Installation of stainless steel solar piping and threaded fittings

We are still preparing products.

But you can look at other categories.

Installation of stainless steel solar pipe and threaded nuts: complete technical guide

A solar system is only as reliable as its weakest link. And in practice, surprisingly often, that link is not the collector or the control system, but the pipe connection. An improperly tightened nut, a forgotten sealing washer, a poor orientation of the corrugation, or a too short bend radius – each of these errors will eventually result in a leak, corrosion, or pressure loss in the system. Yet, proper installation of stainless steel pipe is not rocket science – it is enough to understand a few principles and consistently follow them.

This article is intended for installers and technically skilled DIY enthusiasts who are planning to install solar piping themselves or want to check the work of a construction company. We will cover the process from measuring the route through the selection of the correct pipe, preparatory work, proper tightening of nuts, pressure testing, and even common mistakes we see in customer projects almost every year.

Before installation: what you need to prepare

According to field experience, a simple rule applies: half of the problems during the installation of solar piping arise even before any work has actually started. Either a part is missing, an unsuitable tool is used, or the surface preparation is underestimated. Therefore, before you reach for the wrench, check the following.

List of required materials and tools

For a standard installation of solar piping, you will need:

Solar pipe – depending on the length of the route and the pipe diameter; for typical single-family homes (1–3 collectors), DN16 is most commonly used; for longer routes or more collectors, DN20
Threaded nuts for the specific pipe diameter (always check that the nut fits the corrugation you are working with)
Sealing washers – EPDM or graphite (not traditional hemp/Teflon tape)
Fittings and elbows with the correct thread (G 3/4" or G 1" depending on the system)
Pipe wrenches – at least two (one holds the fitting, the other tightens the nut); ideally a flat wrench 24/27 or 30/32 mm depending on the nut used
Clamps and supports for securing the pipe
Pressure pump for pressure testing
Manometer
Solar medium (glycol mixture) or nitrogen for pressure testing

When selecting the pipe itself, the decision usually narrows down to two options: flexible double pipe or rigid (single-purpose) stainless steel corrugated pipe. If you are not yet decided, I recommend reading the article How to choose solar pipe: flexible hose vs. rigid corrugated pipe, where both types are compared from a practical perspective.

Types of solar pipe and their installation specifics

Not all stainless steel solar pipes are installed the same way. The procedure and installation requirements vary depending on the type of pipe you are using.

Flexible solar double pipe (2 in 1)

The so-called "2 in 1" configuration, i.e., two pipes in one insulation jacket, is today the most widespread method of installation for single-family homes. The manufacturer delivers the pipe wound on a reel, and you simply unwind it along the route from the collector to the storage tank. On Solar flexible hose 2 in 1, 10 m length, you will appreciate the compact packaging suitable for shorter installations (typically a collector on the roof directly above the technical room). For longer routes, Solar flexible hose 2 in 1, 20 m length is available, which covers even more complex routes with redirection through corridors or mezzanines.

The installation of flexible double pipe is faster compared to rigid corrugated pipe, as there is no need to connect multiple short pieces. Nevertheless, a few rules should be followed:

Minimum bend radius: for DN16 it is usually 50–60 mm, for DN20 around 70–80 mm. A sharp bend reduces the cross-section and can damage the corrugated structure.
Never fold the pipe into sharp edges or tie it up – even a seemingly "soft" fold in the insulation can be a sharp bend inside the pipe.
Make sure the double pipe is not twisted (twisted along the axis) when unwinding. Torsion can manifest as a gradual loosening of connections at operating temperatures (60–180 °C).

Stainless steel pipe – rigid corrugated pipe (DN16, DN20)

Rigid stainless steel corrugated pipe is suitable where the route is straight, or where precise elbows and pre-prepared parts are required. Working with it requires a bit more experience, but the result is more robust and long-term reliable, especially for outdoor installations.

From the atria.sk range, stainless steel pipe in the basic configuration is available, as well as specialized sizes DN16 and DN20. The choice of diameter depends on the hydraulic performance of the system – more information can be found in the article What diameter of solar pipe do I need: DN12, DN16 or DN20.

Detailed installation procedure – step by step

We are now approaching the core of the topic. The following procedure describes the installation of a stainless steel solar pipe with threaded connections, which is the most common way of connecting to fittings and valves in solar systems.

Step 1: Measuring and cutting the pipe

Before you measure the length of the pipe, measure the actual route, not just the straight-line distance. Take into account every bend, every elbow, and every space where the pipe must bypass an obstacle. This topic is discussed in more detail in the article Length of solar pipe: how to correctly measure the route from the collector to the tank. Practical tip: always add 5–10 % extra as a reserve – with a pipe that is too short, you can do nothing, but the excess is usually put to good use.

To cut corrugated pipe, use only tools designed for cutting stainless steel tubes – either a tube cutter with a carbide wheel or a grinding disc with a stainless steel disc. A standard metal saw may leave burrs and metal chips on the edge, which are unacceptable in a solar system. After cutting, always check the cut: it must be perpendicular (not slanted), the edge must be free of burrs and visible deformations of the corrugated structure.

Step 2: Putting on the nut before sealing

This is a step that is most frequently forgotten in customer orders, and yet it is crucial. Before you put on any sealing or attempt to connect the corrugated pipe to a fitting, you must have the nut already slid onto the pipe. Once the corrugated pipe is connected, the nut cannot be inserted through the flanged end – it is physically blocked by the expansion at the end of the pipe.

Procedure: take the nut and slide it onto the corrugated pipe with the threaded hole facing toward the free end of the pipe (i.e., the thread will point outward, toward the fitting). Slide the nut about 15–20 cm from the end, so you have enough space for the next steps. Never force the nut through the stainless steel end – if it does not go easily, check whether the internal diameter of the nut is correct for the given DN.

Step 3: Preparing the sealing element

Sealing inserts for solar pipes must withstand temperatures up to 200 °C and be compatible with glycol-based solar mixtures. Therefore, standard rubber O-rings are not used in solar systems, but seals made of:

EPDM (ethylene propylene diene rubber) – resistant to glycol-based media, suitable up to 150 °C in long-term operation, a common choice for most systems
Graphite seals – suitable for higher temperatures (up to 280 °C), used in systems with a drain system, where the medium can reach stagnation temperatures close to 200 °C
Teflon (PTFE) – suitable only for supplementary threaded sealing, not as a main sealing insert in the flanged connection of corrugated pipe

Insert the sealing insert into the seat of the fitting or connector, or put it on the end of the corrugated pipe according to the construction of the given valve. The seal must sit evenly, without any sign of tilting or breaking.

Step 4: Connecting the corrugated pipe to the fitting

Insert the free end of the corrugated pipe into the fitting so that the flanged limit (expansion at the end of the pipe) rests against the seat in the fitting. Do not try to push the corrugated pipe in forcefully – if it is difficult, something is wrong (wrong size, foreign object, wrong orientation of the nut). Once the corrugated pipe is properly seated, manually tighten the nut toward the fitting and begin screwing the thread manually – without tools – at least 3–4 turns. If the nut does not start screwing smoothly, stop and check the perpendicularity – a tilted nut will cross-thread the thread and damage the fitting.

Step 5: Tightening the nut with a wrench

This is the step where installers most often make mistakes in both directions – either they do not tighten the nut enough, or they over-tighten it. Both lead to leakage, but for different reasons.

The correct procedure for tightening the nut on a stainless steel corrugated pipe:

First, tighten the nut manually until you feel slight resistance from the seal (approximately 4–6 turns of the thread by hand)
Then take wrench No. 1 – hold it firmly on the fitting or valve to which the nut is being tightened
With wrench No. 2, tighten the nut by an additional 1.5 to 2 turns (for DN16 with EPDM sealing); for graphite sealing, 1.0–1.5 turns are sufficient
Never tighten the nut without counterholding – you might rotate and damage the fitting or another connection
Never tighten the nut more than the recommended number of turns at once – over-tightened sealing loses elasticity and does not seal, and graphite may crumble

An interesting practice from the field: after tightening the nut, mark a small line (or write with a marker) on the position of the nut relative to the fitting. After the pressure test and the first heating of the system, return and check whether the nut has rotated. Thermal expansion can loosen the nuts by millimeters if they were not sufficiently tightened.

Installation in specific practical situations

Scenario A: Collector on a sloped roof, tank in the boiler room two floors below

This is by far the most common case we deal with in family homes. The route usually goes through the attic structure, then through an installation shaft or gap in the masonry down to the technical room. The total length of the route is usually 10–18 m, which is covered by the 10-meter or 20-meter flexible hose 2 in 1.

When routing the pipe through the attic, pay attention to:

The pipe must not remain unsupported for sections longer than 1.0–1.2 m – its own weight + the weight of the medium and insulation accumulate during operation and can cause vibrations during flow
Always use a UV-resistant and temperature-resistant penetration sleeve (gland) when passing through a roof or wall – not regular PE foil
Prefer vertical sections with a slight slope (1–2 %) towards the storage tank – helps with air venting

Scenario B: Flat roof, long horizontal route, external installation

On flat roofs (apartment buildings, commercial buildings), an external route can reach 30–50 m. In most cases, it is necessary to combine multiple pipe sections and use rigid corrugated sections with fittings. Although connecting multiple corrugated sections in sequence is technically possible, each additional connection is a potential weak point. Therefore, the rule applies: minimize the number of connections wherever possible.

When installing externally on a flat roof, pay attention to:

Insulation protected by UV-stable cladding (otherwise the insulation will disintegrate within 2–3 years)
Fastening every 80–100 cm against uplift by wind (if the fastening is interrupted, the insulation can deform in warm weather)
Expansion loops on long straight sections (every approx. 10 m) – thermal expansion of stainless steel is 0.017 mm/(m·K), at a 100 °C temperature difference on a 20 m section this means ~34 mm length change

Scenario C: Installation on a balcony or façade (compact systems)

Small balcony solar kits usually have routes under 5 m. Here, the problem is more about spatial limitations and the aesthetics of pipe routing. Stainless steel corrugated pipe is ideal here due to its ability to be shaped into space-saving configurations. Be careful to ensure that bends are not hidden behind wall corners without access – if the pipe ever leaks, you must have access to the connection.

Working with fittings: threads, types and compatibility

Solar corrugated pipe fittings are not standardized across all manufacturers, and this is where a lot of confusion arises. The basic types of threads you will encounter in solar systems:

G 3/4" (BSP) – most common for DN16 pipes
G 1" (BSP) – standard for DN20 and larger
M24×1.5 – metric thread, found in some European manufacturers

Important: the fitting must always be made of the same material as the corrugated pipe – i.e., stainless steel (AISI 304 or 316L). A combination of brass and stainless steel is problematic in solar systems due to galvanic corrosion, especially when the medium contains glycol-based inhibitors. In practice, this means visible green or white deposits around the connection and gradual thread degradation.

A more detailed comparison of fittings and seals for different diameters can be found in the article Sealing of connections and selection of fittings for solar pipes DN12 and DN16.

Pressure test after installation: how and why

After completing all connections, a pressure test follows. This is a mandatory step – not optional. If you skip it and directly fill the system with solar fluid, any leak will not only cause media loss but also glycol contamination of the insulation, which is practically irreparable without replacing the entire insulation section.

Pressure test procedure for solar piping:

Use nitrogen (N₂) or dry air for the test – not water. Water in stainless steel solar piping can start to corrode in thread gaps during stagnation (if the system is not immediately started).
Test pressure: 1.5 times the operating pressure, at least 6 bar (operating pressure of most solar systems is 2.5–4 bar)
Hold the pressure test for at least 30 minutes. If the pressure does not drop by more than 0.1 bar, the system is tight.
Visually inspect the connections (by sight) and by hand – during gas leakage you can feel a slight flow. For certainty, you can use a foaming leak detector (available at any professional store).

From practice: the most common leaks after installation occur at connections where the installer forgot to install a gasket (the connection looks fine at a glance, but immediately reveals the issue under pressure), or at a fitting that was not tightened evenly (misalignment of the gasket during tightening). Both issues are immediately detectable by a pressure test before filling the system.

Securing and routing the pipe during installation

Even perfect connections will not help if the pipe is improperly secured. Solar stainless steel piping operates in a temperature range from -30 °C (winter, stagnation without medium) to +200 °C (summer stagnation in an empty system). Such a temperature range causes significant length expansion, and if the pipe has nowhere to "move", the forces are transferred to the connections.

Securing principles:

A fixed point (a fixed clamp that does not allow movement) only at one location of the section – usually at the inlet to the storage tank or at the bend location
All other clamps must be sliding or must have at least 3–5 mm clearance in the direction of expansion
Between the fixed point and each end of the section, there must be an expansion loop or compensator (usually formed by the flexibility of the corrugated pipe itself)
Vertical sections: clamps every 1.2 m
Horizontal sections: clamps every 1.0 m (weight of the medium)

Typical installation errors and how to avoid them

Over the years of contract work, the same mistakes keep recurring. Here is an overview with explanations of what exactly goes wrong and how to fix it even during installation:

Forgotten nut before installation: The most common mistake of beginners. Solution: you have to disconnect and cut the pipe, and repeat the whole process from the beginning.
Inappropriate gasket: Using a standard rubber gasket (e.g., from a plumbing installation) – it will degrade at temperatures above 90 °C. Always use EPDM or graphite certified for solar systems.
Over-tightened nut: You can tell this by the gasket being "pushed" inward or outward from the seat. The solution is to replace the gasket and retighten it with the correct torque.
Missing pressure test: The system is filled with the medium, and a leak appears during the first heating. Repairing it is several times more expensive and labor-intensive than the test itself.
Torsion of flexible pipe: Sometimes the pipe twists along its axis when unwinding the coil. After pressurization, the twist tries to equalize – and transfers torque to the connections.
Too sharp a bend: The corrugation deforms, narrows the cross-section, increases hydraulic resistance of the system, and weakens the pipe wall. In extreme cases, it may crack under stagnation pressure.

A more detailed analysis of operational problems can be found in the article Common problems with solar piping: overheating, leaks and stainless steel corrosion.

Most frequently asked questions (FAQ)

Can I use Teflon tape instead of an EPDM gasket for a flanged connection of a corrugated pipe?

No. Teflon (PTFE tape) is intended for sealing conical threads (e.g., NPT or conical G-threads). A flanged connection of a stainless steel corrugated pipe works on the principle of compressing the sealing insert between the flanged end of the pipe and the socket of the fitting – that is, not a threaded seal, but a flat one. PTFE tape would not provide any sealing here and would only complicate tightening. Always use the correct sealing insert in the socket.

How many turns of the wrench are enough to seal a nut at DN16?

For an EPDM gasket at DN16, the general recommendation is: after manual tightening (until you feel resistance from the gasket), add 1.5 to 2 turns with the wrench. For graphite gaskets, 1.0–1.5 turns are sufficient, as graphite compresses faster and cracks easily when over-tightened. After the pressure test, you can "tighten" the connection by a maximum of half a turn if the pressure drops.

Is it possible to connect stainless steel corrugated pipes from different manufacturers (e.g., a nut from one, a pipe from another)?

Theoretically yes, if the dimensions and thread are compatible (e.g., both G 3/4" at DN16). In practice, however, I recommend limiting such combinations – different manufacturers may have slightly different tolerances on the flanged ends, different sockets in fittings, and a gasket from one manufacturer may not fit perfectly into the socket from another. If you do combine them, always perform a pressure test and be prepared to repeat it after the first heating.

What to do if one connection still leaks after the pressure test, even after tightening the nut?

If tightening doesn't help, the cause is usually either a damaged gasket (crushed or incorrectly seated), or a damaged socket in the fitting (e.g., from previous dry tightening without a gasket). Solution: unscrew the nut, remove the gasket, and visually inspect both. If the socket is damaged (grooves, scratches), the fitting must be replaced – non-compressible water between metal will always find a way out. If the socket is in good condition, insert a new gasket and repeat the whole process.

How long will a stainless steel corrugated pipe last in a solar system?

With proper installation, the correct type of medium (pH 7–8, inhibitor glycol mixture changed every 3–5 years), and adherence to the minimum bending radius, a quality stainless steel corrugated pipe can last 25 years or more. The main causes of premature failure are: corrosion from contact with chlorides (e.g., salt used near the facade), material fatigue from sharp bends with vibrations, and acidity from degraded glycol medium. How to properly extend the life of the piping is discussed in the article Maintenance and inspection of solar piping: how to extend the life of corrugated pipe.

Can I leave solar piping uninsulated inside the building?

Technically it is not prohibited for short sections (up to 0.5 m at a bend or at the inlet to the tank), but it is not recommended from a professional standpoint. Even a short uninsulated section in a technical room will transfer heat loss, which, in year-round operation, represents several percent of the system's total output. In the case of storage tank systems, an uninsulated section also heats the boiler room, which is undesirable in summer. Details can be found in the article Insulation of solar piping: what not to forget when installing outdoors and indoors.

Conclusion: installation of solar piping is not complicated – but it must be thorough

Correct installation of stainless steel solar piping and threaded nuts is not a technically demanding task – it can be handled by any skilled plumber or experienced DIY enthusiast. The key is understanding a few basic principles: why you need to install the nut before the gasket, why you must always hold the fitting while tightening, why the pressure test is not just a formality, and why the choice of sealing materials should not be underestimated.

These principles apply to every type of stainless steel solar piping – whether it's a flexible double pipe 10 m, longer 20 m version, or rigid DN16 and DN20 corrugated pipes. A properly executed installation will ensure trouble-free operation without leaks, without corrosion, and without the need to interfere with the system for many years.

If you are unsure about the correct diameter or length of pipe for your specific route, I recommend checking the overview of all available solar pipes and stainless steel corrugated pipes – you will find the current range together with technical specifications that will help you make the right decision before purchasing.

Do you have a question about this topic?

Not sure how to proceed or dealing with a specific situation in your home? Write to us – we are happy to help.