Installation of floor heating pipes: spacing, laying, and fastening

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Installation of floor heating pipes: spacing, laying and fixing

Floor heating is now a standard part of new buildings and renovations, but its real quality is determined not by the choice of boiler or manifold, but by the actual laying of the pipes into the floor. It is precisely here that the most common mistakes are made, which only become apparent after years – uneven heating, cold spots, pipe cracking in concrete or locally overheated floor surfaces. In this part of the Knowledge Center, we will go through the installation from the basics: correct spacing of loops, individual laying methods, fixing systems and technological procedures that really work in practice.

If you have not yet selected the type of pipe or are deciding which diameter to use, I recommend first reading the article How to choose pipe for floor heating: PEX vs. multilayer and Which pipe diameter to choose for floor heating (16, 17, 18, 20, 25 mm). Here we will assume that you have already selected the pipe and we will go directly to the installation.

Why pipe spacing is not just an approximate number

The spacing of the pipe loops (also called raster or pitch) directly determines how evenly the floor will distribute heat. The larger the spacing, the greater the temperature differences on the floor surface – in extreme cases, so-called "ribbing" is created, which is visible under thin coverings (vinyl floors, laminate) or noticeable to the bare foot.

At the same time, there is a mathematical relationship: smaller spacing = more pipe = lower water temperature required to achieve the same performance. Systems with a raster of 10–12 cm usually operate with a supply temperature of 30–35 °C, which is an ideal range for condensing boilers and heat pumps. Systems with a raster of 25–30 cm require a higher supply temperature for the same performance, which reduces the efficiency of the heat source.

Recommended spacing for different types of rooms

In practice, the following approximate values are used, which have been verified by dozens of customer projects in single-family homes and apartment units:

Living room, bedroom, children's room (normal heat loss): spacing 15–20 cm – sufficient performance, pleasant temperature profile
Bathroom, WC: spacing 10–15 cm – higher heat loss through tiles, lower thermal mass, dense laying required
Entrance, hallway: spacing 10–15 cm – short-term load from opening doors, faster response desired
Winter garden, large glazed area: spacing 10 cm, sometimes edge zones with denser spacing down to 7.5 cm
Garage (if heated): spacing 20–25 cm, higher supply temperature
Edge zone near an external wall or balcony doors: regardless of the room type, the first 0.5–1 m from the window/wall is laid with a 10 cm spacing (so-called edge zone, border zone)

The edge zone is something that many installers neglect or underestimate. The result is visible: the corner of the room near the outside is colder, there is a noticeable temperature difference on the floor near the window. A properly designed system takes into account that a higher specific output is needed near the external wall, and adjusts the laying density accordingly – not the supply water temperature.

Laying patterns: snail, snake and combinations

There are two basic patterns for laying pipe into the floor area. Each has its place and in practice they are often combined within one room.

Snail (spiral pattern, bifilar)

The snail pattern is better in terms of thermal uniformity. The supply (hot) and return (cold) pipes alternate throughout the entire area, which balances the temperature differences. The floor surface temperature is almost uniform. Disadvantage: more complex laying, requires more space for turns and is harder to improvise with at an atypical floor plan.

Serpentine is ideal for: living rooms, bedrooms, spaces with furniture, where the customer requires absolutely even heat. In long loops (over 80–100 m), it is also more advantageous from a hydraulic point of view, because pressure losses are lower than in meanders of the same length.

Snake (meandering pattern, serpentine)

Meandering laying is simpler to implement, but has a significant disadvantage: the water at the end of the loop is cooler than at the beginning, creating a temperature gradient across the entire room. With a spacing of 15 cm and a loop of 60 m, the difference between the supply and return temperature can be 5–10 °C, which is reflected on the floor surface as a difference of 2–4 °C. This is noticeable in some rooms (e.g., a bathroom).

The meandering pattern is suitable for: narrow corridors and foyers, floor plans where a spiral technically does not work (L-shape, T-shape), or as an edge zone in front of windows, where the first passes with the warmest water are placed right against the outer wall.

Combined approach

In practice, the most common approach is a combination: the edge zone (first 0.5–1 m from the outer wall) is laid in a meandering pattern with a spacing of 10 cm, with the first passes – the warmest – placed near the window. The rest of the area is laid in a spiral pattern with a spacing of 15–20 cm. The result is optimal performance and evenness.

Fixing systems: anchoring strips, clamps, foils, and system boards

The pipe must be firmly fixed in the floor before concreting or anhydrite pouring. If not, shifting occurs during pouring, which changes the actual spacing and can cause kinks – areas with increased resistance and increased mechanical stress.

Perforated anchoring strips

The most widespread and cheapest system. Plastic perforated strips are fixed directly to the thermal insulation (or to the reinforcing mesh) using special nails or screws. The pipe is then clipped into the strip at the required spacing. Advantage: the strips can be cut to size, work with any spacing, and are cheap. Disadvantage: setting the spacing requires precise measuring and marking, and the installation is slower than with system boards.

Estimated material waste with laid strips: a standard perforated strip has a hole spacing of 5 cm, which allows for spacing in multiples of 5 cm. For a spacing of 15 cm, every third hole is sufficient. Strips are laid perpendicular to the pipe direction, and the mutual spacing of the strips is usually 50–100 cm – depending on the pipe stiffness (stiffer PEX-b or PEX-c require the strips to be closer together than softer PEX-a).

System boards with protrusions (nipples)

System boards are molded thermal insulation (usually EPS or XPS) with plastic protrusions arranged in a grid, into which the pipe is simply pressed. They are available in grids of 5 cm (protrusions every 5 cm = pipe spacing in multiples of 5 cm). A major advantage is the speed of installation – an experienced worker lays a system board 3–5× faster than anchoring strips. Disadvantage: higher cost, limited spacing variability (only multiples of 5 cm), problematic atypical floor plans and small curve radii.

Important note from practice: with system boards, it is crucial to correctly join the boards at the contact surfaces. If the boards cross expansion joints or the room layout is not rectangular, the boards need to be cut – and each cut reduces the speed advantage of the system. For more complex floor plans, strips are still a better choice.

Fixing to reinforcing steel mesh

In some implementations, the pipe is fastened with plastic clamps directly to the reinforcing steel mesh, which lies on the thermal insulation. The mesh also serves as reinforcement in the concrete screed. This procedure is common in thicker constructions (garages, industrial floors), where the screed is reinforced for static reasons anyway. Disadvantage: the mesh lies directly on the insulation, which is not ideal from a thermal point of view (the mesh should be elevated on spacers to be embedded in the middle third of the screed – the same applies to the pipe).

Correct pipe position in the structural cross-section of the floor

The pipe should not lie directly on the thermal insulation, but at a height of at least 20–30 mm above it, so that it is poured with concrete from all sides. If the pipe lies on the insulation, the heat from it is directed mainly upwards (which is correct), but the lower part of the pipe is not surrounded by concrete – it creates air pockets, worsens heat transfer and mechanically supports the pipe worse under floor loading.

Minimum pipe cover in the screed

The standard STN EN 1264 (and its European equivalents) defines the minimum cover – i.e., the thickness of the screed layer above the pipe. For standard cement screeds (concrete C16/20 or similar), the following applies:

Minimum cover above the pipe: 30 mm – for normal loading (residential construction)
For industrial floors and garages: 45–50 mm above the pipe
With anhydrite screed: some anhydrite manufacturers allow a cover of 20–25 mm, but only when using special high-strength anhydrite

Therefore, a simple logic applies: if you use a pipe of 17×2 mm (outer diameter 17 mm) and want a minimum cover of 30 mm, the total thickness of the screed from the top edge of the insulation must be at least 17 + 30 = 47 mm. In practice, a screed of 55–65 mm is made, which provides sufficient reserve even with minor height irregularities in the pipe position.

Selection of pipe diameter and its impact on installation

The pipe diameter affects laying directly – a larger diameter means a larger minimum bend radius, longer loops for the same spacing (more area per loop), and greater hydraulic capacity (suitable for larger areas without a hydraulic distributor). We will discuss this topic in more detail in the article What pipe diameter to choose for floor heating, here we focus on the practical consequences for installation.

For most residential projects in Slovakia, the standard is a diameter of 17×2 mm. This dimension offers a good balance between the hydraulic resistance of the loop, the minimum bend radius, and overall rigidity. For example, PEX pipe 17×2 in a 240 m reel will cover about three loops at a spacing of 15 cm and an average loop length of 70 m – corresponding to three medium-sized rooms from one manifold. For single-family homes, there is also PEX pipe 17×2 in an economical 600 m reel, which covers the entire house from one delivery and eliminates the need to join remnants from multiple reels.

For larger rooms (over 25 m²) or low-temperature systems with long loops, PEX pipe 20×2, reel 200 m is suitable – a larger diameter allows longer loops (up to 100–120 m) without excessive hydraulic resistance, which reduces the number of circuits on the manifold and the overall system complexity.

Minimum bend radius and corner laying

PEX-a pipe has the best flexibility of all types of PE pipes – due to the molecular cross-linked structure, it can be bent without cracking even at relatively small radii. Approximate values of minimum cold bend radii (without heat):

PEX 17×2 mm: minimum bend radius approx. 5× outer diameter = ~85 mm (in practice, 100–120 mm is used)
PEX 20×2 mm: minimum bend radius approx. 5× outer diameter = ~100 mm (in practice, 120–150 mm is used)
PEX 25×2,3 mm: minimum bend radius approx. 5× outer diameter = ~125 mm (in practice, 150–200 mm is used)

In practice, this means that with a spacing of 10 cm (10 cm spacing), a loop turn (~20 cm diameter turn) with a diameter of 17 mm is technically possible, but tight. With a 20 mm pipe, a 10 cm spacing is at the limit, and when turning, you sometimes have to temporarily heat the pipe with a hair dryer or hot water to get it into the desired radius without whitening the material. Therefore, a 10 cm spacing in combination with a 17 mm diameter is much more comfortable than with a 20 mm diameter.

If you need a 10 cm spacing and long loops, the solution is to use multiple shorter loops with a diameter of 17 mm instead of one long loop with a diameter of 20 mm. The result is hydraulically equivalent.

Expansion joints: where, why, and how to solve them

Concrete screed expands when heated. If not properly divided by expansion joints, uncontrolled cracks will occur. The rules for expansion joints in floor heating are stricter than for regular floors:

Maximum area of one expansion field: 40 m² (some regulations 30 m²)
Maximum side dimension of the field: 8 m, maximum side ratio 1:2 (i.e. no narrow long strips)
Expansion joint always at each transition to another room through a threshold
Expansion joint always at a change of direction (L-shape, T-shape room)

The pipe must cross the expansion joint perpendicularly and in a protective sleeve. The sleeve (plastic corrugated tube) must be at least 0.5 m long on each side of the joint – thus a minimum of 1 m of sleeve at each crossing. The pipe inside the sleeve is not fixed to the screed, it can move freely during thermal expansion of the structure.

The expansion edge strip (foam strip around the perimeter of each room) must be glued or fixed before laying the pipe. Thickness of the edge strip: at least 8–10 mm, height must exceed the entire thickness of the screed including the floor covering. The excess part of the strip extending above the finished floor is cut off after the work is completed.

Installation process step by step

I will describe a standard procedure from practice, which works for a typical family house implementation:

Subfloor inspection: The subfloor must be dry (moisture content below 4 % CM for concrete), without sharp protrusions and significant irregularities (max. ±5 mm on a 2 m straightedge). Irregularities larger than 10 mm must be leveled with self-leveling compound before thermal insulation.
Separation film: A PE film with a minimum thickness of 0.2 mm is laid on the subfloor. Overlaps of at least 200 mm, joints taped. The film prevents moisture migration from the screed into the insulation.
Expansion edge strip: Glued or nailed along the entire perimeter of the walls in the room, before insulation. It must be continuous.
Thermal insulation: EPS or XPS in the thickness required by the project (minimum according to STN 73 0540: for floor above unheated space minimum 80–100 mm, for floor on ground minimum 100–120 mm, for intermediate floor minimum 30–50 mm). Boards are laid in bond (not cross joints), joined with adhesive.
Mounting strips or system board: Mounting strips are applied perpendicular to the direction of pipe laying, at intervals of 50–100 cm, and fixed into the insulation or through the insulation into the subfloor.
Laying the pipe: Always starts from the manifold. Supply and return lines run next to each other to the room (in sleeves where they cross other rooms). The pipe must not cross in one plane (if crossing is necessary, one loop must go over the other – this is solved with system boards by inserting a higher board piece).
Pressure test: A pressure test is mandatory before pouring concrete. The system is filled with water and pressurized to 1.5 times the operating pressure (typically 6 bar for a system PN6), for at least 24 hours. During the concrete pouring and curing of the screed, the system must remain under pressure (min. 3 bar), to detect any damage during the work.
Pouring the screed: Concrete or anhydrite is poured carefully, without mechanical vibrations directly onto the pipe (vibrator must not be applied to the pipe). The screed is densely troweled near the pipe to prevent air pockets.
Technological drying of the screed: No heating immediately after pouring. Concrete screed needs at least 21–28 days to cure, anhydrite at least 7–10 days, depending on thickness and conditions. Only after this period can heating be started according to the ramping protocol (gradual increase of temperature by 5 °C per day from 25 °C to maximum temperature).

Loop length and hydraulic balance

Each loop connected to the manifold must have as similar hydraulic resistance as possible, to balance the flow without excessive adjustment of balancing valves. Practical maximum loop lengths:

Pipe 17×2 mm: max. recommended loop length 80–90 m (at normal operating pressure 1.5–2 bar pressure losses per loop approx. 20–30 kPa)
Pipe 20×2 mm: max. recommended loop length 100–120 m
Pipe 25×2.3 mm: max. recommended loop length 120–150 m

The difference in length between the shortest and longest loop on one manifold should not exceed 30–40 %. If it is more, the pressure loss difference is too large for the manifold valve to handle. In such a case, it is better to split the longer loop into two shorter ones, or conversely, extend the shorter loop by meandering in the edge zone.

A detailed procedure for calculating loop lengths can be found in the article How to calculate the pipe length needed for floor heating – there we deal with formulas, practical tables and examples for the whole house.

Typical installation errors and how to avoid them

Over the years of practice, the same problems repeat. Here are the most common ones:

Pipe joints in the screed: Floor heating pipe must be in the floor plane without joints. Any joint (crimped, compression, threaded) buried in concrete is a potential leak point. If the cutting disc is not enough, it is necessary to design the area so that the end of the pipe is in the manifold, not in the floor. If a joint cannot be avoided, it must be accessible via a maintenance opening.
Under-screeding of the pipe: Pipe lying directly on the insulation and not covered with concrete from below loses thermal contact with the screed, which reduces performance and can cause local overheating. Solution: ensure sufficient concrete filling under the pipe during laying – sometimes it is necessary to manually "lead" concrete under each pipe branch before mechanical processing.
Skipping the edge zone: Customers later complain about cold feet near the windows. The solution is to always densify the spacing in the first strip from the outside wall.
Too long loops: Loops longer than 90–100 m at a diameter of 17 mm cause too large pressure losses and the pump cannot maintain the required flow. Result: the floor at the end of the loop is cold, although the boiler and manifold are working properly. Always calculate the loop length from the project documentation or from the article How to calculate the pipe length needed for floor heating before laying.
Insufficient pressure test: The pressure test is done formally – the system is filled, the manometer is checked and immediately concrete pouring starts. Correctly, it must be under pressure for 24 hours – the pressure must not drop more than 0.1–0.2 bar (a drop is normal due to temperature changes, but must not be significant).
Early heating start: Sometimes heating is turned on too early to speed up drying. Result: the screed cracks, cracks appear above the pipe, in extreme cases delamination of layers occurs. The ramping protocol must really be followed.

Material quantities and purchase planning

A typical mistake when buying pipe is forgetting the connections – i.e. the length of pipe from the manifold to the beginning of the loop in the floor and back. These connections can be 3–15 m per loop and with a house with 8–10 loops that is an additional 60–150 m of pipe that must be included in the order.

For a smaller apartment or renovation of one room, it is suitable to order PEX pipe 17×2, 120 m roll – it covers one larger loop with a reserve. For a whole family house with five to eight rooms, it is more economical to order PEX pipe 17×2, 600 m roll, where the price per meter is significantly lower and there are no unnecessary leftovers from multiple smaller rolls, which are hard to use.

If the project requires long loops or larger rooms (over 25 m²) and you want to reduce the number of circuits on the manifold, consider the PEX Pipe 25×2.3, 200 m pack – a larger diameter allows loops up to 130–150 m with an acceptable hydraulic resistance.

Frequently Asked Questions (FAQ)

Can I connect pipe leftovers in the floor if one reel is not enough?

No. Pipe joints in a concrete screed are absolutely prohibited – each joint is a potential leak point that would require complete removal of the floor. If one reel is not enough, you must design the loops so that each loop corresponds to one reel (or one continuous section from a larger reel). The only exception is if the joint is accessible via a maintenance shaft or access opening in the floor – but this is rarely done in residential practice.

What is the practical difference between 10 cm and 15 cm spacing – is it worth densifying?

The performance difference is noticeable: with the same supply water temperature and the same screed thickness, a 10 cm spacing provides about 20–30 % higher specific output (W/m²) than a 15 cm spacing. This means that with a system using 10 cm spacing, you can heat at a lower supply temperature – which is beneficial for condensing boilers and heat pumps. On the other hand, 10 cm spacing requires 50 % more pipe than 15 cm spacing for the same area. For well-insulated standard houses, 15 cm spacing is usually sufficient; we recommend 10 cm spacing for higher heat loss or for low-temperature sources (heat pumps).

Must the pipe be under pressure before being embedded?

Yes, for two reasons. First, overpressure in the system (min. 3–4 bar) keeps the pipe in a round cross-section and prevents flattening during embedding. Second, if during the concreting someone damages the pipe (e.g., steps on it, hits it with a tool), the pressure drop is immediately visible on the pressure gauge, allowing the fault to be located and repaired before the concrete hardens. Embedding pipe without overpressure is very risky.

How long should you wait before starting heating after pouring the concrete?

For cement-based screed, at least 21 days at a temperature above 5 °C, ideally 28 days. For anhydrite-based screed, at least 7 days at air humidity up to 65 %. After this period, the commissioning protocol begins: the first day, supply temperature 25 °C for 3 days, then each day increase by 5 °C up to the maximum project temperature, where it is maintained for another 3–4 days. The entire commissioning usually takes 7–14 days. After commissioning, check the expansion joints in the floor (they may slightly open), and only then lay the final floor covering.

What to do if my loops are too long and too short on one manifold?

The ideal situation is when all loops on one manifold have similar lengths (deviation up to 30 %). If this is not possible (a large room next to a small bathroom), the solution is to give the larger room two shorter loops instead of one long one, or for a very short loop (e.g., WC, 15–20 m), use a smaller pipe diameter or add hydraulic resistance (via a shut-off valve on the manifold). Never leave a difference of more than 50 % between loops without compensation – uneven flow is a source of problems, as discussed in the article Common faults and leaks in underfloor heating pipes.

Is PEX pipe suitable for renovations with limited construction height?

Yes, but you must account for the minimum screed thickness above the pipe (30 mm) plus the pipe diameter (17 mm for PEX 17×2), giving a minimum screed thickness of 47 mm, plus thermal insulation (at least 30–50 mm for interfloor spaces). The total construction height without the final covering is therefore at least 80–100 mm. For renovations where this is not possible, there are dry mounting systems (aluminum distribution plates without concrete) – these have a total height of 25–40 mm, but they only work with especially low-temperature sources and require special pipe with a smaller diameter. We also cover this topic in the article PEX pipe vs. multilayer pipe: differences, advantages and disadvantages.

Conclusion

Do you have a question about this topic?

Not sure or dealing with a specific situation in your home? Write to us – we are happy to help.