you in bed – it slows down heat transfer. The higher the thermal resistance Rλ,B of the floor covering, the lower the performance of the floor at the same temperature drop.

Approximate values of thermal resistance for common floor coverings:

Ceramic and stone tiles (10 mm): Rλ,B ≈ 0.01 – 0.02 m²K/W – ideal for underfloor heating
Vinyl and LVT floors (3–5 mm): Rλ,B ≈ 0.02 – 0.05 m²K/W – most are suitable, always check the manufacturer's marking
Laminate (8–12 mm): Rλ,B ≈ 0.04 – 0.08 m²K/W – it is necessary to use laminate approved for underfloor heating
Solid wood (15–22 mm): Rλ,B ≈ 0.07 – 0.12 m²K/W – the performance of the floor drops significantly, it is necessary to increase the water temperature or reduce the spacing
Carpet with underlay: Rλ,B ≈ 0.10 – 0.17 m²K/W – not recommended in most cases, the performance can decrease by up to 30–40 %

This practically means: if you are designing a floor with solid wood instead of tiles and keep the spacing at 150 mm, your system may not be able to cover the heat losses of the room. The solution is either to reduce the spacing to 100 mm or to increase the supply temperature by 5–8 °C.

Pipe Fixing – How It Relates to Spacing

Once you know the correct diameter and spacing, you must properly fix the pipes in the floor. This is where the fixing system comes into play, which directly depends on the chosen spacing.

The simplest and most commonly used method in practice is the fixing strip. Fixing strip for pipe diameter 16–18 mm is anchored into the insulation, separation film, or system board. The strip has locks at 50 mm intervals, allowing you to freely adjust the pipe spacing in 50 mm increments – 100, 150, 200, 250, 300 mm. It is a fast and reliable solution for straight sections.

For curves and direction changes, pipe fixing curves for PEX pipe with diameter 16–18 mm are used. They ensure the correct bend radius, prevent the pipe from cracking during installation (PEX has a memory effect and you are trying to bend it into a tight curve), and hold the position until the anhydrite is poured. Without curves at tight direction changes, you may unintentionally kink the pipe and create a weak spot that will only become apparent after years of operation – curves are essential for every turn with a radius smaller than 5× the outer diameter of the pipe.

For point fixing on straight sections where you do not want to use a strip, pipe fixing clips with 50 mm spacing are suitable. They are inserted into the insulation and secure the pipe at a precisely defined location. The recommended spacing of clips on a straight section is 500–600 mm, in curves every 200–300 mm depending on the curve diameter.

The choice between strips, clips, and fixing curves depends on the laying system. More on this topic can be found in a separate article Fixing of floor heating pipes – Clips, strips, and curves.

Separation Film and Its Relation to Pipe Laying

Before laying the pipes themselves, the separation film must be properly installed on the thermal insulation – either ribbed (with a marked grid) or smooth. Ribbed separation film 0.1×1030 mm with an aluminum layer serves several functions at once: it separates the anhydrite from the thermal insulation, prevents moisture penetration, and thanks to the embossed grid, it facilitates orientation during pipe laying at the correct spacing. The aluminum (metallic) layer also reflects thermal radiation upward.

The film strips are laid with an overlap of at least 80–100 mm and joined with metallic tape for joining films (55 mm × 50 m). This tape has an aluminum surface treatment and strong adhesive – it ensures that the film remains intact even during anhydrite movement during setting. A common mistake is using standard adhesive tape, which peels off immediately during anhydrite pouring.

The ribbed film also functions as an aid for spacing – if you see an embossed grid in 50 mm increments on the film, it is very easy to count the loops and check whether you are maintaining the chosen grid. It is a subtle but practically valuable aid that you will quickly get used to.

Comparison: Fixed vs. Variable Spacing in One Room

In some rooms, it is appropriate to combine different spacings within one loop. This is called a variable grid and allows you to respond to the actual heat loss of the room without unnecessarily dividing it into multiple loops.

A typical example: a living room with a large French window. The designer does not want to overload the manifold with additional loops and decides on a variable grid: the first 0.8 m from the window at 75 mm spacing, then 1.5 m at 100 mm spacing, and the rest of the room at 150 mm spacing. The entire area is served by one loop, and the performance is evenly distributed according to heat losses. This method requires an experienced installer and good design documentation, but the result is elegant.

Hydraulic Balancing and Its Relation to Diameter and Spacing

When you have multiple loops with different lengths and different spacings, it is essential to hydraulically balance the entire system. A longer loop has a higher hydraulic resistance, a shorter one has lower – if you do not intervene, the water will prefer the shorter loop and the longer one will be only partially warm.

This is solved on the manifold with flow meters or balancing valves. The designer specifies the required flow for each loop (in liters per hour), and the installer sets it on the manifold. The flow depends on the thermal output of the loop and the temperature difference: Q [l/h] = P [W] / (1160 × ΔT [K]), where ΔT is the difference between the supply and return temperatures (typically 5–10 K).

Example: a loop with a power of 1 000 W, temperature difference 40/32 °C (ΔT = 8 K): Q = 1000 / (1160 × 8) = 0.108 l/s = 6.5 l/h. This number is set on the manifold and ensures that this loop will be properly supplied with heat.

This implies a direct relationship: if you change the pipe spacing without recalculating, the loop performance will change, and so will the required flow – you must also readjust the manifold. This is why improvisation with spacing on the site without consulting the designer can result in a system that will never function properly.

Common Mistakes in Choosing Diameter and Spacing in Practice

Over the years of work in this field, we have seen recurring mistakes. We mention them not for criticism, but so that you can avoid them in advance:

Same spacing throughout the entire apartment regardless of the room – a bathroom and a living room have different heat losses, different floor layers, and different normative limits for surface temperature. A uniform spacing usually means that some rooms are over-dimensioned and others under-dimensioned.
Use of 14 mm pipe in a large room with 150 mm spacing – the maximum length of a ∅14 mm loop is 60–80 m. If the room is 30 m² and the spacing is 150 mm, you need a loop of ~200 m, which is twice as much as the pipe can handle without an unacceptable pressure drop.
Not taking the floor layer into account – the project was designed for tiles, but the investor chose solid oak at the last minute. No one recalculated the spacing or temperatures, and the system cannot cover the heat losses.
Missing edge zone – with large glass areas, it is cold near the windows, even though the system otherwise works.
Too long loops with ∅16 mm over 120 m – the pressure drop is too high, the pump operates at the edge of its capacity, and the last few meters of the loop are cold.

More about errors, their causes and solutions can be found in the article Common mistakes in laying floor heating pipes and how to avoid them.

Cross-section of floor composition – where the pipes are located

For completeness, let's look at where the pipes are physically located in the floor composition and how the thickness of individual layers affects heat flow. This has a direct impact on what pipe diameter is realistically possible to use.

The standard composition for an anhydrite screed looks like this (from bottom to top):

Load-bearing structure (ceiling or foundation slab)
Hydro-insulation layer, if needed (more in the article Hydro-insulation film under floor heating – when and how to use it?)
Thermal insulation (EPS, PIR, XPS) – thickness 60–150 mm according to the project
Separation film (PE film or corrugated ALU film)
Floor heating pipes – lying on the insulation, fixed with strips or clips
Anhydrite or cement screed – minimum 45 mm above the top edge of the pipe for ∅16 mm
Self-leveling compound (if needed)
Walking surface

The total thickness of the anhydrite layer must be at least 45 mm above the top surface of the pipe. For ∅16 mm, this means at least 16 + 45 = 61 mm of anhydrite. For ∅20 mm, at least 65 mm. If the screed thickness is too small, thermal bridges arise and the floor surface has visible heat stripes – precise traces of the pipes marked by a darker floor coloring. More information about the type of insulation can be found in the article How to choose an insulation board for floor heating – thickness, material and requirements.

System board versus grid film – and what it changes for the spacing

When choosing a pipe fixing system, you have two main options: a system board (terazzo or channel type) or a grid film with classic fixing. Each has a different approach to determining the spacing.

System boards have a fixed grid – most commonly 50 mm or 75 mm modulation of the protrusions. This means you can lay the pipe only in multiples of this module: 50, 100, 150, 200, 250, 300 mm. For most projects, this is sufficient. The advantage is the speed of installation and the precise position of the pipe.

Grid film with fixing strips (or clips) gives more freedom – you can basically choose any spacing, which is advantageous for variable grids or non-standard requirements. Disadvantage: it requires more attention from the installer. A detailed comparison can be found in the article System board vs. grid film – what is better for your subfloor?

Most frequently asked questions (FAQ)

What pipe diameter is best for a typical single-family house?

For most single-family houses, the standard choice is a pipe with an outer diameter of 16 mm (16×2 mm). It is a compromise between hydraulic resistance, maximum loop length (80–100 m), flexibility in laying and availability of fittings. 14 mm pipe is used in bathrooms and small rooms with a tight grid, while 20 mm pipe is more common in industrial applications.

Can I combine different spacings in one room?

Yes, and in many cases it is even desirable. A typical example is the edge zone near external walls and windows, where a spacing of 75 mm is used instead of the standard 150 mm in the rest of the room. A variable grid in one loop is a common and legitimate practice, but it requires attention during installation – the pipe must be well fixed in each part.

How much does the walking surface affect the performance of the floor?

Significantly. Switching from ceramic tile (Rλ,B ≈ 0.01 m²K/W) to solid wood (Rλ,B ≈ 0.10 m²K/W) can reduce the floor's performance by 25–35 % at the same temperature difference. If the project was calculated with tile and you choose wood, you must either increase the supply temperature, reduce the pipe spacing, or accept lower performance. Always consult a change in the walking surface with the system designer.

What is the maximum recommended spacing for a passive house?

In passive houses with very low heat losses (typically 10–20 W/m²), a spacing of 200–250 mm with 16 mm pipe is commonly used. Some designers go as far as 300 mm with perfectly insulated building envelopes. At such spacing, the floor functions more as a tempering system than as a full-fledged heat source, which is sufficient for a passive house. Lower floor performance also means lower surface temperature – which is an advantage in terms of comfort, not a disadvantage.

What happens if the floor heating loop is too long?

A loop that is too long has excessive hydraulic resistance. Result: the pump does not deliver sufficient flow, the water temperature drops more along the loop than planned, and the last third of the loop is significantly colder than the first. This is manifested on the floor as uneven heating – warm at the edge of the room, cold at the other edge. Solution: do not exceed the maximum recommended loop lengths during design (∅14 mm max. 70–80 m, ∅16 mm max. 90–110 m, ∅20 mm max. 130–150 m) and, in case of doubts, shorten the loop and add a new one to the manifold.

Do I need to recalculate the manifold settings when changing the spacing?

Yes, always. Changing the spacing changes the loop length, and thus also its hydraulic resistance and thermal performance. A change in performance directly changes the required water flow through the loop. If you do not recalculate and readjust the flow at the manifold, the entire system will be hydraulically unbalanced – some loops will be overflown, others underflown. Hydraulic balancing is the basis of proper system function and must be observed whenever anything in the loop design changes.

Conclusion – diameter and spacing are not details, they are the foundations

The diameter and spacing of floor heating pipes are parameters from which the entire thermal and hydraulic design of the system is derived. It is not a matter of taste or the experience of the installer on site – it is a matter of physics, calculations and standards. A properly designed system with a 16 mm diameter and 150 mm spacing in living areas and 100 mm in bathrooms, with a 75 mm edge zone near external walls, will reliably, comfortably and efficiently function for decades. A system designed by eye may work only partially – and repairing embedded pipes in the floor is a matter of demolition.

If you are unsure about the design, use the project service of the supplier or have a hydraulic calculation done. The cost of project documentation is negligible compared to the potential cost of correcting damage. And if you want to learn more about other aspects of installation, we recommend the article Installation of floor heating step by step – from insulation to pouring, where the entire process is described from start to finish.

Do you have a question about this topic?

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