How to choose pipe for heating, water or floor heating
Why choosing the pipe is one of the most important decisions in heating
The pipe is a part of the installation that, once embedded in a wall, in a floor slab or in an installation shaft, will not be visible anymore and no one will be able to access it for years, maybe even decades, without demolition. That is why the choice of the correct type, diameter and class of pipe is a decision that is very difficult and expensive to correct later. While a wrongly chosen radiator or circulation pump can be replaced within an hour, a leaking or undersized pipe embedded in the floor or built into a partition wall may in the worst case mean cutting concrete, replacing the floor and additional weeks of work.
In practice, when designing pipe distribution systems, we encounter three basic questions that every investor and construction company must answer: what material to use for the pipe, what diameter to choose for a given type of distribution and what pressure and temperature class is needed for a given application. This article deals with all three questions in depth, with specific numbers, dimensional recommendations and practical experiences from real projects. Other related topics - installation, fittings, storage or troubleshooting - are discussed separately in other articles of the Knowledge Center, to which we naturally refer in the text.
What types of pipes are used in practice
Today, two main materials have practically become established on the market for heating, hot and cold water distribution and floor heating - PEX (cross-linked polyethylene) and multi-layer pipe with an aluminum layer (often referred to as PEX-AL-PEX or multi-layer). Classic copper and steel pipes are used only in specific cases.
PEX pipe - cross-linked polyethylene
PEX (cross-linked polyethylene) is polyethylene that has been reinforced by an additional "cross-linking" (networking) process of the molecules, giving it significantly better resistance to temperature, pressure and long-term deformation (creep) compared to standard PE pipe. According to the cross-linking method, the following types are distinguished:
- PEX-a - cross-linking with peroxide (Engel method), the most homogeneous structure, the best flexibility, shape memory (the pipe returns to its original circular cross-section after deformation), the highest resistance to creep. This is the highest quality and at the same time the most expensive type.
- PEX-b - cross-linking with silane, slightly stiffer, slightly worse resistance to point loading, but still very good properties and a lower price.
- PEX-c - cross-linking with electron beams, the cheapest technology, used less for heating applications.
The majority of professional heating and floor heating systems in our country are implemented in the PEX-a class, because it combines flexibility (it bends well directly from the reel without the need for heating), high resistance to temperature cycles and long service life (manufacturers commonly guarantee 50 years at an operating temperature of around 60 °C).
Multi-layer (PEX-AL-PEX) pipe
Multi-layer pipe has a thin aluminum layer in the middle, welded longitudinally or overlapped, covered on both sides by a layer of PEX or PE-RT. The aluminum layer brings three essential advantages: practically zero oxygen diffusion (no separate oxygen barrier is needed), minimal thermal expansion (the pipe "does not remember" the shape of the bend and remains exactly in the position you shape it to), and higher stiffness, which is advantageous for visible distribution systems and for feeding to manifolds, where you need to bend the pipe into an exact shape and it will maintain this shape even without supports.
The disadvantage is a slightly higher sensitivity to repeated bending in the same place (aluminum can develop micro-cracks due to repeated deformation) and a higher price compared to pure PEX. A detailed comparison of both materials, their mechanical properties and recommendations for use are discussed in a separate article PEX vs multi-layer pipe - differences and use.
Copper and steel - when do they still make sense
Copper pipe is used today mainly for gas distribution, for boiler feeding in machine rooms, for aesthetic visible distribution systems or in reconstructions, where we connect to an existing copper system. Steel pipe (most often threaded or welded) is used in industrial boiler rooms, for large diameters above DN50 or for high-pressure and high-temperature applications, where plastic systems no longer meet the parameters. However, for the residential and family segment, which is the focus of most of this article, PEX and multi-layer pipes are today clearly the dominant and recommended solution - they are cheaper in material and installation, faster to connect and have lower thermal losses due to heat conduction through the material.
Cross-section of the pipe construction - what the wall is made of
To clarify why different pipes have different properties, let's look at a typical cross-section of a PEX-AL-PEX pipe used for heating and floor heating systems.
In the case of pure PEX pipe without an aluminum layer, the construction is simpler - a single-layer PEX wall with a diffusion barrier against oxygen (EVOH layer) on the outer surface, which prevents oxygen from entering the water and thus limits corrosion of steel parts of the system (circulation pumps, steel radiators, heat exchangers). This EVOH barrier is the reason why you should never use ordinary (non-barrier) PE pipe intended only for cold water distribution in a heating circuit.
What diameter to choose according to the type of distribution
The pipe diameter is selected according to the flow of the medium that needs to be transported through a given section and according to the acceptable pressure loss. A detailed calculation procedure with flow tables can be found in the separate article What pipe diameter do I need according to the type of distribution. Here we present a practical overview of the most common dimensions that we encounter most frequently in practice:
| Diameter (mm) | Typical use | Note |
|---|---|---|
| 16x2 / 17x2 | Floor heating - heating loops in the floor | The most common size for floor heating loops in living areas |
| 20x2 | Connections to manifolds, shorter risers, bathroom heating | Higher flow, suitable for feeding multiple loops as well |
| 25x2,3 | Main distribution to manifolds, boiler connection, risers in apartment buildings | Transfers higher performance over longer routes |
| 32 and more | Main distribution in boiler rooms, larger buildings | Used mainly for larger outputs above 15-20 kW on one route |
For standard floor heating in a family house, the dimensions 16x2 or 17x2 mm are used in the vast majority of cases - the difference between them is in the wall thickness and internal diameter, which affects the pressure loss on longer loops. For connections between the boiler, manifolds and main risers, 20x2 or 25x2,3 mm are commonly used.
In our range you will find for example PEX pipe 17x2 mm on a 120 m spool, which is a typical dimension for smaller floor loops, or a larger spool PEX pipe 17x2 mm - 600 m for larger buildings, where it is more cost-effective to buy pipe in a large spool with a lower price per meter. For main connections and distribution lines, the suitable options are PEX pipe 25x2.3 mm - 200 m or a smaller PEX pipe 20x2 mm - 200 m.
Pressure and temperature - class PN6, PN10 and what these numbers mean
The designation PN (nominal pressure) indicates the maximum working pressure the pipe can safely withstand at a given temperature - and this is exactly the detail that is often overlooked when choosing the pipe. The resistance of plastic pipe to pressure decreases significantly with increasing temperature, so the manufacturer always specifies a combination of pressure/temperature, not just a single number. A common designation on PEX pipes in our range is PN6 at a temperature of +90 °C and PN10 at a temperature of +60 °C.
This means in practice: at the normal operating temperature of a floor heating system (typically 35-45 °C at the inlet), the pipe has a much higher pressure reserve than during short-term overloads of the system up to 90 °C, for example during a regulation failure or when connected to a high-temperature source (solid fuel boiler without a mixing valve). Therefore, it is important to always consider the worst realistic operating condition when choosing the pipe, not just the average temperature.
Practical recommendation: for standard hydronic floor heating (connected via a manifold with a mixing valve or thermostatic heads, where the water temperature in the loop does not exceed about 45-50 °C), the combination of PN6/PN10 is sufficiently dimensioned with a large reserve. For radiator circuits supplied directly from the boiler, where the temperature can temporarily rise to 80-90 °C (for example during a circulation pump failure or during transient conditions of a solid fuel boiler), it is necessary to consider that the system will sometimes operate close to the upper limit of the pressure-temperature diagram, and therefore it is important to also pay attention to the correct dimensioning of the safety valve and expansion tank, not just the pipe itself. Precise numerical values for specific pipe types can be found in the article "What pressure and temperature can PEX pipe withstand (PN6, PN10)."
Floor heating - specifics of pipe selection
In floor heating, the pipe is laid in loops (meanders or spirals) directly into the layer of screed above the system insulation board. There are several rules that repeatedly appear in the design of distribution systems in practice:
- The length of one loop should generally not exceed 100-120 m for a diameter of 16-17 mm, because longer loops increase pressure loss and the circulation pump in the manifold may no longer ensure uniform flow.
- The spacing of the pipe (distance between individual "branches" of the loop) typically ranges between 10 and 25 cm, denser near windows and sparser in the center of the room.
- The pipe must have an oxygen barrier (EVOH or aluminum layer) - otherwise, corrosion of steel components in the system, circulation pumps and heat exchangers in the boiler occurs gradually.
- It is recommended to buy pipe from one spool for the entire loop, so that no joining is required in the floor - each joint in the floor is a potential risk and should be only exceptional and properly documented in the project documentation.
From the last point, there is a practical reason why we sell pipe in large spools as well - for example, the 600 meter spool PEX pipe 17x2 mm - 600 m allows the installer to cut the required length of the loop directly on site without unnecessary waste and without the need to connect shorter pieces.
The loop in the image is a simplified schematic of a meandering layout - in practice, it is combined with a denser spiral method in the edge (cooler) zones to equalize the floor surface temperature and eliminate the so-called "cold strip" near the façade.
Radiator and wall heating distribution
In classic radiator heating systems (wall or floor-mounted to radiators, not floor heating), the most common dimensions are 16-20 mm for individual radiator connections and 20-25 mm for main distribution lines between the boiler and individual branches or manifolds. In a "star" system (each radiator has its own pair of pipes led from a central manifold, similar to floor heating), the most common diameter is 16-17 mm for individual loops, which allows simple flow regulation at each radiator individually using a manifold with flow meters.
In the older (and still common) "T-piece" or two-pipe system running along the walls, the main line is dimensioned to 20-25 mm and the branches to individual radiators to 16 mm. It is important to remember that in multi-layer pipe, due to low thermal expansion, the pipe can be run without expansion loops on longer straight sections, while with pure PEX pipe, greater expansion must be considered and when running in wall grooves, insulation against direct contact with plaster is recommended.
Drinking water distribution
Similar principles apply to hot and cold drinking water distribution as for heating, with two differences. First, the material must be certified for contact with drinking water (hygienic certificate), which is a common feature of quality PEX and multi-layer pipes intended for universal use. Second, the diameter is not determined by power in kW, but by the required flow rate in l/s according to the number and type of outlets (sink, bath, shower, WC, kitchen sink) - a detailed calculation procedure is again in the article on pipe diameter selection.
In apartment and residential distribution systems, 16-20 mm is commonly used for connecting individual equipment items, while 25 mm and larger is used for risers and main distribution lines in the building. When designing, it is also important to consider the simultaneity of consumption (not all outlets are running at the same time) and pressure conditions in the network, to prevent significant pressure drops at the farthest point from the source during full load.
Bars or spools - how to choose the delivery form
Pipe is supplied to the market either in bars (straight pieces, usually 5-6 m in length), or in spools (coils) of various lengths - commonly 50, 100, 120, 200 up to 600 meters. The choice of delivery form depends on the purpose:
- Spools are clearly more suitable for floor heating and for long straight runs, where you want to minimize the number of joints. An advantage is also the lower price per meter for larger spools and easier transport and handling on the construction site (the spool is unwound gradually directly from the pallet or from an unwinding stand).
- Bars are mainly used for multi-layer pipe in visible or precisely shaped distribution systems, where it is an advantage that the material retains the exact shape after bending without the need for mechanical fastening every 20-30 cm.
When ordering spooled pipe, we recommend always allowing for a certain reserve (5-10 % extra) to account for measurement errors, possible repairs and technological losses when passing through building structures. Proper storage of spools (they must not be compressed for long periods, nor exposed to direct sunlight or frost during handling) is discussed in a separate article on Storage and Handling of Pipe in Bars and Spools.
Practical examples from everyday implementation practice
Example 1 - single-family house, floor heating throughout the ground floor. For an area of 90 m² with an average loop length of 90 m and a laying spacing of 15 cm, the total pipe consumption is approximately 550-600 m of 17x2 mm dimensions. In such a case, it clearly pays off to buy one large 600 m spool instead of several smaller spools, as this saves on the price per meter and also on the number of joints between spools (ideally no joints in the floor).
Example 2 - apartment renovation, radiator heating in a star system. An apartment with 5 radiators requires 5 pairs of loops led from a distributor located in the installation core. With an average loop length of 15 m per radiator (10 loops in total), the consumption is approximately 150-160 m of 16-17 mm pipe, plus the connection of the distributor to the boiler dimensioned for 20-25 mm in length according to the distance from the boiler room to the distributor.
Example 3 - new construction with a combined system. In practice, we increasingly encounter a combination of floor heating in living areas and radiators (e.g., panel radiators) in bathrooms, as floor heating alone in a small bathroom with tiles may not be sufficient for a quick temperature rise. In such cases, the design must account for both types of loops on a common distributor and properly set the flows using flow meters to distribute heat according to the actual needs of individual zones.
Example 4 - incorrect dimensioning from practice. We often encounter situations where an installer, in order to save on material, chooses the same 16 mm diameter for the main connection between the boiler and the distributor as for individual loops, even though this route must carry the sum of all loop flows at once. The result is increased pressure loss, noise from the circulation pump and insufficient performance under full system load. The solution is always to dimension the main distribution lines at least one to two size steps higher than individual loops.
What to pay attention to when ordering pipe
- Check whether the pipe is intended for heating (with an oxygen barrier), not just for cold water distribution - visually it is often not distinguishable, the technical sheet and certificate decide.
- Check the pressure-temperature marking (PN6/PN10 at the respective temperature) and compare it with the actual operating conditions of your system, not just with the average temperature.
- When ordering for floor heating, allow for a reserve and try to choose a spool length that covers the entire loop without the need for a joint in the floor.
- Do not combine pipes from different manufacturers with different dimensional series in one system without consulting the supplier of fittings - the outer diameter may slightly differ and the fittings may not fit.
- Think in advance about the type of fittings and tools you will need for installation - in detail in the article on Which Fittings and Connectors to Use for PEX Pipe and Step-by-Step Installation of PEX and Multi-Layer Pipe.
If you are considering which specific dimension is most suitable for your project, we also recommend reading a separate article on dimensioning the diameter according to the type of distribution - there you will also find approximate tables of performance in kW for individual dimensions at various temperature differences.
Frequently asked questions about heating, water and floor heating pipes
Can I use the same pipe for floor heating and for potable water distribution?
Yes, provided the pipe has the appropriate hygiene certificate for contact with drinking water and at the same time an oxygen barrier required for a closed heating circuit. Many universal PEX and multi-layer pipes in the professional range meet these properties simultaneously, but you must verify this in the technical sheet of the specific product.
What is the difference between PN6 and PN10 for the same pipe diameter?
The PN marking indicates the maximum operating pressure at a specific temperature - PN6 at 90 °C means that the pipe can safely withstand a pressure of up to 6 bar at this high temperature, PN10 at 60 °C means a higher allowable pressure of 10 bar, but only at a lower temperature. It is the same physical property of the material expressed for two different operating points, not two different products.
Is it better to buy pipe in one large spool or in several smaller ones?
For floor heating and long straight runs, it is always better to choose one sufficiently large spool to avoid joints embedded in the floor or wall. For smaller jobs or where you need different lengths for several shorter loops, it pays to calculate the total consumption and choose a combination of spools with the least waste.
Must floor heating pipe have an oxygen barrier?
Yes, definitely. Without an oxygen barrier (EVOH layer in pure PEX or an aluminum layer in multi-layer pipe), there is a slow but permanent penetration of oxygen into the heating water, which causes corrosion of steel parts of the system - circulation pumps, heat exchangers in the boiler and steel radiators. This property is standard for professional heating pipes, but in cheaper universal PE pipes intended only for water, you do not need to look for it.
What pipe diameter do I need for a typical single-family house with floor heating?
For the heating loops in the floor, 16 or 17 mm is used in the vast majority of cases. The exact diameter and number of loops, however, depends on the heat loss of individual rooms, the length of the loops and the laying spacing - an approximate calculation procedure can be found in the article on selecting the pipe diameter according to the type of distribution.
Can PEX pipe be straightened again if it deforms during handling?
Yes, for pure PEX-a pipe - thanks to the so-called shape memory, the material returns to its original circular cross-section after mild heating (e.g. with warm air) or even on its own over time. In multi-layer pipe with an aluminum layer, this property is missing, the aluminum remains in the new shape after deformation, so you must handle it more carefully during handling. Common errors during handling and their solutions are discussed in the article on Common Mistakes and Problems During Pipe Installation.
Conclusion
Selecting pipe for heating, water distribution or floor heating is not just a matter of price per meter, but a combination of several technical parameters - material (PEX or multi-layer), diameter corresponding to the actual flow and performance, pressure-temperature class matching the operating conditions of the system, and delivery form (bar or spool) matching the characteristics of the route. An investment in a slightly more expensive but correctly dimensioned and certified pipe always pays off in the form of trouble-free, long-term operation without the risk of leaks, corrosion or insufficient performance. If you are unsure which specific type and dimension is the right one for your project, we recommend also reading other related topics in this Knowledge Center - especially the comparison of PEX and multi-layer pipe, calculation of the required diameter and step-by-step installation procedure - or to consult directly with our technical team before ordering the material.
Do you have a question on this topic?
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