Common mistakes when laying underfloor heating pipes and how to avoid them

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Why mistakes in the installation of underfloor heating pipes are so expensive

Underfloor heating is among the most comfortable and energy-efficient ways to heat residential homes and commercial spaces. However, precisely because the entire system is embedded in concrete or anhydrite, installation errors are particularly tricky – most of them are discovered by the owners only during the first heating season or when they receive a shocking energy bill. Fixing an error after the pipes have been poured means tearing up the floor, which in practice is an expensive and painful matter.

Over the years of experience in this field, I have seen dozens of projects where the same mistakes were repeated over and over – by professional companies as well as by skilled DIY builders. This article is not a collection of theory, but a catalog of real problems, with specific values, dimensions, and practical solutions. If you plan to install underfloor heating, read it before you unroll the first meter of pipe.

Mistake No. 1: Missing or incorrectly installed separation film

This is a mistake that seems trivial, but in practice it has serious consequences. The separation film performs several functions at once: it separates the thermal insulation from the screed (concrete or anhydrite will not penetrate into the insulation and will not impair its thermal insulation properties), it creates a smooth base for even pipe laying, and in some types of films with a grid pattern, it also helps with spacing orientation.

The most common variant of this mistake: the film is laid, but individual strips are not overlapped sufficiently or are not joined at all. The result? Wet concrete during pouring penetrates the joints and flows under the film, locally damaging the insulation. A thermal bridge that arises can result in a loss of several percent of heat, but on a larger area, it means significantly higher heating costs every season.

Correct procedure: separation grid film is laid with an overlap of at least 5 cm on each joint. The joints are then taped with metallic tape for joining films – this tape is resistant to the alkaline environment of concrete and ensures that the joints will last for decades under the screed. The film must be pulled up along the walls at least to the height of the entire floor construction (typically 10–15 cm) and sealed there with expansion tape.

Another variant of this mistake: the film is laid only along the perimeter of the room up to the wall without being lifted upwards. Concrete then comes into direct contact with the wall and creates a thermal bridge along the edge of the floor. I have seen this on a project in a new residential house – in the corners of the rooms, the floor temperature was 2–3 °C lower than in the center during winter, because the thermal bridge was carrying heat away to the foundation slab.

Mistake No. 2: Incorrect spacing and length of the loop – the math you can't afford to ignore

The spacing of the pipes and the maximum length of one loop are quantities that are interdependent and directly affect the performance and pressure loss of the entire system. This is one of the most underestimated mistakes – the installer "by eye" stretches the pipe as the leftover material allows, and the result is a system that either doesn't heat properly or has an overly hot floor in some rooms and a cold one in others.

Practical values for DN 16 mm pipe (the most common diameter for residential homes):

Spacing 10 cm – very dense, used for heat losses above 60–70 W/m², with low screed thickness, or in bathrooms
Spacing 15 cm – standard for most living rooms with normal insulation
Spacing 20 cm – for very well insulated new builds (passive house), where a lower specific output is sufficient
Maximum loop length for DN 16 mm – approx. 80–100 m for PE-X pipe, otherwise the pressure loss increases to such an extent that the circulation pump cannot overcome it or it operates with very low flow
For DN 20 mm the loop can be longer, typically 100–120 m

Real mistake from practice: a project – a five-room residential house, living room 45 m². The installer made one loop with a spacing of 15 cm, resulting in a pipe length of approx. 135 m. The result? The first quarter of the room near the manifold was warm (even overheated), the second half was only warm, and the end of the loop hardly heated at all. Cause: enormous pressure loss on the long loop, low flow, large temperature drop. The solution would have been to divide the loop into two shorter ones. More on this topic is written in the article What pipe diameter and spacing do I need for underfloor heating? in our Knowledge Center.

Mistake No. 3: Poor or insufficient pipe fixation

Pipes made of PE-X or PE-Xc material have a shape memory – after unrolling from the reel, they tend to return to their original shape. If you don't fix them sufficiently before pouring, during the pouring of the concrete screed or anhydrite, they will lift, change position, or the spacing will change. This has a direct impact on the system's performance and the even distribution of heat.

There are three basic ways of fixing:

Fixing strips – the fastest way for large areas

Pipe fixing strip (length 1 m) is placed directly on the separation film in the direction of pipe laying. The pipe is simply clicked into the teeth of the strip at the desired spacing. Strips are designed for pipe diameters of 16–18 mm. Advantage: fast laying, even spacing, the strip holds itself on the film. Disadvantage: at curves and direction changes, a combination with curves or fasteners is needed.

Fixing clips – point fixing, ideal combination

Pipe fixing clip (straight, 50 mm) is used for point fixing the pipe into the insulation. It is driven directly into polystyrene or mineral insulation and reliably holds the pipe during pouring. Recommended clip density: 1 piece per approx. 50 cm of straight sections, 2–3 clips per one bend at bends.

Fixing bends – solution for direction changes without cracking

PEX pipe fixing bend is a special element for 16–18 mm pipe that protects the pipe from breaking or having too small a bend radius when turning 180° (type "hanger"). The minimum bend radius of PE-X pipe DN 16 is 5× diameter, i.e. 80 mm – with a closer spacing of 10 cm it is exactly on the limit and without a bend there is a risk of mechanical damage to the pipe.

Mistake I have seen on several projects: pipe fixed with clips every 2–3 meters, resulting in it lifting and floating during pouring. Concrete or anhydrite is relatively fluid during pouring and creates an uplifting force – without sufficient fixing, the entire pipe pattern will shift. Moreover, if the pipe lifts, the thickness of the cover layer (covering concrete) above the pipe will decrease, which negatively affects the floor's thermal capacity and can lead to cracking of the screed during temperature changes.

Mistake No. 4: Skipping the tightness test before pouring

This is perhaps the most critical mistake – and yet even experienced plumbers make it when under time pressure or when they think "nothing can leak from PE-Xc". It can. Joints, adapters, or mechanically damaged areas (e.g., from careless movement of heavy tools on the floor) can cause water leaks – and after pouring, you will only discover it when the wet stain has already spread through the floor covering.

Correct procedure for pressure testing:

Fill the system with water and bleed all circuits
Pressurize to 1.5 times the working pressure (3–6 bar for standard systems), for at least 24 hours, ideally 48 hours
Check all joints and transitions through walls/valve manifold
Record the pressure test in the installation documentation (date, pressure, signature) – this documentation is later proof in case of claims
Maintain pressure in the system during concrete pouring and for at least 24 hours after pouring

Why maintain pressure during concrete pouring? Because pressurized pipe retains its shape and is more resistant to mechanical damage from stepping or pressure from concrete. In addition, you can immediately see on the manometer if damage occurs during concreting.

Mistake No. 5: Ignoring expansion joints and expansion tape

Concrete screed expands when heated. Anhydrite even more so. When the room area exceeds 40 m², or when the room is longer than 8 meters, or if it has a non-standard shape with narrow necks (L-shape, T-shape), the screed must be divided into expansion joints. These must be marked before pouring using an expansion profile or foam tape.

The mistake is that the pipe passes through the expansion joint location without protection. The screed moves – and the pipe rubs against the edge of the joint. After 5–10 years of thermal stress (expansion and contraction with each heating season), mechanical fatigue of the material can occur and the pipe can be damaged precisely at this point.

Solution: every pipe passing through an expansion joint must be wrapped with a protective sleeve (saddle) of at least 30 cm on each side of the expansion line. The sleeve ensures that the pipe is not in direct contact with the screed in the critical movement zone. Joints and connections must never be placed in the expansion joint or in its immediate vicinity.

Error No. 6: Pipe routing in the edge zone and under fixed furniture

Every room has so-called edge zone – a strip near the exterior walls and windows of width 0.5–1 m, where heat loss is highest. In this zone, a denser pipe spacing is recommended (7.5–10 cm instead of 15 cm). The mistake occurs when this zone is overlooked and the entire room is looped with the same spacing – the result is that the edges are cold and the center is overheated.

Equally serious mistake: routing the pipe under fixed built-in furniture, kitchen countertops, bathroom cabinets, or under the stairs. In these areas, heat cannot dissipate into the room, accumulates, and over longer operation can cause cracking of the screed or damage to the furniture. The pipe in these zones must either be rerouted or the entire area must be thermally insulated from above (cork underlay, wooden structures), which is rarely done. The cleanest solution: have an accurate plan before laying with all fixed furniture marked, and simply avoid these areas when laying the pipe.

Error No. 7: Insufficient thickness of the covering screed above the pipe

The thickness of the screed above the pipe has a direct impact on the uniformity of surface heat. If it is too thin, the floor shows so-called "snake" – visible or palpable thermal strips copying the pipe route. If it is too thick, the system is slower, thermal inertia is higher, and temperature control is more complicated.

Recommended minimum thickness values of the screed above the top of the pipe:

Concrete screed C20/25 – minimum 45 mm above the top of the pipe (total screed thickness for DN 17 thus minimum 62 mm)
Anhydrite screed – minimum 30 mm above the top of the pipe (due to better thermal conductivity, less is sufficient)
Lightweight concrete (Litobeton) – minimum 45–50 mm above the top of the pipe, depends on the ratio

How does this error occur? Most often when the planning of sub-heights (layer thicknesses) was not calculated correctly: the insulation is thicker than planned, door heights do not match reality, and the screed has to be "saved" by several centimeters. The result is a screed only 15–20 mm thick above the pipe – and that is not enough even for anhydrite.

Error No. 8: Missing or incorrect balancing of circuits on the manifold

A floor heating system with multiple circuits of different lengths is not automatically balanced. A shorter circuit offers less hydraulic resistance – more water flows through it, it heats up more and emits more heat. A longer circuit is, on the other hand, "throttled" by higher resistance. Result without hydraulic balancing: one room is overheated, the other is underheated.

Correct procedure: each circuit on the manifold must be set to an individual flow rate using an adjustable manifold valve. The flow is set according to a hydraulic calculation, or according to the power calculation of each room. As a rough rule: longer circuit = larger valve opening, shorter circuit = smaller opening (throttling).

In practice, the setting is done either manually using flow meters on the manifold (you can see the exact flow in l/min for each circuit), or automatically using control heads. Without flow meters, the setting is only "by eye" and is not accurate. This topic is related to the issue of the correct selection of the manifold, which is discussed in further articles of our Knowledge Center.

Error No. 9: Inappropriate floor covering reducing the system performance

This is not an error in the laying of the pipe itself, but it is an error in the overall design – and technically it is closely related to the laying. A floor covering has a thermal resistance (value R in m²K/W), which directly affects how much heat passes through the floor into the room. The higher the thermal resistance of the covering, the less heat reaches the room at the same medium temperature.

Ceramic tiles or stone – R ≈ 0.01–0.02 m²K/W – ideal for floor heating, very good thermal conductivity
Laminate or wooden parquet – R ≈ 0.05–0.10 m²K/W – acceptable, but system performance is reduced; maximum allowed resistance for floor heating is R = 0.15 m²K/W
Solid wood 22 mm thick – R ≈ 0.15 m²K/W – on the edge, it is necessary to increase the medium temperature
PVC, vinyl (LVT) – R ≈ 0.01–0.05 m²K/W – depends on the product, always check the manufacturer's certificate for floor heating
Carpet – R ≈ 0.10–0.25 m²K/W – very unsuitable for floor heating, strongly reduces performance

The problem arises when the design was dimensioned for ceramic tiles and the investor chooses thick wooden parquet or carpet at the last minute. The system does not heat sufficiently, increasing the medium temperature is limited (maximum floor surface temperature 29 °C in living rooms, 35 °C in bathrooms according to EN 1264), and the problem cannot be solved otherwise than by replacing the floor covering.

Error No. 10: Underestimating the first commissioning of the system

After pouring and drying of the screed (and before laying the final floor covering), it is essential to perform the correct first heating of the system – so-called functional heating. Most plumbers know that they have to wait until the screed dries, but they do not know (or do not implement) functional heating according to the standard.

Gradual heating of anhydrite (according to EN 1264-4):

Start no earlier than 21 days after pouring the anhydrite screed (28 days for concrete)
First 3 days: maximum medium temperature 25 °C
Subsequent days: increase by max. 5 °C per day
Keep the maximum design medium temperature for at least 4 days
The whole process takes at least 10–14 days

If this gradual heating is skipped and the system is switched to full power immediately, the screed will crack – in a wet screed, sudden thermal expansion occurs, microcracks weaken the screed and it gradually degrades. The result will become apparent after years: cracking floor, tiles with loose grout, loose parquet.

The whole process from insulation through installation to the first heating is thoroughly discussed in the article Underfloor heating installation step by step – from insulation to pouring in the Knowledge Centre.

Mistake No. 11: Hidden joints and connectors in the screed

According to technical standards and pipe manufacturer requirements, no mechanical joints, crimped connectors or other fittings must be hidden in the concrete or anhydrite screed – with the exception of certified special joints for embedding. Every joint is a potential weak point. The screed must always contain one continuous pipe section without any joint.

If the pipe from the reel is too short and does not reach the full circuit, the solution is not to connect it in the screed – you should increase the reel or redesign the circuit route. Joints are allowed only in visible and accessible areas: on the manifold, or in a shaft or inspection opening with appropriate documentation.

Mistake No. 12: Missing documentation and photo documentation

This is not a technical mistake in the narrow sense of pipe installation, but it is a mistake that will show up at the worst possible moment – when you need to resolve a fault or a warranty claim. Without photo documentation of the pipe layout before the screed is poured, no one knows exactly where the pipe lies. You drill into the floor, search for the pipe, and either the drill hits it (and you have a breakdown), or it misses and you don't know what is happening.

Minimum documentation you need to have:

Photos of the entire pipe installation before pouring the screed (from multiple angles, with reference points – room corners, door openings)
Diagram of the circuits with lengths, spacing and identification of each circuit on the manifold
Pressure test report
Functional heating report
Technical data sheets of the materials used (pipe, insulation, screed)

Frequently asked questions (FAQ)

Do I need to use a metal tape to join the foil, or is regular adhesive tape enough?

No, regular adhesive tape will not hold. Wet concrete is a highly alkaline environment (pH 12–13) and common adhesives degrade quickly in it – the tape will peel off during the screed pouring or shortly after. Metallic tape for joining foils is resistant to alkaline environments and ensures a long-term hermetic connection between the foil strips.

How can I find out where the pipe runs in the floor if I don't have documentation?

The most reliable method is a thermal camera – with the system heated, the pipe route is clearly visible as a warmer pattern on the floor surface. Another option is an inspection camera inserted into the pipe via the manifold (with a GPS locator). Both methods are expensive and time-consuming, which is why photo documentation before pouring is absolutely essential.

Can I save money by skipping fixing arcs in areas with small bending radius?

No, this is a direct path to problems. The minimum bending radius for PE-X pipe DN 16 is 80 mm (5 times the diameter). If the pipe is bent to a smaller radius, it is locally stressed – the inner side of the bend is compressed, the outer side is stretched. After repeated thermal cycling (every heating season = hundreds of cycles), material fatigue and microcracks will occur. Fixing arcs for PEX for DN 16–18 mm are specifically designed for this – they ensure the correct bending radius even with dense spacing.

What is the difference between a fixing strip and a fixing clip – when to use which?

Fixing strip is suitable for long straight sections, where you need to quickly maintain an exact spacing – simply click the pipe into the strip and it holds the entire length. Fixing clip (50 mm, straight) is ideal where the strip is not sufficient – for arcs, when crossing expansion joints, for fixing the pipe in irregular areas or in places where a strip cannot be used. In practice, these two methods are combined: strips for straight sections, clips for arcs and special situations.

When do I need to address waterproofing under the underfloor heating?

The obligation for waterproofing arises in areas where water may occur – bathrooms, kitchens, technical rooms. Separation foil only serves a separation function, not a waterproofing one. In wet areas, a certified waterproofing layer under the entire floor construction is required. This topic is covered in a separate article Waterproofing foil under underfloor heating – when and how to use it? in the Knowledge Centre.

Can an underfloor heating system also be used for cooling in summer?

Yes, but only if the floor construction is dimensioned for it and the system is equipped with dew point control to prevent condensation on the cold floor. The floor surface temperature during cooling must not drop below the dew point of the air in the room. More on this topic is covered in the article Underfloor heating and cooling – what must the floor construction meet?

Conclusion: A systematic approach instead of improvisation

Most mistakes in underfloor heating pipe installation do not arise from a lack of knowledge of the technology, but from underestimating the details and time pressure. The installer knows what to do – but when there are few clips in stock, the customer is pushing for a deadline and the anhydrite order is scheduled for tomorrow, many things are "skipped" or done half-heartedly.

The results usually do not appear immediately – the first summer and winter the system works fine. Problems arise after 3–7 years, when accumulated mechanical fatigue, slow material degradation in the wrong environment or incorrect hydraulic balancing become fully apparent. At that time, the dependency on the system is much greater, the customer has forgotten who installed it, and the repair is many times more expensive than the original savings.

Proper installation is not complicated – it just requires diligence, the right tools and following proven procedures. Investment in quality fixing elements, proper separation foil and quality documentation is minimal compared to the overall construction costs, and its value will be apparent for decades in a functioning and trouble-free system.

If you are planning installation and want to make sure you have chosen the right materials and procedures, also check related topics in our Knowledge Centre: System board vs. grid foil – which is better for your subfloor?, Fixing underfloor heating pipes – clips, strips and arcs, How to choose insulation board for underfloor heating – thickness, material and requirements and Maintenance and air venting of underfloor heating system.

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.