What diameter of solar pipe do I need: DN12, DN16 or DN20

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What solar pipe diameter do I need: DN12, DN16 or DN20?

What seems like a simple question, but upon closer inspection divides plumbers into two camps: those who choose the diameter intuitively based on habit, and those who think about hydraulics, pipe length, and system performance. In practice, the difference between the correct and incorrect solar pipe diameter is visible on the electricity bill (the pump works unnecessarily hard), on the lifespan of the corrugated pipe (excessive flow speed causes erosion), or on the efficiency of the entire solar system (overcooling of the collector at low flow rates). This article will help you understand what the numbers DN12, DN16 and DN20 actually mean, and how to decide for a specific job.

What does the DN marking mean and how it differs from the actual pipe diameter

The abbreviation DN comes from the French Diamètre Nominal and in Slovak corresponds to the term nominal diameter or nominal bore. It is important to understand that DN is not the outer diameter of the pipe – it is a rounded nominal value that is mainly used to standardize fittings, bends and connecting nuts throughout Europe.

In practice, for stainless steel corrugated pipes in solar systems, this means the following:

DN12 – the outer diameter of the corrugated pipe is typically 12–13 mm, the inner bore (ID) is around 9–10 mm
DN16 – the outer diameter of the corrugated pipe is typically 16–17 mm, the inner bore is around 13–14 mm
DN20 – the outer diameter of the corrugated pipe is typically 20–22 mm, the inner bore is around 16–17 mm

These differences may seem small, but when you realize that the flow through a circular cross-section grows with the square of the radius, the difference between DN12 and DN20 is not three times, but almost five times in terms of cross-sectional area. In other words: if you plan a flow of, for example, 3 l/min, the DN12 pipe will carry it with a much higher flow velocity than DN20 – and it is precisely here that hydraulic problems arise.

Basic hydraulic rules that determine the choice of diameter

Solar systems operate with so-called glycol mixture (propylene glycol + water, usually in a ratio of 40–50 % glycol). This mixture has a higher viscosity than pure water, which means you need a more powerful pump to overcome hydraulic resistance – or a larger pipe diameter. Therefore, one of the basic rules of solar hydraulics applies:

The recommended flow velocity in solar piping is 0.3 – 1.0 m/s. Below this limit, stagnation and poor circulation are at risk; above it, increased noise, erosion of the corrugated pipe wall and unnecessary resistance that the circulation pump must overcome occur.

Another key parameter is the specific pressure drop, i.e. the pressure loss per 1 meter of pipe. The recommended value for solar systems ranges between 100–300 Pa/m. If I choose a too narrow pipe, I easily get to 500–700 Pa/m and more, which practically means a significantly larger pump, higher electricity consumption and faster system wear.

Specific flow rates with which solar systems operate:

Small family systems (1–2 flat collectors, 200–300 l storage tank): 1.5 – 3 l/min
Medium family systems (2–4 collectors, 300–500 l storage tank): 3 – 6 l/min
Larger systems (4–8 collectors, community or commercial use): 6 – 15 l/min

DN12: when is this diameter suitable and when not

The DN12 diameter has historically been the most popular for small family solar systems in Germany and Austria, from where solar technologies came to us. The reason is simple: a compact double pipe DN12 can be bent and led through narrow gaps in the masonry or roof, insulated and fixed much easier than thicker versions.

The DN12 diameter is suitable if:

You have 1 to a maximum of 2 flat collectors or 1 vacuum collector (total collector area up to approx. 5 m²)
The length of the route from the collector to the storage tank does not exceed 10–12 meters (total length of both branches – supply and return)
The planned flow is less than 2.5 l/min
The lift height (elevation from the pump to the collector) is up to 6–8 meters

Where DN12 is not sufficient: if you have 3 or more collectors, a longer route, or a building with multiple floors, DN12 will put you in the range of too high pressure loss and you will either have to significantly increase the pump power (and thus increase operating costs), or replace the pipe. From practice I know cases where the installer "saved" on DN12 instead of DN16, and the customer then wondered why his circulation pump was running constantly at full power and the storage tank was charging slower than the neighbor's with a smaller collector. The diagnosis was always the same: undersized pipe.

DN16: the golden middle path for most single-family homes

If I had to recommend one pipe diameter for a "standard single-family home with a regular solar system," it would be DN16 without hesitation. The reason is simple: most series-produced solar pump units (e.g., from Resol, Grundfos, Wilo) are dimensioned exactly for DN16 hydraulic systems. Tank connections, pump unit inlets, collectors – everything is standardized around this nominal diameter.

DN16 works reliably at:

2 to 4 flat plate collectors or 2–3 vacuum tube collectors
Total collector area 4–10 m²
Pipe route length 8–25 meters (total length of both branches)
Planned flow rate 2–6 l/min
Lift height up to 12–15 meters

For a specific installation: a bungalow with a gable roof, 3 flat plate collectors on the roof, a 400 l tank in the boiler room in the basement, pipe route length 14 meters. This is a typical job where the installer will take Stainless steel pipe DN16 without thinking and be sure that the hydraulics will be fine at a flow rate of 3–4 l/min.

Practical note from the field: DN16 is also advantageous in terms of stock availability of fittings and nuts. Whereas with DN12 you sometimes have to order special reductions to connect to the pump unit, DN16 fits directly. You will learn more about this issue in the article Sealing of connections and selection of nuts for solar pipes DN12 and DN16.

DN20: when you really need it

The DN20 diameter comes into play when the residential solar system grows into the area of a larger installation or when the pipe route is unusually long. Specific situations where DN20 makes sense:

Systems with 5 or more collectors (area over 12–15 m²)
Community houses, apartment buildings, hotels, agricultural enterprises
Pipe routes longer than 25–30 meters
Flow rate over 6–8 l/min
Systems with parallel-connected circuits, where DN20 pipe is the primary distribution before branching off into individual collector fields

In practice, DN20 appears, for example, in an installation on a farm, where 8 vacuum collectors supply hot water for workers and also preheat water for a technological process. The total collector area is 32 m², the tank is 2 000 l, and the route from the barn roof to the heat exchanger station is 35 meters. Here, DN16 would not be sufficient – the pressure loss would be too high and the pump would have to be over-dimensioned to such an extent that it would break the system's overall economics.

For such systems, Stainless steel pipe DN20 is available, which can handle these more demanding applications.

Important note: DN20 is not automatically "better" than DN16 just because it is larger. At low flow rates (up to 2 l/min), you would have too slow a flow in DN20, which could cause stratification and uneven heating. The system would not function efficiently. The pipe diameter must correspond to the actual flow rate – not too small, not too large.

Flexi-tube 2 in 1: both diameters in one product

Solar pipe for single-family homes is most commonly delivered as a so-called twin pipe – that is, the supply and return hose are combined into one unit and insulated together. This solution saves time during installation, reduces the risk of mixing up the supply and return lines, and significantly simplifies routing the pipe through walls, ceilings, or roof structures.

For most single-family homes, the ideal is Solar Flexi-tube 2 in 1, 10 m – that is, a twin pipe 10 meters long, which covers the standard route in a single-family home. If your installation is larger or the route is longer (for example, in holiday homes with a greater distance between the collector and the technical room), you will use Solar Flexi-tube 2 in 1, 20 m.

These flexible hoses are standardly available in DN16 size, which corresponds to the most common requirement in practice. For a comparison of versions and more detailed information on why to choose a flexible hose instead of a rigid corrugated pipe, I recommend reading the article How to choose solar piping: flexible hose vs. rigid corrugated pipe.

Rigid corrugated pipe vs. flexible hose: impact on pipe diameter selection

A rigid stainless steel corrugated pipe and a flexible double-wall pipe differ not only in physical design but also in hydraulic properties. The corrugated structure (surface undulations) causes slightly higher flow resistance compared to a smooth pipe of the same internal diameter. This effect is evident in practice, as when calculating the hydraulics of a solar system, you must consider the so-called equivalent length of the corrugated pipe.

As a general rule: at the same flow rate, a corrugated pipe has a 15–25 % higher pressure loss per meter of length compared to a smooth pipe of the same nominal diameter. For dimensioning purposes, this means that for long runs (over 20 m), you should be somewhat more conservative and preferably choose a larger diameter.

In places where the pipe route passes through an opening in a wall or ceiling slab and where it is not possible to use bends of the flexible hose, a rigid Stainless steel corrugated pipe is installed, which is connected to the flexible hose using appropriate connecting nuts. In this case, make sure that the diameter of the rigid corrugated pipe matches the diameter of the flexible hose – i.e., for a DN16 flexible hose, you must also use a DN16 corrugated pipe; otherwise, hydraulic jumps and sealing problems at the connections will occur.

Practical examples from everyday customer practice

Case 1: Single-story family house, 2 flat collectors, 200 l storage tank

The route from the roof collectors to the storage tank in the bathroom passes through the ceiling and one wall, total length 8 meters. Planned flow: 2 l/min. Selection: DN12 or DN16. Given the short route and low flow, DN12 would technically be sufficient, but the customer planned to add another collector in the future. The installer correctly chose DN16 with a reserve – after two years, an additional collector was added and the system operated without any pipe modifications.

Case 2: Two-story house, 4 flat collectors, 400 l storage tank in the basement

The route from the roof collectors through the attic, intermediate floor, and basement – total length 22 meters. Elevation of the collector above the storage tank 9 meters. Planned flow 4–5 l/min. Selection: clearly DN16. The pressure loss at this flow rate over 22 meters of DN16 corrugated pipe is still within a comfortable range, and a standard Grundfos Solar pump group handles it without problems.

Case 3: Agricultural enterprise, 8 vacuum collectors, 2 000 l storage tank in an adjacent building

The pipe route runs through the outdoor environment, partially underground, total length 38 meters. Planned flow 10–12 l/min. In this case, the selection of DN20 was the only reasonable solution. Rigid DN20 corrugated pipes were placed in an insulated protective channel, and the connection to the collector field and the storage tank group was realized using DN20 fittings.

Case 4: Renovation – transition from DN12 to DN16

The customer bought a house with an existing solar system from 2008, where a DN12 corrugated pipe 18 meters long was installed. The system had 3 collectors and a 300 l storage tank, but the pump group was running at maximum capacity and the tank never heated up properly above 55 °C. After diagnosis, we found that the pressure loss on DN12 at a flow rate of 3.5 l/min was almost 450 Pa/m – almost double the recommended maximum. Replacing the pipe with DN16 took one day and the tank began regularly reaching 65–70 °C.

How to correctly measure the route and use the length when selecting the diameter

One of the most common mistakes in dimensioning solar piping is measuring the "air distance" instead of the actual route length. The air distance from the collector to the storage tank may be 6 meters, but the actual corrugated pipe route through the ceiling slab, sloped roof, horizontal section along the attic, and vertical drop into the technical room can easily be 16–18 meters. And it is precisely this real length that determines the dimensioning.

Therefore, I strongly recommend reading the article Length of solar piping: how to correctly measure the route from the collector to the storage tank, where you will find a step-by-step measurement procedure including the consideration of bends and fittings, which also add hydraulic resistance.

As an approximate rule for the "equivalent length" of fittings for stainless steel corrugated pipe, the following applies:

90° elbow: + 0.5–1 m equivalent length
T-piece: + 1–1.5 m equivalent length
Ball valve DN16: + 0.3–0.5 m equivalent length
Check valve: + 1–2 m equivalent length

For a specific example: a system with 18 meters of pipe and 4 90° elbows + 2 ball valves has an equivalent hydraulic length of about 21–22 meters. This difference can, when dimensioning at the limit, decide between DN16 and DN20.

Temperatures, pressures and other parameters that influence the choice of diameter

Along with the diameter, you must always verify the temperature and pressure parameters of the corrugated pipe for solar piping. Solar systems operate with medium temperatures up to 180–200 °C (in stagnation, when the storage tank is fully charged and the pump has stopped) and pressures of 3–6 bar. Stainless steel corrugated pipes are suitable for these conditions, but note – plastic or copper piping is not suitable for such a system.

All three diameters (DN12, DN16 and DN20) in the stainless steel version usually meet the following requirements:

Operating temperature: –20 °C to +200 °C
Operating pressure: min. 16 bar (PN16)
Material: AISI 304 or AISI 316 (resistance to glycol and solar medium)

About the pitfalls of stainless steel corrosion in specific environments (salty air near the sea, aggressive water) you will read in the article Common problems with solar piping: overheating, leaks and stainless steel corrosion.

Reference table: selecting the diameter according to key parameters

Parameter	DN12	DN16	DN20
Number of collectors	1–2	2–4	5 and more
Collector area	up to 5 m²	5–12 m²	over 12 m²
Length of route (total)	up to 12 m	12–25 m	over 25 m
Flow rate	up to 2.5 l/min	2–6 l/min	6–15 l/min
Typical use	small house, garden house	standard house	larger building, commercial
TÚV tank	up to 200 l	200–500 l	500 l and more
Lift height	up to 6 m	up to 15 m	up to 25 m
Recommended flow velocity	0.3–0.8 m/s	0.3–1.0 m/s	0.3–1.0 m/s

Mistakes that repeat on customer orders again and again

Based on experience from many implementations, we repeatedly encounter the same mistakes. I mention them so you can avoid them:

Selecting a pipe that is too small due to cost. The price difference between DN12 and DN16 is relatively small, but the cost of replacing the pipe after one year of operation is many times higher. Always dimension with a slight reserve.
Ignoring the length of the route. Customers often think that a 15-meter route is "short", but for DN12 it is already at or beyond the limit.
Mixing the total length of both branches with the length of one branch. If the supply branch is 10 m and the return branch is 10 m, the total hydraulic length is 20 m, not 10 m. Pressure loss is calculated for the entire loop length.
Not maintaining a consistent diameter throughout the entire loop. If you have a DN16 flexible hose and you connect it to a DN12 corrugated pipe via a reducer, you are creating a hydraulic bottleneck. The entire loop should have the same diameter.
Selecting DN20 for a small system "just to be safe". Excessive diameter also causes problems – low flow velocity in DN20 at low flow can cause issues with air bubble venting.

Combination of different diameters in one system

In some situations, it is legitimate to combine different diameters in one solar system. The most common case: a system with multiple collector fields, where each collector field has its own DN16 pipe, but these are connected on the collector to a common primary pipe DN20, which goes to the tank.

This is a hydraulically correct solution, which copies the principle of a tree-like distribution system: from the details (individual collectors) through branches (collector fields in DN16) to the main trunk (DN20 to the tank). It is important that each change in diameter is implemented via the correct reducing fitting and that the material consistency is maintained (stainless steel – stainless steel, not stainless steel – copper).

More about the practical side of installation and connecting different diameters can be read in the article Installation of solar stainless steel piping and threaded fittings.

Common questions (FAQ)

Can I use DN12 pipe with two collectors and a 15-meter route?

Technically yes, but hydraulically it is on the edge. At a flow rate of 2 l/min and a length of 15 m on DN12 corrugated pipe, you will get a pressure loss of around 350–400 Pa/m, which is above the recommended maximum. The pump unit can overcome this resistance, but it will work at a higher power and consume more electricity. If you do not plan to expand the system, DN12 can work – but I recommend investing in DN16 and having a reserve.

What is the difference between DN16 flexible hose and DN16 rigid corrugated pipe from a hydraulic point of view?

Both have the same nominal diameter, but the flexible hose is usually slightly more flexible and has a slightly smoother inner profile. The rigid corrugated pipe has more pronounced corrugation, which can cause 10–20 % higher pressure loss per meter. In practice, this difference is negligible for normal lengths (up to 25 m), but for long routes it should be considered.

Do I need to calculate the diameter separately for each branch (supply + return)?

No, both branches of the solar loop must have the same diameter – this is a condition for proper hydraulic function. Since the same amount of medium that comes in via the supply branch from the collector must go back out via the return branch, the hydraulic calculation is done for the entire closed loop, not for one branch separately. That is why we talk about the total loop length, which is the sum of the supply and return branches.

Can I add another collector to an existing DN12 pipe without replacing the pipe?

In most cases no, or only at the cost of reduced efficiency. Adding another collector increases the required flow and the pressure loss on DN12 will rise above the acceptable limit. In practice, we either replace the entire pipe with DN16 (recommended) or keep the original flow and accept lower performance of the new collector (suboptimal). Replacing the pipe during reconstruction is always more cost-effective than paying higher operating costs over the years.

Is DN20 suitable for underground solar piping between buildings?

Yes, DN20 is standard for underground piping between buildings. It is important that the pipe is protected in a protective pipe or channel, properly insulated (minimum 19 mm insulation) and protected against moisture. Insulation and protection of the pipe during outdoor and underground installation are described in detail in the article Insulation of solar piping: what not to forget during installation outdoors and indoors.

What if the collector manufacturer specifies a particular pipe diameter?

Always follow the manufacturer's requirements stated in the installation instructions. Some manufacturers explicitly specify a minimum internal diameter of the connecting pipe (e.g., ID ≥ 14 mm, which corresponds to DN16). If you were to use a smaller diameter, you might lose the collector warranty and also risk insufficient flow through the collector array.

Conclusion: investing in the correct diameter pays off immediately

Selecting the correct diameter for the solar pipe is not just a technical detail – it is a decision that affects the overall efficiency of the system, the lifespan of the components, the electricity consumption of the circulation pump, and the customer satisfaction throughout the entire lifetime of the installation (20–30 years). An investment in the correct diameter – usually DN16 instead of DN12 in borderline cases – pays for itself within the first year of operation through electricity savings on the pump and higher solar gains from a properly functioning system.

For most standard single-family homes, DN16 is the correct choice. For small installations with short runs and a single collector, DN12 is sufficient. For larger systems, long runs, or commercial applications, go for DN20. And if you are unsure – better choose a larger diameter. In solar hydraulics, overdimensioning is a much cheaper solution than underdimensioning.

For an overview of available products, visit the main category solar pipes and stainless steel corrugated hoses, where you can select the correct length and diameter for your specific application.

Do you have a question on this topic?

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