How to choose solar pipe: flexible hose vs. rigid corrugated pipe
How to choose solar pipe: flexible hose vs. rigid corrugated pipe
When you are dealing with solar water heating and have already selected a collector and a storage tank, it is time to decide how to actually connect these two components. Choosing between a flexible hose and a rigid corrugated pipe may seem trivial at first glance – after all, it is just a piece of pipe. In practice, however, this is a decision that will affect the installation, long-term reliability, heat loss, and how you will feel during the first repair in five years. Over the years I have been working with solar system installations, I have seen both types of installations, both successful and problematic. This article will help you understand how these two categories differ, where each one is justified, and how to avoid mistakes that even experienced plumbers often make.
Basic terminology: what is what
Before we start comparing, let's clarify the terms. By the term solar flexible hose (also known in practice as "double pipe"), we mean a double-walled flexible conductor usually made of copper or stainless steel inner pipes, wrapped in thermal insulation and an outer protective layer. The whole package forms a compact, easily bendable route that can be shaped manually during installation without special tools.
Rigid corrugated pipe (stainless steel corrugated pipe), on the other hand, is a single pipe made of stainless steel with a corrugated surface. The corrugation provides a certain degree of flexibility, but not in the true sense of the word – the corrugated pipe retains its shape, the bending radii are limited, and for larger directional changes, it is necessary to use shaping tools or fittings. Corrugated pipe is sold either as bare (without insulation) or with pre-installed insulation.
In the category of solar double pipes and stainless steel corrugated pipes, you will find both variants. Each has its own logic of use, and we will now examine each in detail.
Solar flexible hose 2 in 1: strengths and typical use
The flexible hose in a double-pipe configuration currently represents the most widespread solution for single-family homes with roof-mounted collectors and a storage tank in the technical room. The reason is simple: installation speed. Instead of pulling two separate pipes (output and return), insulating them, and securing them along the entire route, a single operation is sufficient – from the collector to the storage tank, a single flexible element containing both pipes and insulation in one package is used.
A typical Solar flexible hose 2 in 1 in length 10 m covers most single-family home installations where the collector is mounted on a gable roof and the storage tank is in the boiler room on the ground floor or in the basement – the distance of the route along the wall, ceiling, and shaft usually does not exceed 8–9 meters of clean length. If the distance is greater or you need to solve a more complex route (for example, a collector on the side wall of a garage, a storage tank in the opposite corner of the house), it is better to choose Solar flexible hose 2 in 1 in length 20 m. Better to have an extra meter or two than to have the route tight, because connecting two pieces of flexible hose carries the risk of leakage at the joints and interruption of the insulation at the joint.
Where flexible hose excels
- Simple passage through walls and ceilings: You drill one hole with a diameter of about 50–60 mm and the hose passes through without problems. If you were to pull two separate corrugated pipes, you would need either two holes or one significantly larger one.
- Changes in direction without fittings: Flexible hose can handle a bending radius of usually 15 cm or more, depending on the specific type. Changes in direction are made manually directly during installation, without elbows and without additional joints, which are potential leakage points.
- Pre-installed insulation: The insulation is already integrated, so the later step of insulating the pipe is omitted. On the roof, where the external insulation must be resistant to UV radiation and mechanical stress, this is a huge advantage.
- Fast installation: An experienced installer can complete a flexible hose route in an hour, whereas with separate pipes it would take significantly longer.
Where flexible hose hits its limits
Flexible hose is not suitable for every installation. The first limitation is temperature resistance. Standard solar flexible hoses are dimensioned for continuous operation up to 150 °C and short-term temperature peaks up to 200–210 °C. Most flat plate collectors do not exceed these temperatures in a properly dimensioned system, but vacuum tube collectors in a stagnation state can generate higher temperatures. Always check what the hose manufacturer guarantees.
The second limitation is the length available as one piece. Flexible hoses are sold in standardized lengths (10 m, 20 m), which can be a disadvantage in complicated routes – either you waste material or it is not enough and you have to connect pieces.
The third aspect is repairability. If the outer sheath or the inner pipe is damaged at a specific location, you usually replace the entire hose from the collector to the storage tank. With rigid corrugated pipes, you can repair locally in principle.
Stainless steel corrugated pipe: when is rigid piping the right choice
Stainless steel corrugated pipe is a material-wise clever solution. The corrugated wall made of austenitic stainless steel (typically AISI 304 or 316L) gives it corrosion resistance while maintaining a certain degree of formability. Unlike copper pipes, corrugated pipe is not prone to degradation by aggressive solar heat transfer mixtures (glycol mixtures can have a corrosive effect over time on certain copper alloys).
Stainless steel corrugated pipe is especially popular in cases where the route runs partially or fully through the interior (e.g., in the technical shaft of a residential building), where a flexible hose is not desired for aesthetic or spatial reasons, or where the system requires a higher nominal diameter for long runs with greater flow capacity.
Corrugated pipe is available in various sizes – in practice, for solar systems in single-family homes, we most commonly encounter DN12, DN16 and DN20. For medium-sized systems, stainless steel pipe DN16 is a kind of standard – it offers a good compromise between flow resistance and compactness. For long runs (over 15–20 m one-way length) or for larger solar systems with higher flow, go for stainless steel pipe DN20, which significantly reduces pressure losses along the route. We elaborate on this decision-making in a separate article What solar pipe diameter do I need: DN12, DN16 or DN20 in this Knowledge Center.
Typical scenarios for stainless steel corrugated pipe
Scenario 1 – Apartment building, shared solar system. You have five apartments, each with its own storage tank, and on the roof there is a field of four collectors. The routes run through a common installation shaft and later branch out into individual apartments. Here, rigid piping in the shaft is a necessity – it is easier to mount on brackets and rails, easier to label (colored bands for each branch), and in the case of a failure in one branch, it is easier to locate and replace the section.
Scenario 2 – Industrial building, long run. A production hall has a collector on the roof and a storage tank in the basement at a distance of 30 meters. A flexible hose in one piece would not work here (length), and connecting two pieces is a compromise we want to avoid. Rigid corrugated pipe sized to DN20 is the clear choice here.
Scenario 3 – Renovation, existing copper system. The customer has existing copper piping in the house and the solar system is added as a supplement. They want visual unity and technical compatibility. Stainless steel corrugated pipe is connected here to copper components via transition fittings – a common solution, no problem.
Technical parameters that matter
Temperature and pressure
The solar circuit operates with a heat transfer mixture (usually water + propylene glycol mixture in a ratio of about 40:60 for our climatic conditions), which reaches temperatures of 60–90 °C during operation. During stagnation (the collector is hot, the pump is not sufficient, the storage tank is full), the temperature in the primary circuit can jump to 130–160 °C and in the case of long-term stagnation with vacuum tubes even higher. The pressure in the primary circuit typically ranges from 2–3 bar.
Stainless steel corrugated pipe can easily handle a working pressure of 25 bar and a temperature up to 200 °C – so there is a significant reserve for a solar system. Flexible hoses are usually dimensioned for 10–16 bar working pressure (depending on the manufacturer) and temperatures of 150–200 °C. Both types are therefore sufficient for a standard solar system, but for systems with vacuum tubes, where stagnation generates high temperatures, always check the specific specification of the selected hose.
Flow and pressure losses
Flow in a solar system is a key parameter. For standard flat collectors, a specific flow of 30–50 liters per hour per square meter of collector area is recommended. One panel with an area of 2.5 m² thus requires a flow of 75–125 l/h. Pressure losses depend on the pipe diameter, route length, number of bends, and fittings used.
Corrugated pipe has an increased surface roughness compared to a smooth pipe (due to the corrugation), which means a 20–40 % higher pressure resistance at the same diameter and length compared to a smooth straight pipe. A flexible hose also has bends integrated into the route, which further slightly increases resistance. Therefore, for long runs, always choose a larger diameter than the theoretical calculation would suggest. A more detailed calculation procedure can be found in the article What solar pipe diameter do I need: DN12, DN16 or DN20.
Lifespan and corrosion
Stainless steel corrugated pipe made of AISI 316L is significantly more resistant to aggressive media than AISI 304. For solar systems with glycol-water mixtures, both types are mutually compatible, but if the local water is aggressive (chlorides over 100 mg/l), 316L will be more durable in the long run. In practice, both types are commonly available and, with proper installation and control of the medium's pH (the value should be in the range of 7–8), they will last 25–30 years without problems. Flexible hoses usually have a shorter guaranteed lifespan of the outer sheath (10–15 years with outdoor exposure), while the inner tubes are usually made of stainless steel or copper with the same long-term stability.
Installation differences in practice
Route preparation and measurement
Before any installation, you must measure the actual length of the route – not the straight line from the collector to the tank, but the actual path with bends, wall crossings, and vertical sections along the wall. Underestimating this length is the most common mistake I encounter. I recommend going with a string along the planned route and measuring it physically, not just estimating. This topic is discussed in detail in the article Length of solar pipe: how to correctly measure the route from the collector to the tank.
When using flexible hose, add at least 0.5 m to the measured length at each end for convenient fitting onto connections and possible directional adjustments. When using corrugated pipe, account for material length for bends – a 90° bend in corrugated pipe consumes more material than a geometric calculation would suggest.
Mounting and routing
Flexible hose is fastened with clamps that have sufficiently soft inserts (rubber or silicone) to prevent insulation from being crushed. A typical distance between mounting points is 0.8–1.5 m on horizontal sections, and up to 1.5–2 m on vertical sections. Corrugated pipe is mounted separately for each pipe, with a similar spacing. An advantage of corrugated pipe is that it is much easier to route in technical strips or pipe channels.
Never mount the pipe in a way that it is stretched or forced into a shape – the system must allow for movement due to thermal expansion (pipe lengthens when heated and contracts when cooled). Stainless steel corrugated pipe has a thermal expansion coefficient of approximately 16–17 µm/(m·K) – meaning that a 10 m long pipe will extend by about 16–17 mm when the temperature rises by 100 °C. This expansion must go somewhere – either into a pre-designed expansion loop or into bends in the route.
Connections, nuts, and sealing
This is the point where the precision of the installer is most critical. Stainless steel corrugated pipe is connected to the collector and pump or tank using either compression fittings (two-part nuts with a crimping ring) or push-fit fittings specifically designed for corrugated pipe. The detailed selection of the correct nuts for a given corrugated pipe dimension is discussed in the article Sealing of connections and selection of nuts for solar pipe DN12 and DN16.
Flexible hoses usually have fittings pre-mounted on both ends with external or internal threads, which significantly simplifies installation. Always use sealing materials suitable for high temperatures – standard rubber seals degrade quickly at 150 °C. For solar applications, Teflon/PTFE tapes or silicone seals with a minimum temperature resistance of 200 °C are suitable. A more detailed discussion of the installation of stainless steel piping is provided in the article Installation of stainless steel solar pipe and threaded nuts.
Insulation: a crucial, often underestimated step
Flexible hose has integrated insulation. Corrugated pipe does not – you must add insulation separately. For outdoor routing, foam elastomer insulation (such as Armaflex) or mineral wool in an aluminum jacket is commonly used. In a solar circuit, the pipe temperature is high, and without insulation, losses due to radiation and convection are enormous if the pipe runs outside (e.g., along a façade).
The minimum insulation thickness for an outdoor route should be 25–30 mm for DN16 and 30–40 mm for DN20. In the interior, 13–20 mm is sufficient. The insulation must be resistant to UV radiation (for outdoor installation) – without UV protection, elastomer insulation will break down within 2–3 years. The topic of insulation is discussed in detail in the article Insulation of solar pipe: what not to forget when installing outdoors and indoors.
Summary: when to choose flexible hose and when to choose corrugated pipe
| Criterion | Flexible hose 2 in 1 | Stainless steel corrugated pipe |
|---|---|---|
| Typical route length | up to 20 m (1 piece) | any (connectable) |
| Installation speed | very fast | slower, more steps |
| Insulation | integrated | must be added separately |
| Number of connections (leak risk) | minimum (only at the ends) | more (fittings, connections) |
| Repairability | lower (whole unit must be replaced) | higher (local repair possible) |
| Temperatures above 200 °C (vacuum tubes) | check specifications! | no problem |
| Routing in strip / shaft | more complicated | easy, flexible |
| Diameter (choice) | limited (pipe type) | DN12, DN16, DN20 |
| Material cost | medium | lower (without insulation) |
| Total costs including labor | lower (labor savings) | higher for the same route |
Most Common Mistakes in Selection and Installation
Over the years of practice, I have repeatedly seen the following mistakes:
- Too short flexible hose: The customer measures the air distance of 7 meters, orders a 10 m hose, and during installation realizes that the actual route with all bends and transitions is 11.5 m. Result? A joint in the middle of the route, which is a weak point of the entire system.
- Wrong diameter: DN12 for a route longer than 10 m causes significant pressure losses, the pump operates at the edge of its performance, and the system's efficiency drops dramatically.
- Lack of UV protection for insulation of corrugated pipe: A bare corrugated pipe is insulated with Armaflex without an outer jacket, and after three years, the insulation is already deteriorated.
- Sealing unsuitable for temperature: A standard rubber gasket in the joints, after the first stagnation event, the system starts to leak.
- Undesigned expansion for corrugated pipes: A long straight section of corrugated pipe without an expansion bend – the pipe tries to extend when heated, the pressure on the clamps and joints leads to media leakage or insulation damage.
- Mixing materials without transition fittings: Direct connection of an aluminum fitting to a stainless steel pipe via a steel bolt without a dielectric insert – galvanic corrosion over the years damages the joint.
These faults are discussed in more detail in the article Common Problems with Solar Piping: Overheating, Leaks, and Stainless Steel Corrosion and also in Maintenance and Inspection of Solar Piping: How to Extend the Lifespan of Corrugated Pipe.
Practical Decision-Making Process – Step by Step
If you are facing a choice and cannot decide, I recommend the following procedure:
- Measure the actual length of the route physically (string along the planned route).
- If the route is up to 18 meters and it is a family house with a flat collector on the roof – a 2-in-1 flexible hose is practically always faster and more cost-effective choice.
- If the route is longer, runs through an interior shaft, or it is a vacuum tube collector with high stagnation temperature – go for corrugated pipe.
- Determine the DN diameter according to the length of the route and flow requirements (see the article on diameter).
- Verify temperatures and pressures against the specifications of the selected product.
- Plan the insulation – for corrugated pipes, do not forget to include it in the budget and work schedule.
- Check the compatibility of threads and materials of the connections on the collector and the tank.
Frequently Asked Questions (FAQ)
Can I connect two pieces of flexible hose if one is not enough for the entire route?
Technically yes, but I do not recommend it as the primary choice. Every joint is a potential leak point – and the solar medium under pressure of 2–3 bar and at a temperature of 80–150 °C can be very dangerous in case of a leak. If the route exceeds 20 meters (the maximum length of one piece), it is more reasonable to switch to corrugated pipe, which is easier to connect in any sections with more reliable joints. If you really cannot avoid connecting flexible hoses, use a certified coupling for solar applications, not a standard plumbing one.
What is the difference between DN16 and DN20 in terms of daily operation?
DN16 has an inner diameter of about 16 mm, DN20 about 20 mm. For short routes (up to 10 m), the difference in pressure losses is minimal and DN16 is sufficient. For routes of 15–25 m with a flow rate over 150 l/h, DN16 generates significantly higher pressure resistance, which forces the circulation pump to work at a higher performance and reduces the overall efficiency of the system. In such cases, DN20 is a wise investment, which pays off with lower pump consumption over the season. A detailed calculation is in the article What Diameter of Solar Pipe Do I Need: DN12, DN16 or DN20.
Can stainless steel corrugated pipe corrode if it is outside?
AISI 304 and 316L stainless steel are very resistant to atmospheric corrosion. Problems arise only in environments with high concentration of chlorides (seawater air, industrial emissions) or in long-term contact with aggressive condensate. In most Slovak conditions, corrugated pipe made of 304 or 316L is fully sufficient for outdoor installation – but it must be properly insulated, because the insulation protects it not only thermally, but also from condensation and direct wetting. More in the article Common Problems with Solar Piping: Overheating, Leaks, and Corrosion of Stainless Steel.
Is the insulation of a flexible hose sufficient for an outdoor installation along the entire route?
It depends on the specific product. Most flexible hoses intended for solar applications have an outer jacket resistant to UV radiation (usually EPDM, silicone or aluminum jacket). Always check whether the manufacturer explicitly states UV resistance of the outer jacket. Without this property, the jacket will disintegrate in the sun within 2–3 years and the exposed inner insulation loses its thermal insulation properties and absorbs moisture. If the manufacturer does not specify UV resistance, install the hose in a protected conduit or additionally treat it with UV-stable coating or cladding.
How long does a solar flexible hose last compared to a stainless steel corrugated pipe?
The inner tubes of both types (stainless steel or copper) have a comparable lifespan – with proper installation and maintenance, easily 25–30 years. The difference is in the insulation and outer jacket. A stainless steel corrugated pipe with quality elastomeric insulation and UV protective jacket will last 20–25 years without intervention. The outer jacket of a flexible hose is more susceptible to mechanical damage and UV degradation, so the realistic lifespan in exposed conditions is rather 15–20 years. The reason for replacing a flexible hose is rarely the inner tube – almost always it is the damaged outer casing. Regular inspection once a year significantly extends the lifespan – the procedure is described in the article Maintenance and Inspection of Solar Piping: How to Extend the Lifespan of Corrugated Pipe.
Can I use the same pipe for systems with different types of solar media?
Stainless steel corrugated pipe is compatible with propylene glycol/water mix, synthetic solar media and pure water. Problems arise only when using ethylene glycol mix (toxic, less suitable for closed residential systems) or when the wrong ratio is used (too concentrated glycol solution can carbonize and deposit in the pipe during stagnation). Flexible hoses with copper inner tubes may be limited in combination with certain synthetic media – always check compatibility in the technical documentation of the media.
Conclusion
Choosing between a solar flexible hose and a stainless steel corrugated pipe is not a question of what is "better" – it is a question of what is more suitable for a specific project. A flexible hose wins in speed, simplicity and compactness for standard family house installations with a route up to 20 meters. A corrugated pipe wins where longer routes, precise diameter dimensioning, routing in shafts, easier repairability and longer performance under extreme conditions are needed.
In any case, the rule applies: do not buy piping only by price. A solar circuit works with hot liquid under pressure, sometimes in places difficult to access for repairs. Saving 20 euros on piping will not be returned to you if you have to dismantle the roof structure in five years due to a leak at the joint. Invest in quality material, thorough installation and regular maintenance – this is the surest way to a solar system that will work quietly and reliably for the next twenty years.
Do you have a question on this topic?
Can't decide or are you dealing with a specific situation in your household? Write to us – we are happy to help.