How to correctly dimension the length of PEX pipe coil for floor heating
Dimensioning the length of the PEX pipe coil is a key step that directly affects the hydraulic balance and energy efficiency of the entire floor heating system. Any deviation from the recommended limits can lead to an unbalanced system, where some rooms are overheated and others are insufficiently thermally loaded.
Hydraulic limits and maximum branch length
The main limiting factor in calculating the length of the coil is not the boiler capacity, but the pressure loss due to water friction against the pipe walls. For standard home systems, a length of one coil up to 100 meters is considered safe and optimal, while the ideal range for achieving an even temperature field is between 60 to 90 meters.
- Maximum allowed length: 100 m (without compensation).
- Optimal length: 60 – 90 m for even circulation.
- Critical limit: above 110 m there is a significant drop in flow.
Influence of pipe diameter on length capacity
The pipe diameter directly determines the flow speed and thus the pressure loss; the narrower the pipe you use, the shorter the coil must be to maintain sufficient flow. For standard floor heating, the most commonly used diameter is DN15 (internal diameter approx. 16 mm), which is a compromise between material savings and hydraulic resistance, although there are also variants with a larger diameter.
If you need to cover a larger area or have long routes, it is advisable to consider using thicker types that allow more heat to be transferred over the same distance without excessive losses. For example, PEX Pipe - 25x2.3 offers lower flow resistance, which allows for more efficient heat transfer over longer distances, but it requires a more precise calculation of the room's thermal output.
Difference between theoretical and real storage
Theoretical calculations based solely on room geometry often underestimate the actual system needs, as they do not account for the number of bends and fittings, which increase local resistance points. In practice, it is recommended to add approximately 10 % extra length to the total coil length for every five bends and fittings to avoid situations where the system is hydraulically "closed off."
This factor is critical during installation in complex spaces, where the pipe must bend around load-bearing walls or conduits. When selecting the length of the coil, for example, PEX Pipe - 17x2 in 120m, always reserve a small amount for adjustments and repairs that may become necessary during embedding in concrete.
Methods of measuring and calculating length
The most accurate way of measuring is by using digital planning tools that can automatically integrate room geometry with the required pipe spacing. These programs generate an exact length in meters, including a coefficient for bends, thus eliminating human error in manual calculations.
For simpler projects, it is sufficient to use the "unfolding" method, where you draw a coil layout on paper at a scale of 1:50 and measure individual sections using a ruler and template. This method is suitable for small apartments or individual rooms where the geometry is simple and the number of bends is limited.
Practical examples from realizations
In practice, we have encountered a situation where a customer wanted to use a single coil of 120 meters for a living room of 40 m², which led to the second side of the room being cold while the first side was overheated. After measuring and dividing it into two coils of 60 meters each, the temperature difference was eliminated and the system began to function harmoniously.
Conversely, when using PEX Pipe - 17x2 in 240m for large industrial halls, it turned out that the system had to be divided into multiple separate loops, each with its own regulator, to ensure the stability of the entire system and avoid temperature fluctuations.
The role of distributors and hydraulic balance
The length of the coil is not the only factor that determines the system's functionality. The type of distributor (manifold) to which the coil is connected also plays a key role. Each loop must be equipped with its own regulating valve and flow meter to compensate for the different lengths of individual branches.
If you have coils in the system with significantly different lengths – for example, one is 50 meters and the other is 90 meters – an unbalanced hydraulic condition arises. Water naturally chooses the path of least resistance, meaning that the shorter coil will have a higher flow and the water will be hotter, while the longer coil will be under-pressurized and insufficiently heated. Therefore, distributors must allow for flow adjustment for each branch individually, but the ideal is to have coil lengths as balanced as possible, ideally with a difference of no more than 10 %.
Effect of Thermal Output and Pipe Spacing
The length of the coil is directly proportional to the area of the room and inversely depends on the required thermal output. If you need to heat a room with a high thermal load (e.g., large glass surfaces, cold external walls), you must increase the pipe density, i.e., reduce the spacing between them. Reducing the spacing from 20 cm to 15 cm significantly increases the required pipe length per square meter.
This fact often leads to the length of a single coil in small rooms with high heating demands (such as bathrooms or winter gardens) quickly approaching the upper limit of 100 meters. In such cases, it is technically more appropriate to create two shorter coils with a smaller spacing rather than one extremely long coil with a larger spacing, which could cause local "cold spots".
| Pipe Spacing (cm) | Approximate Length per m² | Application |
|---|---|---|
| 20 cm | ~5 m | Standard residential areas |
| 15 cm | ~6.7 m | Rooms with higher heat loss |
| 10 cm | ~10 m | Large windows, cold zones, bathrooms |
Compensation for Thermal Expansion and Safety Reserves
When dimensioning the length of the coil, it is necessary to account not only for hydraulic losses but also for the physical behavior of the material. PEX pipe has significant thermal expansion, which is approximately 1.5 to 2 mm per meter at operating temperature. For a 100-meter coil, the total length of the pipe can increase by more than 15 cm, creating internal stresses in the concrete.
To avoid cracking or deformation of the floor, it is advisable to consider the installation of expansion joints or to divide the coil into two separate loops for very long coils (over 80 meters). In addition, we always recommend purchasing a package with at least a 5-meter safety reserve. This reserve is useful for repairing damaged sections during installation, which cannot be easily soldered and must be replaced, or for adjusting the geometry in case of incorrect planning.
Strategic Division of Loops in Large Buildings
In implementations for single-family homes or smaller office spaces, we often encounter the problem that individual rooms are located at different distances from the boiler room. In such cases, the strategy for dividing the coils is based on the logic of the "longest path". It is recommended that the maximum length of the longest coil in the system does not exceed 100 meters and that the difference between the shortest and longest coil is as small as possible.
This means that if you have a room located 80 meters away and another only 30 meters away, you cannot use one coil for both, as it would disrupt the hydraulic balance. The correct solution is to create two separate coils for both rooms, or to connect them via a common manifold with the possibility of individual flow adjustment. This ensures that each coil operates in an optimal mode and the system consumes minimal energy to overcome pressure losses.
Frequently Asked Questions
How is the length of the coil calculated for complex room shapes?
All pipe sections are included in the total length, including bends, with an additional 10 % added for every fifth bend to compensate for pressure losses.
Can I use a longer coil than the recommended 100 meters?
Technically, this is possible only with an increased pump flow and a larger pipe diameter, but in the long term, it leads to system imbalance and higher energy consumption.
How does the pipe spacing affect the length of the coil?
A smaller pipe spacing (e.g., 10 cm) means a greater number of pipes and thus a longer coil for the same area, which increases material costs and hydraulic resistance.
Proper dimensioning of the coil length is the foundation for the long-term and energy-efficient operation of your floor heating system, so it is advisable to invest time in an accurate calculation before installation.
Do you have a question on this topic?
Struggling to decide or dealing with a specific situation in your home? Write to us – we are happy to help.