What pipe diameter do I need based on the type of distribution

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Why the pipe diameter is one of the most important decisions when designing a distribution system

When building or renovating a heating system, water distribution or floor heating, most laymen and many less experienced tradespeople focus mainly on the boiler's output, type of radiators or the brand of the circulation pump. The pipe diameter is often chosen "by eye" - taking what is customary or what was left over from a previous job. In practice, however, it is one of the most critical parameters of the entire system, because an incorrectly chosen diameter becomes apparent only after the building is completed, when the correction involves breaking up floors, cutting into walls or dismantling floor coverings.

An oversized pipe diameter means unnecessarily large water volume in the system, slower response of the regulation, higher material costs and, in the case of floor heating, also worse thermal inertia of the floor. On the other hand, an undersized diameter causes high pressure loss, noise from the flow, insufficient flow to the most distant consumers and, in the final consequence, the radiators in the last room simply won't heat up. Proper dimensioning is therefore not a detail - it is the basis of a functional, quiet and economically operated system.

This article follows previous texts from the Knowledge Center - especially the articles How to choose pipe for heating, water or floor heating and PEX vs multilayer pipe - differences and use. While there we focused on materials and pipe construction, here we focus exclusively on the question - what pipe diameter do I need for a specific type of distribution, and how to arrive at this number without unnecessary guessing.

What actually determines the pipe diameter

The pipe diameter is not an isolated value that you choose from a catalog - it is the result of three interrelated quantities: thermal output (respectively water flow), the speed of the medium flow and the acceptable pressure loss on a given section of the route. If you know these three quantities, the pipe diameter can be calculated quite accurately. Let's look at them individually.

Thermal output and flow

Each section of pipe transfers a certain thermal output (in kW) that a given branch needs - for example, a radiator, a floor heating loop or a group of water outlets. This output is recalculated for a given temperature difference (difference between supply and return temperature) into a mass or volume flow of water. A simplified relationship applies:

Q = m × c × ΔT

where Q is the thermal output (kW), m is the mass flow of water (kg/s), c is the specific heat capacity of water (approximately 4.18 kJ/kg·K) and ΔT is the temperature difference (K). From this formula, the required flow for a given branch can be expressed. The larger the temperature difference you choose (for example, 20 °C instead of 10 °C in floor heating), the smaller the flow is sufficient to transfer the same output - and thus a smaller pipe diameter may be sufficient. This is why low-temperature systems with a larger temperature difference sometimes require thinner piping than might appear at first glance.

Medium flow speed

The second key parameter is the speed of water flow in the pipe. In practice, for different types of distribution, approximate recommended ranges have been established:

Floor heating: 0.2 - 0.6 m/s (low speed due to quiet operation and even temperature distribution in the loop)
Radiator distribution - connections to bodies: 0.3 - 0.8 m/s
Main heating distribution and risers: 0.5 - 1.0 m/s, up to 1.5 m/s in exceptional cases for short sections
Hot and cold water distribution: 0.5 - 1.5 m/s, up to 2 m/s in short sections with larger diameter

If the flow speed exceeds the upper limits of these ranges, noise from the flow, erosion of fittings by the water and unnecessarily high pressure loss occur in the system, which the circulation pump must overcome. If the speed is too low, on the other hand, there is a risk of air and dirt settling in the pipe and, in the case of water distribution, the risk of water stagnation.

Pressure loss and route length

The third factor is the length of the route and the number of fittings (bends, T-pieces, reductions) that offer resistance to the flow. Pressure loss increases with the square of the flow speed and linearly with the length of the pipe. Therefore, in long routes (for example, a distribution to a distant bathroom radiator on a higher floor), it pays to choose a larger diameter, even if a smaller one would be sufficient in terms of pure performance - this will save on pump operating costs and reduce noise.

The above diagram shows the difference between the two most common PEX pipe diameters - 17x2 mm (inner diameter approx. 13 mm) and 20x2 mm (inner diameter approx. 16 mm). At first glance, a small difference in the designation means almost 50 % difference in the cross-sectional area, which directly affects the flow capacity of the pipe.

Pipe diameters for different types of distribution

Floor heating

The situation with floor heating is specific - the pipe is embedded in a thin layer of screed (typically 45-65 mm above the pipe) and must meet a compromise between sufficient flow, flexibility when laying and minimal impact on the floor construction height. In the vast majority of apartment and family homes, therefore, two diameters are used:

16-17 mm (e.g. PEX 17x2) - standard for living rooms, bathrooms, bedrooms. The recommended maximum length of one loop is approximately 80-100 m, with a spacing between pipes of 150-300 mm depending on the room's heat loss.
20 mm (e.g. PEX 20x2) - used for larger areas (halls, industrial floors, large living rooms) or where a higher flow per loop and longer route without unnecessary pressure loss is needed.

For a typical family home, a typical choice is PEX 17x2 pipe in a 120 m coil for smaller loops, or PEX 17x2 pipe in a 240 m coil for larger areas or multiple loops at once without the need to connect the coil. In more extensive projects, for example in multi-storey family homes or larger layouts, a larger coil PEX 17x2 pipe in a 600 m coil is justified, which reduces the number of joints along the route and thus the risk of leaks.

Radiator heating - connections, risers, main distribution

In radiator heating, the diameter changes gradually from the source towards individual bodies - the system branches out and with the decreasing transferred output, the required diameter also decreases. Approximately:

Connection of one radiator (output up to approx. 1.5-2 kW): PEX 17x2, occasionally 16x2 for a short connection and a small radiator
Branch for 2-4 radiators / longer connection with loss: PEX 20x2
Risers and main distribution in a smaller family home (output up to approx. 15-20 kW per branch): PEX 25x2.3
Main distribution from the heat source at higher output (over 20-25 kW): here it is customary to switch to copper or multilayer pipe of larger dimension, or steel pipe in boiler rooms

For a medium-sized family home with multiple radiators, the common combination is therefore PEX Pipe 20x2 in a 200 m coil for branches with multiple bodies and PEX Pipe 25x2.3 in a 200 m coil for the main distribution from the manifold or boiler to the individual floors.

Hot and cold water distribution

For hot and cold water distribution, the diameter is selected based on the number and type of connected outlets (basin, shower, bathtub, WC, kitchen sink, washing machine) and on the simultaneity of their use - it is not realistic that water would flow from all outlets at once, so the so-called simultaneous withdrawal is calculated. Approximately:

Connection of one outlet (basin, WC): 15-16 mm
Connection of a shower, bathtub, or kitchen mixer: 16-17 mm, 20 mm for higher flow
Distribution to a group of outlets in the bathroom: 20 mm
Main vertical distribution in an apartment / family house: 25 mm, 25-32 mm for a larger house or multiple bathrooms

When it comes to water, it is also important to remember that hot water cools down in the system if the route is too long and the diameter is too large - a larger diameter contains a larger volume of water, which the user has to flush out first before hot water actually starts flowing from the tap. Therefore, for DHW it is recommended to choose the middle rather than the upper limit of the recommended diameter and to minimize the length of the route to individual outlets.

Approximate table of diameters according to power and use

The following table summarizes approximate values commonly used in practice for preliminary design (not a substitute for a precise hydraulic calculation for large or complex systems, but sufficient for a standard family house or apartment):

Type of distribution	Approximate power / use	Recommended PEX diameter
Floor heating - loop in a living room	up to approx. 1.5-2 kW per loop, length up to 100 m	16-17 mm
Floor heating - large area / hall	over 2 kW per loop or longer route	20 mm
Connection of a radiator	up to 2 kW	16-17 mm
Branch for multiple radiators	2-6 kW	20 mm
Vertical pipe / main distribution for a smaller house	6-20 kW	25 mm
Main distribution for a larger house / higher power	over 20-25 kW	32 mm and more (other material)
Connection of a water outlet (basin, WC)	1 outlet	15-16 mm
Connection of a shower, bathtub	1-2 outlets	16-20 mm
Distribution to the bathroom	group of outlets	20 mm
Main vertical water distribution	entire apartment / house	25-32 mm

How to calculate the diameter yourself - a simple step-by-step procedure

If you do not want to rely only on tabular values and want to be sure for a specific job, the pipe diameter can be derived relatively easily through your own calculations. The procedure is as follows:

In practice, the simplified formula V = Q / (1.163 × ΔT) is commonly used to calculate the flow rate, where V is the flow in liters per hour, Q is the power in kW and ΔT is the temperature difference in °C (the constant 1.163 comes from the specific heat capacity of water converted to watt units). Once you know the flow rate, the required internal cross-section of the pipe is calculated according to the relation S = V / w, where S is the cross-section and w is the velocity, based on the desired flow velocity (e.g. 0.4 m/s for floor heating). From the cross-section, the internal diameter is then calculated and the wall thickness of the pipe is added to it, resulting in the required external diameter.

In practice, however, very few people calculate this manually for each job - there are online calculators and manufacturer tables that simplify this calculation. It is important to understand the principle: more kW or longer route = need for a larger diameter or larger temperature difference, so that the flow and velocity remain within the recommended range.

Practical examples from everyday practice

Example 1 - Family house 140 m², floor heating

A typical single-story family house with a heat loss of about 8 kW and floor heating in all rooms. When divided into 8-10 loops, the average power per loop is approximately 0.8-1 kW, which is safely within the range for PEX 17x2. In practice, a combination of larger coils is commonly used for such a job, for example, PEX Pipe 17x2 in a 240 m coil, or for a larger number of rooms and longer loops, directly 600 m coil, to minimize the number of joints in the screed - a joint in the screed is the only place that cannot be checked without destroying the floor after pouring.

Example 2 - 3+1 apartment with floor heating in the bathroom and radiators elsewhere

A combined system where the bathroom has a small area of floor heating (one short loop, PEX 17x2 in a 120 m winding is fully sufficient) and the remaining rooms have panel radiators connected via a manifold. Connections to radiators are made using PEX 17x2, and the main distribution from the riser to the apartment manifold is routed in PEX 20x2, or in PEX 25x2,3 in the case of higher overall apartment output (for example, when electric floor heating is combined with a hydronic system).

Example 3 - New build with a heat pump and a low-temperature system

With a heat pump, the temperature of the heating water is lower and the temperature difference is smaller (often only 5-7 °C instead of the classic 10-15 °C with a boiler). A smaller temperature difference means that for the same output, a higher flow rate is needed - therefore, for these systems, it is recommended to use the higher diameter limit from the table, i.e., PEX 20x2 where PEX 17x2 would be sufficient for a boiler system, especially on main lines and longer branches. This is a common mistake we see when retrofitting older systems for heat pumps - the original pipe, dimensioned for a larger temperature difference, becomes a bottleneck in the whole system and the pump has to run at higher speeds than necessary.

Example 4 - Water distribution in a family house with two bathrooms

The main vertical distribution from the water meter assembly is routed in a diameter of 25-32 mm (multilayer or copper pipe), from which it branches to individual bathrooms in a diameter of 20-25 mm and to individual outlets (basin, WC, shower) in a diameter of 15-16 mm. When using a shower and a washing machine simultaneously in one bathroom, it is advisable for the shared distribution before branching to have at least 20 mm, otherwise, during simultaneous draw-off, there will be a noticeable drop in pressure and temperature at both outlets at the same time.

The graph above illustrates why even a small difference in diameter (for example, from 20 mm to 25 mm) means a significant increase in the pipe's flow capacity - at the same flow velocity, the flow increases approximately with the square of the diameter. Therefore, it is worth choosing a slightly larger diameter for main lines and risers, rather than later encountering insufficient performance under full system load.

Most common mistakes when choosing pipe diameter

Over the years of implementation and advisory work, we repeatedly encounter several typical mistakes that can be easily eliminated with simple reasoning in advance:

Using the same diameter throughout the house regardless of branching. It is common to see implementations where the entire distribution from the boiler to the last radiator is routed in the same diameter - either it is unnecessarily oversized at the end branches or undersized at the main distribution.
Underestimating the length of the circuit in floor heating. A loop longer than 100-120 m has a significantly higher pressure loss and uneven temperature distribution along its length - instead of extending the loop, it is better to add another circuit to the manifold or choose a larger diameter.
Not taking the temperature difference into account with heat pumps. As mentioned above, a smaller temperature difference requires a higher flow rate, and therefore often also a larger diameter, than with classic boiler heating.
Underestimating simultaneous water draw-off. For water distribution, it is worth calculating with a realistic usage scenario (for example, morning peak when showering and the washing machine is running simultaneously), not just with a theoretical single draw-off.
Combining diameters without suitable reductions. A transition between different diameters must be solved with a proper reducing coupling, not by forcing a larger pipe into a smaller one or vice versa - suitable couplings and fittings are discussed in a separate article, What couplings and fittings to use for PEX pipe.

These and other installation mistakes are discussed in more detail in the article Common mistakes and problems in pipe installation, where you will also find steps on how to deal with them retroactively if they have already been made.

Impact of pipe material on diameter selection

With the same outer diameter, the inner diameter can vary depending on the wall thickness and the pipe material. PEX pipe typically has a thinner wall than multilayer pipe with an aluminum insert at the same outer diameter, which means a slightly larger internal cross-section and thus a slightly higher flow capacity. On the other hand, multilayer pipe has the advantage of zero oxygen diffusion and greater shape stability, which is beneficial especially for long straight sections above plaster. A detailed comparison of both materials can be found in the article PEX vs multilayer pipe - differences and applications. When choosing a diameter, it is therefore always a good idea to look at the specific technical sheet of the pipe, where the manufacturer lists the exact inner diameter, not just the outer marking.

Equally important is the issue of pressure and temperature resistance - PEX pipes are commonly supplied in PN6 at temperatures up to 90 °C or PN10 at temperatures up to 60 °C, which must be taken into account especially in systems with higher operating temperatures (classic radiator heating with a boiler) compared to low-temperature systems (floor heating, heat pumps). The topic of pressure and temperature resistance is discussed in detail in the article What pressure and temperature can PEX pipe withstand (PN6, PN10).

How diameter also affects storage and handling

A minor detail that is often overlooked in planning is the fact that with increasing diameter, the minimum bending radius of the pipe and the material stiffness also increase. While PEX 17x2 can be shaped relatively freely by hand in small coils during installation, PEX 25x2,3 requires more space for handling and, in the case of sharper bends, also a bending spring to prevent the pipe wall from collapsing. When ordering larger diameters in long coils, it is therefore advisable to plan in advance how the coil will reach the installation site and how it will be unrolled - more on this in the article Storage and handling of pipes in bars and coils.

Common questions about pipe diameter selection

What PEX pipe diameter is the most universal for a typical family home?

For most standard projects with a combination of floor heating and a smaller number of radiators, a combination of PEX 17x2 for end branches (floor heating loops, radiator connections) and PEX 20x2 or 25x2,3 for main lines and risers is sufficient. We recommend this combination as a starting point for most apartments and smaller family homes, of course taking into account the specific heat loss and layout.

Can I use the same pipe diameter for heating and water distribution?

Technically yes, PEX pipe of the appropriate class can be used for both purposes, but from a dimensional standpoint, it is more appropriate to assess both distributions separately - heating is dimensioned according to thermal output and temperature difference, while water supply is dimensioned according to simultaneous draw-off of individual outlets. In practice, the diameters on both distributions often match (for example, 20 mm), but the reason for their selection is different.

What happens if I choose a too small pipe diameter for floor heating?

If the diameter is too small or the loop is too long, the pressure loss increases to such an extent that the circulation pump is no longer able to ensure sufficient flow. This will result in uneven floor heating - the part of the room near the loop inlet will be warmer, while the part near the outlet will be significantly cooler. The solution is either to shorten the loop, divide it into multiple circuits, or switch to a larger diameter, for example from 17x2 to 20 mm.

Is it possible to increase the pipe diameter later without breaking the floor or wall?

Not directly in the embedded route - pipe embedded in the screed or in a groove cannot be "enlarged". The only option is to add a parallel branch during renovation, or in the case of radiator systems, to run a new pipe of a larger diameter alongside the original one, for example in the floor or in a new groove. This is why it is important to carefully plan the dimensioning already during the design phase, when changing the diameter is a matter of ordering a different winding, not an additional construction intervention.

Does the length of the winding (120 m, 200 m, 240 m, 600 m) affect the diameter I should choose?

Not directly - the length of the winding is only a logistical question - how many meters of pipe you need without unnecessary waste and how many connections you want to have along the route. The diameter is selected based on the performance and flow rate, and the length of the winding is then adapted to the actual needs of the specific project, for example a shorter 120 m winding for a smaller bathroom, or a longer 600 m winding for multiple loops at once without the need for connecting.

Is there a difference in pipe diameter between a system with a boiler and a system with a heat pump?

Yes, as mentioned above - heat pumps operate with a lower temperature difference, which, for the same performance, requires a higher flow rate, and thus often also a larger pipe diameter by one size on the main lines, than would be necessary in a classic boiler system with a higher temperature difference.

Summary and recommendation in conclusion

The choice of pipe diameter should never be random or based solely on "what is customary". It is based on the specific thermal output or flow that a given section needs to transfer, the recommended flow velocity for the type of piping, and the length of the route including the number of fittings. For typical implementations in single-family homes and apartments, the following is generally valid: PEX 17x2 for end branches of floor heating and radiator connections, PEX 20x2 for branches with multiple consumers and medium-sized lines, and PEX 25x2,3 for main lines and risers with higher output. For water distribution, diameters are in similar ranges, but for a different calculation reason - according to the current consumption of individual outlets.

If you are unsure about the correct choice for a specific project, it is always better to choose a diameter one size larger than to later encounter insufficient performance, noisy flow or high pressure loss that must be compensated by an oversized circulation pump. The full range of piping, including all the mentioned diameters and winding lengths, can be found in the category Pipes - heating, water, floor, where you can directly compare and combine different dimensions according to the needs of a specific project.

Do you have a question about this topic?

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