Solar 2-in-1 flexible pipe: what does the dual-tube design mean and when is it worth it
Solar Flex Hose 2 in 1: What It Actually Is and Why It Is an Important Solution for Solar Installations
When a beginner installer or a DIY enthusiast first looks at a solar system diagram, they are often surprised by how many pipes actually need to be run between the roof-mounted collector and the tank in the technical room below. A solar circuit always requires two pipes – one for the supply medium (hot heat transfer fluid going from the collector to the tank) and one for the return (cooled heat transfer fluid returning from the tank back to the collector). In a standard installation, this means laying these two pipes separately, protecting them separately, and connecting them separately. A solution that elegantly bypasses this problem is the so-called solar flex hose 2 in 1 – a double-pipe design where both channels are integrated into a single insulated unit.
In this article, we will take a detailed look at what "2 in 1" means technically, what the internal structure of such a pipe is, what are its real advantages and limitations, when it is worth reaching for this solution and when it is better to choose classic separate stainless steel corrugated pipes. We will also look at specific lengths, diameters, temperature ranges and other parameters that determine the correct choice.
What "double-pipe design" means in solar piping
The term "2 in 1" in the context of solar piping means that two separate stainless steel corrugated pipes are placed in one insulated unit. Each of them forms a hydraulically separate channel – supply and return. Both pipes are surrounded by insulating material (most often foam elastomer or UV-resistant polyethylene foam) and the whole is protected by an outer sheath resistant to weather influences.
The key word is separate: although both pipes physically adjoin in one hose, the heat transfer fluid in the supply channel and the heat transfer fluid in the return channel never mix. There is thermal insulation between the pipes that minimizes heat transfer between the hot supply pipe and the cooler return pipe.
The internal stainless steel corrugated pipes are usually DN12 in diameter (internal diameter approx. 12 mm), which corresponds to most residential solar systems with a collector area of 2–6 m². The corrugated shape of the stainless steel pipe serves two functions: it allows the pipe to be bent without deforming the cross-section and compensates for thermal expansion of the material at large temperature differences.
Internal structure and materials: what is in the hose
In order to understand why the solar flex hose 2 in 1 can withstand conditions that would quickly destroy ordinary plastic piping, we need to look at the material composition of each layer:
- Stainless steel corrugated pipes (AISI 304 or AISI 316L): The most important part – the medium carrier. Stainless steel can withstand temperatures up to 200–250 °C for short periods. In solar systems, the operating temperature is usually 120–150 °C and the stagnation temperature can briefly reach 180 °C. The corrugated shape (korrugation) allows bending and works as a thermal expansion damper.
- Thermal insulation: Between the stainless steel pipes and between the pipes and the outer sheath is insulation made of expanded polyethylene (EPE), foam elastomer (e.g. Armaflex HT) or mineral wool. The quality of the insulation determines how much heat is lost along the route and how much is transferred between the supply and return.
- Outer sheath: A braided or sleeve made of UV-stabilized plastic (LDPE, EPDM or a combination) protects the insulation from sunlight, mechanical damage and moisture. In outdoor installations, the UV resistance of the sheath is crucial – cheap solutions crack after 2–3 years and the insulation degrades.
When choosing, it is not enough to look only at the price per meter – it is important to know how thick the insulation is (at least 13–19 mm on each side for quality products) and what temperature resistance the manufacturer guarantees for the outer sheath and the insulation itself.
Connection diagram in a solar system: where the flex hose is located
Flexi-tube 2 in 1 usually forms that part of the route that passes from the technical room through a wall or ceiling to the roof and to the collector. This is the section where you will appreciate the flexibility of the tube – there is no need to create a rigid elbow, the hose will flexibly adapt to the passage through the structure. At both ends, the hose is terminated and connected to a rigid connection – directly to the collector connections at the collector, and at the storage tank to the solar regulation station or directly to the tank.
Available lengths and when to choose which one
In practice, the solar flexi-tube 2 in 1 is most commonly delivered in two standard lengths. You can find them specifically at Atria.sk:
- Solar flexi-tube 2 in 1, 10 m – for shorter routes, typically single-family homes with the collector placed directly above the technical room or in its immediate vicinity
- Solar flexi-tube 2 in 1, 20 m – for longer routes, when the collector is on the opposite roof, on a pergola further from the house, or when the route is not direct
Before ordering, it is essential to measure the actual route with a reserve. Not only the air distance from the tank to the collector, but the entire length along the actual route – including bends when passing through a wall, ceiling, or roofing. Always add at least 0.5–1 m as a technological reserve for connection. If your measured length comes out to 9.3 m, go for the 10 m version – you won't lose anything, but you will save yourself potential worries about a short end.
If your route is longer than 20 m, the flexi-tube 2 in 1 as a single piece ceases to be an ideal solution. In such cases, it is better to combine a shorter section of flexi-tube (for example, for passage through the roof and through the building) with rigid piping for the rest of the route, or use stainless steel corrugated pipe laid in two separate tubes.
Comparison: flexi-tube 2 in 1 vs. two separate tubes
As you can see from the comparison, the flexi-tube 2 in 1 excels in installation simplicity, while two separate tubes offer greater freedom in system design. The choice depends on the specific installation.
When to definitely choose the flexi-tube 2 in 1
From practice, I know that the flexi-tube 2 in 1 is an optimal solution in these scenarios:
- Renovation of an existing building – when you want to run the solar piping through existing masonry or ceiling without breaking a large hole. One cable with both tubes is passed through one hole with a diameter of about 60–80 mm.
- Short to medium length route (up to 20 m) – most single-family homes with panels on a sloped roof directly above the technical room. A vertical section of 5–8 m plus a horizontal section of 2–4 m – this is exactly the domain of the flexi-tube.
- Solo installation (DIY or small contractor) – when the installation is done by one person, it is extremely advantageous to handle one piece instead of two tubes plus insulation sleeves.
- Standard residential solar system 2–6 m² – the performance and flow rates in such systems fully match the DN12 diameter, which is standard in combined hoses.
When it is better to choose separate tubes
On the other hand, there are situations where the flexi-tube 2 in 1 is not the best choice:
- Long route over 20 m – the hydraulic resistance of DN12 at a length of 25–30 m can be a problem for smaller circulation pumps. Either a larger diameter is needed (which means rigid piping DN16 or DN20), or the route should be shortened by another routing.
- Systems with a large collector field – for 10–20 m² of collectors and corresponding flows, a DN16 or even DN20 diameter is required, which is not available in the standard commercial version of the flexi-tube 2 in 1.
- Industrial or commercial installations – where the piping runs through technical ducts, shafts, or along the façade for long distances; there, rigid stainless steel corrugated pipes with insulation are more advantageous.
- Different routes for the supply and return branches – if, for layout reasons, the supply and return channels run different paths (which is rare but occasionally happens), the flexi-tube 2 in 1 naturally does not suit.
Hydraulic parameters and pressure resistance: what it means in practice
One of the disadvantages of corrugated tubes compared to smooth tubes is higher pressure resistance. Corrugations create turbulent flow even at relatively low flows, which is expressed as increased pressure loss per meter of length. This is a crucial piece of information for dimensioning the circulation pump.
As a reference: at a flow rate of 1.5 l/min in a DN12 corrugated tube, the pressure loss is typically 20–35 mbar/m. This is significantly more than in a smooth copper tube of the same diameter (4–8 mbar/m). At a length of 10 m (supply pipe) and 10 m (return pipe), the total loop length is 20 m and the pressure loss from the piping alone can reach 400–700 mbar – which is 0.04–0.07 bar. This is still manageable for standard solar pumps (standard solar pumps in solar stations can handle 1.5–3 bar), but it must be taken into account in the overall hydraulic calculation, including the resistance of the collector, the tank, and the fittings.
If you are interested in larger diameters for less resistance, you can also find stainless steel pipe DN16 or stainless steel pipe DN20 in the category, which are suitable for larger systems and longer routes. DN16 is a compromise between flexibility and hydraulic resistance, while DN20 is intended for the largest residential or small commercial systems.
Heat losses along the route: why insulation thickness is more important than it seems
Heat losses along the pipe are a directly measurable economic issue. For example, a collector may produce 800 W of thermal power – if you lose 6 W/m along 10 m of pipe (i.e., 60 W total on each branch), this is a 15% loss just from the piping. Therefore, insulation is not a luxury, but a necessity.
A high-quality 2-in-1 flexible hose has a minimum insulation thickness of 19 mm (usually indicated as "19 mm" or "HT" version with thermal resistance up to 175 °C). Pay attention to this specification when selecting – cheaper products may have insulation of only 9–13 mm, which is insufficient for outdoor installation. When part of the route runs outside (along the façade, through the roof), a thickness of 19–25 mm on each side is a real foundation. Further tips on this topic can be found in the article Insulation of solar piping: what not to forget when installing outdoors and indoors in our Knowledge Center.
Installation of 2-in-1 flexible hose: practical steps and most common mistakes
The installation of a 2-in-1 solar flexible hose is basically simple, but a few details can decide whether the system will be tight and reliable for 20 years or cause problems after a few years.
Step 1: Preparation of the wall or ceiling passage
The typical outer diameter of the entire 2-in-1 flexible hose is 55–75 mm. The hole through the wall should be 10–15 mm larger than the outer diameter of the hose to allow it to pass through without forced bending. After passing through, the hole is sealed with frost-resistant sealant or a prefabricated passage. Do not forget that the passage must be placed so that any condensation water can drain outside, not inside.
Step 2: Connecting to the collector
The 2-in-1 flexible hose splits into two separate corrugated stainless steel pipes with corresponding fittings at both ends. Each pipe is connected to the corresponding inlet/outlet of the collector. It is important to follow the correct assignment – supply to supply, return to return – and not to confuse which end goes to the collector and which to the storage tank. A good practice is to mark both ends of the hose with colored tape before installation (red = supply, blue = return).
When tightening the fittings, keep in mind: stainless steel corrugated pipes are sealed with a compression joint – the sealing is built into the fitting or an additional gasket is used. Do not over-tighten – corrugated pipes are relatively delicate material and excessive tightening can deform the end of the pipe. More information on this step can be found in the article Installation of solar stainless steel piping and fittings with threads and also in Sealing of connections and selection of fittings for solar piping DN12 and DN16.
Step 3: Routing and fixing the hose
The flexible hose must be fastened at regular intervals – at least every 1–1.5 m on a vertical section, every 0.8–1 m on a horizontal section. Use clamps with rubber inserts (not bare metal clamps, which could damage the outer sheath). When routing along the façade and roof, ensure that the hose cannot vibrate or flap in the wind – this can damage the outer sheath over time.
The minimum bending radius of the flexible hose is usually 5–8 times the outer diameter, so for a diameter of 65 mm, the minimum bending radius is 325–520 mm. A smooth curve prevents deformation of the pipe cross-section and unnecessary increase in pressure resistance at the bend.
Step 4: Pressure test and first filling
Before covering or final fixing, always perform a pressure test of the system. Fill the circuit with water (not yet with glycol mixture) and pressurize to 1.5 times the working pressure, at least 3 bar, and monitor the pressure drop for 30 minutes. Any drop indicates a leak, which must be located and removed before filling with the heat transfer fluid. Further steps can be found in the article Common problems with solar piping: overheating, leaks and stainless steel corrosion.
Stagnation and temperature peaks: how the flexible hose reacts to them
A solar system does not stop the sun from vacationing or during the weekend when no one is drawing hot water. During stagnation (a state when the system is not in operation, but the collector still captures solar radiation), the temperature of the heat transfer fluid in the supply pipe can rise to 150–200 °C. This is a condition that strictly tests every component of the solar circuit.
Stainless steel corrugated pipes themselves handle this condition – their thermal resistance is sufficient. However, the critical component can be the insulation and the outer sheath. With low-quality products, repeated stagnations lead to degradation of the insulation – the filling foams lose volume, the outer sheath cracks. Therefore, look for products with HT (High Temperature) insulation with a resistance value of at least 150–175 °C.
The corrugated shape of the pipe also compensates for thermal expansion: stainless steel expands by approximately 1.2 mm/m with a temperature change of 100 °C. For a length of 10 m, this is 12 mm on each pipe. The corrugation easily absorbs this expansion, which a rigid pipe without an expansion loop could not do.
Comparison with classic stainless steel corrugated pipes
For those who are hesitating between a 2-in-1 flexible hose and a pair of separate stainless steel corrugated pipes, here is a brief overview of practical situations.
Case 1 – renovation of a row house: The technical room is on the ground floor, the collector is on the roof on the opposite side of the roof (north is the storage tank, south is the collector). The route is 12 m through the basement ceiling, perimeter wall, and attic. In this case, a 2-in-1 flexible hose of 20 m is ideal – one piece, one wall passage, minimal work, minimal space in the attic passage.
Case 2 – new construction with an 8 m² collector field: The system performance requires a flow of 3–4 l/min, which is at the limit of DN12. The route is 8 m, but the designer recommends DN16 due to lower pressure resistance and future expansion. In this case, it is better to go for a pair of separate DN16 corrugated pipes with individual insulation.
Case 3 – garden cottage with panels on a pergola: 6 m route, simple passage through wooden cladding, the tank is directly under the pergola in a technical room. Clear choice: 2 in 1 solar flexi-tube in length 10 m, installation in a few hours without special tools.
What to monitor during long-term operation and maintenance
The 2 in 1 solar flexi-tube does not require intensive maintenance with proper installation, but there are a few things to check during the regular annual inspection of the solar system:
- Condition of the outer sheath: Visually check for cracks, bulges or discoloration of the sheath. Cracking indicates degradation of the UV stabilizer or mechanical damage.
- Stiffness and flexibility: Old insulation hardens and loses elasticity. If the flexi-tube "breaks" instead of bending, it is time for replacement.
- Condition of the wall passage: Sealant or wall penetration can dry out and crack, allowing water to enter the structure. Seal it at any suspicion of leakage.
- System pressure: During the annual inspection, check the pressure in the system. A drop of more than 0.3 bar compared to the previous inspection indicates a leak.
More detailed information on this topic can be found in the article Maintenance and inspection of solar piping: how to extend the life of the corrugated tube.
Most frequently asked questions (FAQ)
Can I shorten the 2 in 1 flexi-tube if it is too long?
Yes, the flexi-tube can be shortened – stainless steel corrugated tubes are cut with special corrugated tube scissors or a fine metal file (without jerky movements to avoid deformation of the tube end). The insulation and outer sheath are cut with a knife. After shortening, you must check whether the tube ends are evenly cut and suitable for fitting the compression nut. We recommend leaving at least 30–40 mm of bare corrugated tube for fitting the nut and olive.
What heat transfer fluid can be used in the 2 in 1 flexi-tube?
Stainless steel corrugated tubes are compatible with standard solar heat transfer fluids based on propylene glycol (concentration 30–50 % according to the climatic zone). Ethylene glycol is not recommended in residential systems due to its toxicity. The heat transfer fluid should be inhibited (contain corrosion inhibitors) and regularly checked – degraded glycol solution can be aggressive to sealing materials. Heat transfer fluid replacement is recommended every 5–7 years or according to the manufacturer's instructions.
Is the 2 in 1 flexi-tube suitable for pressure systems with high operating pressure?
Standard 2 in 1 flexi-tubes are dimensioned for operating pressure 6–10 bar (maximum test pressure 15 bar), which is more than sufficient for most closed solar loops with an expansion tank. Operating pressure in a typical residential system ranges between 1.5–3 bar. Always check the technical data sheet of the specific product – the maximum pressure is always stated there.
Can I connect two pieces of flexi-tube in series if one length is not enough?
Technically, it is possible – both ends are connected via standard compression nuts and a T-piece or straight coupling. Practically, however, you should consider that each connection is a potential leak point and each connection increases hydraulic resistance. For routes up to 20 m, one piece without a joint is always better. For longer routes, consider whether the 2 in 1 flexi-tube is the right solution or whether it would be more appropriate to switch to solid stainless steel corrugated piping.
How can I determine which end of the 2 in 1 flexi-tube is the supply and which is the return?
The 2 in 1 flexi-tube is symmetrical from a hydraulic point of view – both channels are the same. You assign "supply/return" during installation according to the flow direction in the system. In most flat collectors, the upper outlet is the supply (hot medium goes out upwards) and the lower one is the return. Check the wiring diagram of the specific collector. It is good to permanently mark the assignment with colored tape or labels for future maintenance.
What if the 2 in 1 flexi-tube is not sufficient for a longer route – what is the alternative?
If your route is longer than 20 m or you need a larger diameter, the best alternative is separate stainless steel corrugated tubes. On Atria.sk you will find stainless steel piping in the form of corrugated tubes in various diameters including DN16 and DN20. You lay these tubes in two parallel routes, each insulated and fastened separately. More about the comparison of both approaches can be found in the article How to choose solar piping: flexi-tube vs. solid corrugated tube.
Conclusion: dual-tube design as an intelligent compromise
The solar flexi-tube 2 in 1 is not a revolutionary technology – it is a practical, well-proven compromise between installation convenience and hydraulic properties. For the vast majority of residential solar systems with flat collector areas up to 6 m² and routes up to 20 m, it is an ideal solution that reduces installation time, simplifies pipe passage through the structure and ensures reliable operation for decades.
It is important to choose a product with sufficient thermal insulation thickness (at least 19 mm), with an outer sheath resistant to UV radiation and with a certified temperature resistance of the insulation at least up to 150 °C. When choosing the length, always measure the actual route and add a reserve – the investment in a longer piece is minimal compared to the problems that arise with too short piping.
If you are unsure about choosing between the 2 in 1 flexi-tube and solid corrugated tubes, also see other articles in our Knowledge Center: What diameter of solar piping do I need: DN12, DN16 or DN20 and Length of solar piping: how to correctly measure the route from the collector to the tank – together they give you a comprehensive view of the correct dimensioning of your entire piping system in your solar loop.
Do you have a question on this topic?
Not sure or dealing with a specific situation in your home? Write to us – we are happy to help.