What pipe diameter do I need for heating: 15, 22 or 28 mm

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What pipe diameter for heating do I need: 15, 22 or 28 mm?

This is one of the most common questions that installers and DIY builders deal with when designing or renovating a heating system. At first glance, this seems to be a simple matter – after all, it's enough to buy some pipe and connect the radiators – but in practice, it determines whether the system will work efficiently, quietly and with low operating costs, or whether you will be dealing with insufficient performance, noise or premature wear of the circulation pump for years. In this article, we will go through all three common diameters – 15, 22 and 28 mm – and show you exactly when to use which one, how to calculate it and what happens if you make a mistake.

If this is your first time here and you want to first understand the basic differences between types of HEPWORTH pipes, I recommend reading the article How to choose the right HEPWORTH pipe for heating in our Knowledge Centre. But now we'll dive straight into the dimensions.

Why the pipe diameter really matters

Heating pipe is not just a "pipe through which water flows". It is the hydraulic highway of the entire system. Each of its sections has a certain flow capacity – the maximum flow that can pass through it without pressure losses rising to unacceptable levels. When the pipe is too narrow for a given flow, several problems occur simultaneously:

The speed of water flow is too high, which causes noise (hissing, cracking, vibrations).
Pressure losses in the pipe are large, the circulation pump has to work at full capacity – higher electricity consumption.
Radiators at the end of the branch do not receive enough heat transfer medium – insufficient performance.
Long-term high speeds erode the inner wall of the plastic pipe – reduced lifespan.

On the other hand, when the pipe is unnecessarily large:

Higher material and fitting costs.
Larger volume of water in the system – slower warm-up, larger expansion tank.
In some cases, lower flow speed can cause problems with system venting.

A golden middle path exists and can be determined quite accurately. Let's get to it.

Basic hydraulic rules for dimensioning

For plastic multilayer pipes for heating (such as HEPWORTH), the same hydraulic laws apply as for copper or steel pipes, with the difference that plastic pipes have a smoother inner wall and therefore slightly lower friction. For practical dimensioning, three parameters are of interest to you:

Thermal power of the branch (W or kW) – how many watts the branch needs to deliver.
Temperature difference of the system (ΔT) – the difference between the temperature of the supply and return water. Classic system: 80/60°C → ΔT = 20 K. Modern condensing boiler: 70/50°C or 55/45°C → ΔT = 10–20 K. Floor heating: 45/35°C → ΔT = 10 K.
Water flow (l/h or m³/h) – it is calculated using the formula: Q = P / (c × ρ × ΔT), where P is the thermal power in W, c = 4 186 J/(kg·K) is the specific heat capacity of water and ρ = approx. 1 kg/l. For routine calculations, a simplified formula is sufficient: Q [l/h] ≈ P [W] / (ΔT [K] × 1,163).

For example: a branch with a power of 5 000 W, temperature difference of 20 K → Q = 5 000 / (20 × 1,163) ≈ 215 l/h. This number is then compared with the capacity of individual diameters.

The recommended flow velocity of water in plastic heating pipes is 0.3 – 0.8 m/s. Below 0.3 m/s, problems with venting and removal of impurities may occur; above 0.8 m/s, noise and erosion occur. Some sources mention a maximum of 1 m/s, but I recommend staying below 0.8 m/s in the long term, especially with plastic pipes.

15 mm pipe – when to use it

Fifteen-millimetre pipe is a real workhorse in heating for the last section of the distribution – so-called branches to radiators. It is the smallest commonly used diameter and has a very clearly defined range of use.

Capacity: At a flow velocity of 0.5 m/s, about 150–180 l/h can flow through a 15 mm pipe, at 0.8 m/s about 250–300 l/h. In thermal power, this corresponds to about 3 500 – 7 000 W at ΔT = 20 K. That sounds quite a lot, but remember that in practice, one branch to a radiator rarely needs more than 1 500 – 2 500 W.

Where 15 mm belongs:

Connections to individual radiators (so-called last section to the radiator, length usually 0.5 – 3 m).
Bathroom manifolds and low-power bathroom radiators (up to 600 W).
Series connection of radiators in a smaller room.
Heating mats and small individual circuits.
Star distribution systems from a distributor (manifold), where each radiator has its own branch and flow is precisely regulated by thermostatic valves.

Where 15 mm does not belong:

Main distribution in the house – even in small houses.
Sections where the combined power of more than two larger radiators is required.
Long horizontal runs (over 6–8 m) without a distributor.

The product we recommend for these purposes: HEPWORTH pipe for heating, 15 mm is available in coils, which is ideal for running under the floor or in the wall without unnecessary joints. For installation in visible or behind-the-wall installations, HEPWORTH pipe for heating in rods, 15 mm is advantageous, where it is easier to cut to precise lengths.

Pipe 22 mm – universal distribution for most homes

The 22 mm pipe is in practice the most commonly used diameter for main distribution in single-family homes. If you are renovating or building a single-family home with central heating, you will almost certainly use 22 mm for most of the horizontal distribution lines.

Capacity: At a flow rate of 0.5 m/s, we are looking at around 350–400 l/h, at 0.8 m/s around 600–700 l/h. In thermal output (ΔT = 20 K), this corresponds to approximately 8 000 – 16 000 W, i.e., 8 – 16 kW. This is sufficient for most single-family homes up to 200 m², where the boiler has a power of 10 – 18 kW.

Where 22 mm belongs:

Main horizontal distribution in a single-family home (horizontal floor distribution).
Vertical pipes in two-story homes, when one floor has a power of up to 8–10 kW.
Supply to a manifold for multiple radiators.
Connection of floor heating to the system – branch to a separate floor heating circuit.
Supply and return pipes to a hot water storage tank (boiler).

Practical example from practice: Heating renovation in a brick house from the 80s, floor area 140 m², boiler Viessmann 18 kW. The entire system was originally made of steel 1" (26 mm) pipes, which were rusty and clogged. We designed the new distribution as follows: main horizontal floor distribution from the boiler throughout the basement in 22 mm, vertical distribution to radiators in 15 mm, branches to individual radiators in 15 mm. Result: the system works quietly, the boiler starts less often and the customer saves about 12 % on gas compared to the previous state (partly due to better hydraulics, partly due to thermostatic heads).

For this type of installation, we recommend Pipe HEPWORTH, heating, 22 mm on spools, or Pipe HEPWORTH for heating in bars, 22 mm for visible basement distribution.

Pipe 28 mm – when a larger diameter is necessary

The 28 mm pipe is reserved in home installations for high-performance systems or long main distribution lines. It is not a daily diameter, but when you need it, there is no substitute for it.

Capacity: At a flow rate of 0.5 m/s around 700–800 l/h, at 0.8 m/s up to 1 100–1 200 l/h. In thermal output (ΔT = 20 K), this corresponds to 16 000 – 28 000 W, i.e., 16 – 28 kW. This is not usually necessary for single-family homes, but it is for larger buildings.

Where 28 mm belongs:

Boiler circuit (so-called boiler distribution) – connection of the boiler to a hydraulic separator or boiler manifold, where the entire boiler output flows.
Main supply to a large manifold in a house with multiple circuits (floor heating + radiators + storage tank).
Buildings with heating output above 20 kW – larger single-family homes above 250 m², multi-generational homes, small commercial spaces.
Connection of two boilers in cascade.
Supply to a storage tank with a heat pump (where the output can be up to 14–20 kW).

Common mistakes with 28 mm: I have seen installations where the installer used 28 mm throughout the house "to have a reserve". The result was that the circulation pump could not maintain sufficient flow speed, air was not being vented, there was constant noise, and the system had to be reworked. A larger diameter does not mean a better system – it must be correctly dimensioned.

For this diameter, I recommend looking at Pipe HEPWORTH, heating, 28 mm.

Table: quick overview – which diameter for what

Diameter	Max. flow (l/h)	Max. thermal output (ΔT 20K)	Typical use
15 mm	up to 300 l/h	up to approx. 7 kW	Connections to radiators, bathroom manifolds, last section of the distribution
22 mm	up to 700 l/h	up to approx. 16 kW	Main distribution in single-family homes up to 200 m², rising branches, supply to manifold
28 mm	up to 1 200 l/h	up to approx. 28 kW	Boiler basement, large family homes, main supply to multi-circuit systems, cascade systems

How sizing differs with various temperature differences

An important factor that is often overlooked when choosing a diameter is the system's temperature difference. Modern condensing boilers and heat pumps operate at lower temperatures and smaller ΔT, which means that for the same thermal output you need a higher flow – and potentially a larger pipe diameter.

Example: a boiler room with a power of 12 kW.

Old radiator system 80/60°C (ΔT = 20 K): Q = 12 000 / (20 × 1,163) ≈ 516 l/h → 22 mm is sufficient.
Modern condensing boiler 70/50°C (ΔT = 20 K): Q ≈ same 516 l/h → 22 mm is sufficient.
Heat pump 45/35°C (ΔT = 10 K): Q = 12 000 / (10 × 1,163) ≈ 1 032 l/h → 22 mm is not sufficient, 28 mm is needed!

This is one of the most common mistakes during renovations: the customer replaces the boiler with a heat pump but keeps the original 22 mm pipes. The result is that the pump cannot deliver enough heat, the system is inefficient, and the customer is puzzled why their electricity bills are high.

Therefore, the rule applies: always size the pipes according to the actual heat source and its operating temperatures, not according to what was originally in the house.

Reducing the diameter along the branch – how it works in practice

A properly designed heating system does not use one diameter from the boiler to the last radiator. On the contrary, the diameter is gradually reduced as branches divert part of the flow to individual radiators. This is called progressive reduction and is the basis of proper hydraulic design.

A typical example for a house with 5 radiators and a 14 kW boiler, ΔT = 20 K, total flow 603 l/h:

Boiler basement (total flow 603 l/h): 28 mm or strong 22 mm.
Main horizontal branch (after 1st branch, ~480 l/h): 22 mm.
Middle branch (after 2nd branch, ~360 l/h): 22 mm.
Shorter branch (after 3rd branch, ~240 l/h): 22 mm or 15 mm.
Last branch (~120 l/h): 15 mm.
Each connection to a radiator: 15 mm.

In practice, this is also called a "tree-like sizing" or "branching system" and is standard for distribution systems without a manifold. In systems with a manifold (distributor), it is simpler: from the boiler to the manifold, 22 or 28 mm is used, and from the manifold to each radiator, 15 mm is used regardless of length.

More about how systems with and without a manifold differ can be found in the article Installation of HEPWORTH pipe for heating step by step in this Knowledge Center.

The influence of pipe length on diameter selection

Along with flow rate, the length of the branch also plays an important role. The longer the pipe, the greater the pressure loss (pressure drop). For long branches, it is valid that even at a lower flow rate, it may be more advantageous to use a larger diameter in order to keep pressure losses within reasonable limits.

Practical rule of thumb:

Branch shorter than 5 m to a radiator: 15 mm is perfectly fine.
Branch 5 – 15 m with multiple radiators: 22 mm for the main part, 15 mm only for the last branches.
Branch longer than 15 m or a main horizontal pipe running through the whole house: 22 mm or 28 mm depending on the capacity.

In practice, I often encounter a situation where the customer wants to save money and runs the entire house in 15 mm. The result is that the boiler, running at full capacity, is unable to push enough water through such narrow pipes, the circulation pump hums, and the last radiators are cold. Paradoxically, the customer ends up paying several times more for service interventions and higher energy consumption for this "savings" on piping.

Coils vs. rods – does the diameter choice depend on it?

When choosing a diameter, you will also encounter the question of packaging form – coils or rods. For everyday use, it holds true that 15 mm and 22 mm are commonly available in both forms, while for larger diameters (28 mm), the rod form is more common due to the material's rigidity.

We discuss this topic in detail in the article HEPWORTH piping in coils vs. in rods – what to choose. In short: coils are advantageous for long runs without joints (under the floor, in the wall), while rods are preferred for visible installations in the basement, boiler room, or under the wall, where you appreciate a straight line.

Mistakes in diameter selection that we see most often

Over the years of practice, the same mistakes repeat themselves. Here are the most common ones to help you avoid them:

Same diameter from the boiler to the last radiator – common in DIY installations. Result: uneven heating, noise, and disproportionate strain on the pump.
Ignoring the temperature drop – calculating the flow without considering the actual operating temperature of the system. Critical when switching to a heat pump.
Too long 15 mm branches – supply lines 8–10 m long to a radiator in 15 mm create enormous pressure losses.
Underestimating future expansions – the customer installs a 15 mm main pipe and then wonders why the system fails after adding another radiator.
Overdimensioning out of "precaution" – the whole house in 28 mm, leading to low flow speeds, problems with air venting, and unnecessarily high costs.
Mixing up external and internal diameter – a 15 mm diameter is the external diameter; the actual internal diameter is only around 11 mm. This is a technical specification you need to consider in calculations.

We discuss these mistakes in more detail in the article Common mistakes in the installation of plastic piping for heating.

Practical 5-step procedure for selecting the diameter

If you want to choose the right diameter without unnecessary hydraulic calculations, proceed as follows:

Determine the boiler capacity and temperature drop (from the boiler's technical sheet or design).
Calculate the total flow rate: Q = P / (ΔT × 1,163).
Distribute the flow rate to the branches according to the capacity of each branch and divide into partial flow rates.
Assign a diameter to each branch according to the table above, respecting a flow velocity of 0.3–0.8 m/s.
Check for gradual reduction – the diameter decreases towards the radiators, never increases.

For most standard single-family homes, it holds true: boiler main pipe 22 or 28 mm, main runs 22 mm, radiator connections 15 mm. This rule covers about 80% of standard installations.

Frequently asked questions (FAQ)

Can I use 15 mm piping for an entire small apartment with 3 radiators?

It depends on the radiator output and the length of the runs. If you have a boiler up to 6 kW, a temperature drop of 20 K, and all runs are shorter than 5–6 m, it is technically on the edge of feasibility. In practice, this almost always results in noise and uneven heating. I recommend at least the main loop or horizontal run to be done in 22 mm and only the short connections to radiators (1–2 m) to remain in 15 mm. You will save less on material than you think, and the system will operate more reliably.

Do I need to change the piping if I replace the boiler with a heat pump?

Very likely yes, at least partially. Heat pumps operate at lower temperatures and a smaller ΔT (typically 5–10 K instead of 20 K), which means that for the same output, they require 2–4 times higher flow. If your current runs are in 22 mm, it may be necessary to at least expand the boiler main to 28 mm. This needs to be assessed individually for each building.

What is the difference between the external diameter 15/22/28 mm and the actual flow cross-section?

The numerical designation of the diameter (15, 22, 28 mm) always refers to the external diameter. The internal diameter is smaller by the wall thickness (including the aluminum layer and plastic). For HEPWORTH 15 mm piping, the internal diameter is approximately 11 mm, for 22 mm it is approximately 16 mm, and for 28 mm it is approximately 20 mm. These values are crucial for hydraulic calculations, so always work with the technical sheets of the specific product.

Can I combine piping in coils and rods in one installation?

Yes, without any problem. Both forms are made of the same material and can be connected with the same fittings. A common practice is: horizontal runs in the basement and boiler room from rods (straight lines, easier to look at), and runs under the floor and in walls from coils (no joints, easier to pull through transitions). More on this can be found in the article HEPWORTH piping in coils vs. in rods – what to choose.

How can I find out what diameter is installed in my house if I don't have the original documentation?

The easiest way is to directly measure the external diameter with a sliding caliper (vernier caliper) on a visible section of the pipe – for example, in the basement. If the pipe is multilayer plastic, 15 mm corresponds to the width of a thumb, 22 mm is about 2.2 cm, and 28 mm is about 2.8 cm. Another option is to compare the dimensions with the fittings – fittings are usually marked with the diameter directly on the body.

Is 22 mm piping always sufficient for a single-family house?

For most single-family houses up to 200 m² with a gas or electric boiler up to 18–20 kW and a classic temperature drop of 20 K, 22 mm is sufficient for the main runs. Exceptions include: large houses over 250 m², systems with heat pumps (low ΔT), cascade boiler systems with multiple boilers, and systems where the main horizontal pipe is longer than 20 m. In such cases, at least partial dimensioning in 28 mm is necessary.

Conclusion: the rule of three diameters in practice

Selecting the correct diameter for heating piping is not rocket science, but it does require some systematic thinking. The basic rule that works for most single-family houses is: 28 mm for the boiler main at capacities over 16 kW, 22 mm for the main horizontal and vertical runs, and 15 mm for each radiator connection. If you are unsure, it is better to invest in a larger diameter for the main run – fittings and labor for rework are much more expensive than the price difference per meter of pipe.

For every installation, whether it is a new build or a renovation, I recommend always starting with specific numbers: boiler capacity, temperature drop, and calculated flow rate.

Do you have a question about this topic?

Having trouble deciding or dealing with a specific situation in your home? Write to us – we are happy to help.