Sealing of connections and selection of nuts for solar pipe DN12 and DN16
Tightness of connections and selection of nuts for solar pipe DN12 and DN16
If you have ever encountered a leak of solar fluid in a newly installed system, you know what I'm talking about. A leaky connection on a solar pipe is not just an aesthetic issue – it means loss of medium, pressure drop, and in the worst case, insulation damage and corrosion of connections in places where you would least expect it. And in most cases, the cause is trivial: the wrong nut, a poorly tightened connection, or a forgotten sealing olive.
In this article, I cover everything you need to know about the tightness of stainless steel solar pipe connections with diameters DN12 and DN16 – from understanding the geometry of the connection, through the selection of the right nuts and seals, to specific tightening moments and typical mistakes from practice. This article is primarily intended for those who install solar systems themselves or want to understand what the installer is doing with their system.
Why the tightness of solar connections is more demanding than in a standard water pipe
The solar circuit operates under conditions that are significantly more aggressive than a standard water supply. The heat transfer medium – usually a mixture of propylene glycol and water in a ratio of 1:1 to 2:3 – has a lower surface tension than pure water. In practice, this means that the solar fluid "seeps" through leaks much more easily than water. A connection that would last with water does not seal with the solar medium.
Temperature cycles also come into play: during the day, the pipe heats up to 150–180 °C in stagnation (a state where the storage tank does not want to accept heat and the collectors overheat), and at night the temperature drops to 10–20 °C. These cycles cause thermal expansion, which gradually loosen the connections if they are not properly designed. The corrugated stainless steel pipe absorbs these expansions with its own flexibility, but the connections themselves must be robust.
The solar fluid is also chemically active – its pH is around 7–9 and contains corrosion inhibitors that require compatible sealing materials. EPDM, FKM (Viton), and PTFE are suitable materials. Common rubbers or unsuitable plastics degrade with the solar medium.
Connection geometry DN12 and DN16 – what do these markings mean
The marking DN (Diameter Nominal) is the nominal pipe diameter. For stainless steel corrugated solar pipes, it applies:
- DN12: the outer diameter of the corrugated pipe is usually 13–14 mm, the inner clear diameter is about 12 mm. The thread of the fitting is most often 1/2" BSP (Gas) or M16×1 according to the manufacturer.
- DN16: the outer diameter of the corrugated pipe is about 17–18 mm, the inner diameter is 16 mm. The thread of the fitting is typically 3/4" BSP or M22×1.
These dimensions are important not only for the hydraulic calculation of the system – we elaborate on this in the article What diameter of solar pipe do I need: DN12, DN16 or DN20 – but primarily for the selection of the correct nut. A nut that fits DN12 does not fit DN16, and the final sealing (olive or flat gasket) must fit precisely to the geometry of the corrugated pipe neck.
In practice, I have encountered cases where a customer bought nuts for DN16 and tried to fit them on a DN12 corrugated pipe. The nut did fit, but the olive could not seal the narrowed neck properly – the result was a leak immediately after the first pressurization of the system.
Types of nuts and seals for solar pipe
1. Compression (clamping) nut with olive
This is the most common type of connection for stainless steel corrugated pipe. It works on the principle of pressure deformation of the olive (ring): when you tighten the nut, the olive is pressed into the groove on the valve neck and at the same time wraps the outer wall of the corrugated pipe. A metal-to-metal connection is created with minimal risk of degradation.
Key parameters:
- The olive must be made of stainless steel (AISI 304 or AISI 316) or brass – for solar applications, stainless steel 316 is recommended for higher resistance to glycol medium.
- The nut must have an internal thread matching the thread of the fitting: for DN12 typically 1/2", for DN16 typically 3/4".
- Tightening torque: for DN12 usually 25–35 Nm, for DN16 35–50 Nm. Always follow the manufacturer's instructions for the specific nut.
2. Nut with flat gasket (face sealing)
Some valves and solar distributors use a flat seat with face sealing. The corrugated pipe has a flat flange (enlarged neck) at the end, which is pressed against the valve seat by a flat gasket made of EPDM or PTFE. The nut serves only as a tightening element.
This type of connection is simpler to install, but more sensitive to the flatness of the face. If the corrugated pipe neck is slightly deformed (bent, compressed), the sealing will not be uniform and a leak is inevitable. The advantage is that the gasket is replaceable without replacing the entire connection.
3. Threaded connection with a threaded neck (direct nut)
Less common, but occurs mainly when connecting to solar stations, where the corrugated pipe is terminated with a forged threaded neck. Here it is more of a standard threaded sealing: hemp rope with sealing paste or PTFE tape. Note – PTFE tape must be high quality, low-quality tapes cannot withstand temperatures of 150 °C in solar medium.
Material compatibility of nuts and seals with solar medium
This is an area where mistakes are most common. The customer buys standard compression nuts from local DIY stores – they are cheap and available in every hardware store – and after two seasons they start to leak. Why? Because the olive is made of brass with a higher zinc content, which gradually dezincifies (corrodes) when in contact with glycol medium at temperatures above 80 °C.
Recommended materials for solar applications:
| Component | Suitable material | Unsuitable material |
|---|---|---|
| Olive | Stainless steel AISI 316, stainless steel 304 | Zinc brass, copper without coating |
| Nut | Stainless steel, dezincified brass (DZR) | Standard brass, aluminium |
| Flat gasket | EPDM, FKM (Viton), PTFE | NBR (nitrile rubber), CR (neoprene) |
| PTFE tape | Dense, min. 0.1 mm, temperature resistance 200 °C+ | Thin standard tape up to 120 °C |
| Sealing paste | PTFE-based paste for glycol | Paste for water/gas only |
An interesting fact is that EPDM gaskets, which are recommended for solar systems, are not suitable for oil. If you were to ever transfer heat from a boiler using an oil additive into a circuit, the combination of EPDM and mineral oil would end in disaster. For solar systems with propylene glycol, however, this is an ideal choice – EPDM withstands temperatures up to 150–160 °C and is chemically inert to glycols.
Dimension table of nuts for DN12 and DN16
To avoid confusion, here are typical dimensions you will encounter with standard stainless steel corrugated pipes:
| Parameter | DN12 | DN16 | DN20 |
|---|---|---|---|
| Outer diameter of corrugated pipe | 13–14 mm | 17–18 mm | 22–23 mm |
| Typical valve thread | 1/2" BSP | 3/4" BSP | 1" BSP |
| Nut opening (wrench) | 22–24 mm | 27–30 mm | 36–38 mm |
| Tightening torque | 25–35 Nm | 35–50 Nm | 50–70 Nm |
| Olive diameter (outer) | 13–14 mm | 17–18 mm | 22 mm |
Note: These dimensions are typical for most European manufacturers, but each manufacturer may have slight variations. Always verify the exact dimensions for a specific product. For example, for Stainless steel pipe, corrugated and Stainless steel pipe DN16, these data are specified in the manufacturer's technical documentation.
Step-by-step procedure for correct joint assembly
Here is the procedure I use for every solar pipe installation, and which has not caused a single repeat visit due to leakage in years of practice:
Step 1: Preparing the end of the corrugated pipe
Cut the corrugated pipe to the required length using special shears or a fine-toothed angle grinder. Never use a grinding wheel – sparks and metal dust will get inside the pipe. After cutting, check that the end is straight and perpendicular to the pipe axis. An uneven cut = unevenly loaded olive = leakage.
Step 2: Putting on the nut and olive
First, slide the nut onto the corrugated pipe (with the internal thread facing the end of the pipe), then the olive. Remember this order: nut – olive – end. If you forget and tighten the joint without the olive, you will have to disconnect the entire pipe and start over. I have seen even experienced installers make this mistake under time pressure.
Step 3: Inserting into the valve
Insert the end of the corrugated pipe into the valve until it reaches the stop. Most valves have a stop or a visible groove. The olive must sit precisely in the conical seat of the valve. Check visually before tightening the nut.
Step 4: Pre-tightening by hand
First, screw the nut on by hand a few turns until you feel resistance. This will tell you whether the thread is going correctly (parallel) or whether you have slightly "crossed" the nut. A crossed nut is a typical cause of thread damage and an irreparable leak.
Step 5: Tightening with a wrench
Use a properly sized box wrench for the final tightening. For DN12, use a 22 mm wrench; for DN16, use a 27 mm or 30 mm wrench depending on the manufacturer of the nut. Hold the valve firmly with an opposite wrench – otherwise, instead of tightening the nut, you will turn the valve, which can damage the thread on the valve side.
Tighten in 1/4 turn increments. After each 1/4 turn, check whether the nut moves smoothly. When you feel a noticeable increase in resistance, fix it with another 1/8 turn. For compression joints with an olive, it is not suitable to use a torque wrench – the olive can be deformed and crack during thermal expansion.
Pressure test after installation – don’t skip it
The pressure test is the final step, without which the installation makes no sense. It is carried out before insulating the pipe – if you detect a leak only after insulation, you will spend hours removing the insulation. Solar systems are tested at 6–8 bar (operating pressure is typically 2.5–4 bar, the test pressure is double the maximum operating pressure). Nitrogen or air is used – never water without freeze protection, because in the case of frost you may destroy the system before you even start it.
Let the pressure act for at least 30 minutes, ideally 2 hours. A pressure drop of more than 0.2 bar indicates a leak. Locate the leak with a foaming detection spray – you cannot do this with saliva or regular soap foam, because these methods do not always detect micro-leaks at low pressures, which may appear only under thermal stress.
Common mistakes and how to avoid them
From practice I know that most connection problems stem from a few recurring causes:
- Forgotten olive or incorrect order of installation: Since the corrugated pipe and valve are often installed in a confined space, it is easy to forget the olive. Solution: always prepare the complete set (nut + olive) and, for certainty, thread them onto the corrugated pipe in advance before installation.
- Wrong size of olive: DN12 and DN16 olives differ visually only slightly. Help yourself by marking them, or directly measure the outer diameter of the corrugated pipe with a caliper and compare it with the olive.
- Nut tightened on an angled thread: A nut tightened at an angle is easy to tighten (the thread itself cuts the metal), but the result is a damaged thread and a leak that cannot be repaired without replacing the valve. Solution: always thread the nut on first and turn it a few turns manually without tools.
- Use of regular PTFE tape on solar threads: Regular thin tape melts or loses elasticity at 150 °C. Invest in a thicker heat-resistant tape (100 % PTFE, min. thickness 0.12 mm).
- Over-tightening: Intuitively, it seems that the tighter, the better. This does not apply to compression joints – exceeding the recommended torque flattens the olive and instead of an elastic sealing joint, a brittle, cracking joint is created. On aluminum valves, you may also damage the thread.
- Dirt in the valve seat: Metal chips, construction dirt, remnants of old sealing. Always clean the valve seat with a soft cloth and check whether there are grooves or scratches from previous installation.
Specific situations – DN12 and DN16 corrugated pipes in practice
With the Solar corrugated pipe 2 in 1 (10 m) or its longer version – Solar corrugated pipe 2 in 1 (20 m) – you work with a double construction: two stainless steel pipes (output and input) are integrated in a common insulation. Each pipe has its own end for a compression joint, with the threads usually being 3/4" on the tank side and 1/2" on the control side (depending on the specific manufacturer).
Installation of corrugated pipe has a special feature: the pipe is long and rigid in the insulation. You must lead the ends to the valves without applying lateral forces – this would bend the end of the pipe off-axis and the olive would not sit evenly. I recommend always suspending/supporting the pipe before tightening the joint so that the end points directly into the axis of the valve.
Another thing: with insulated corrugated pipe, you cannot visually check the ends and the olive after installation. Therefore, the rule applies twice: prepare the olive and nut before guiding the end into the installation space.
Information on sizing and selecting the correct diameter can be found in the related article What solar pipe diameter do I need: DN12, DN16 or DN20. The overall installation logic including measuring the route is described in the article Length of solar pipe: how to correctly measure the route from the collector to the tank.
Re-sealing – when not and when yes
If a joint turns out to be leaky, the first instinctive reaction is to tighten it a bit more. This is the correct reaction for flat EPDM gaskets and for threaded joints with PTFE tape, where tightening can solve the problem. With compression joints with an olive, this may not be the case – if the olive has already been properly compressed, further tightening may deform it into an irreparable state.
With compression joints where a leak occurs:
- Loosen the nut completely and pull out the end of the corrugated pipe.
- Check the olive – if it is evenly compressed without cracks, you can sometimes reuse it, but it is usually better to use a new one.
- Check the valve seat for scratches and grooves.
- Replace the olive with a new one of the same size and material, and reassemble the joint according to the procedure.
Reapplying thread sealant on threaded connections with hemp rope is a standard practice: clean the old rope, apply a new layer, and apply sealing paste. Replace the entire PTFE tape – it makes no sense to stick new tape onto the old one.
Solar pipe DN20 – when and how
For completeness: Stainless steel pipe DN20 is used in higher performance systems, where hydraulic loss on DN16 would be too high (simplified: longer runs over 15 meters, a higher number of collectors). Nuts for DN20 are analogous to DN12 and DN16, but the thread is typically 1" BSP and the wrench size is 36–38 mm. The installation procedure is the same, only the tightening forces are higher, and therefore the wrench on the fitting is even more important.
Care for connections during operation
After the first season of operation, it is advisable to perform an inspection. Thermal cycles can slightly loosen a connection that was on the edge of sealing. Visually check for signs of glycol leakage around the nuts (white or yellowish sediment). On a pressurized system, you can place a dry cloth – micro-leakage will show up as moisture, not dripping.
Regular maintenance of solar connections is part of the overall system maintenance, which we discuss in more detail in the article Maintenance and inspection of solar piping: how to extend the lifespan of corrugated pipe. Issues with corrosion of stainless steel parts are discussed in the article Common problems with solar piping: overheating, leaks and stainless steel corrosion.
Most frequently asked questions (FAQ)
Can I use water supply pipe nuts for a solar system?
No, and for several reasons. Standard water supply nuts are dimensioned for lower temperatures (max. 70–90 °C) and lower pressure. The material of olive-shaped seals made from galvanized brass corrodes when in contact with solar glycol. The angle of the nut's wrench flats may differ from that of solar fittings. Always use nuts certified for solar applications – the manufacturer must state that they are suitable for propylene glycol and a temperature range of at least 150 °C.
What is better: an olive seal or a flat EPDM gasket?
It depends on the fitting design – you cannot choose arbitrarily. If the fitting has a conical seat for an olive seal, you must use an olive seal. If it has a flat seat with a face seal, you must use a flat gasket. In terms of reliability, both types are equally reliable when installed correctly. A flat EPDM gasket has the advantage of easier replacement, while an olive seal is more durable under vibrations.
How many times can I disassemble and reassemble the same connection with an olive seal?
Theoretically once or twice, if the olive seal was not deformed. In practice, I recommend visually inspecting the olive seal after each disassembly – if it is evenly compressed, without cracks or deformations, you can reuse it. In case of doubts, always use a new one. Olive seals are cheap – don’t save on them, especially if the connection is not accessible after insulation.
Why is my connection leaking right after installation, even though I followed the procedure?
The most common causes: (1) the olive seal has a different outer diameter than the corrugated pipe – check the dimensions for consistency, (2) the fitting seat has a groove or dirt, (3) the nut thread was twisted at an angle and damaged the fitting thread, (4) the solar medium is cold and the leak appears only after heating – this indicates that the seal is not sufficiently compressed. Disassemble the connection and start over with a new olive seal.
Can I use PTFE tape on compression nuts with an olive seal?
No. PTFE tape is used on threaded connections where it ensures sealing on the side of the thread. In a compression connection with an olive seal, the seal is not formed on the thread of the nut, but on the seat of the olive. Applying PTFE tape on the thread of the nut will not improve the seal and may even prevent the nut from being properly seated on the conical seat of the fitting.
What wrench do I need for DN12 and DN16 nuts?
For DN12: a box wrench of 22 mm or 24 mm (depends on the manufacturer), and a wrench on the fitting according to its size. For DN16: a wrench of 27 mm or 30 mm. I recommend always having both wrenches at hand and never tightening a nut without a wrench on the fitting. An adjustable wrench (Swedish wrench) is a substitute in an emergency – its jaws slightly open under load and can damage the edges of the nut, which is especially important for stainless steel nuts.
Conclusion
Sealing of solar pipe connections is not a matter of chance – it is the result of the correct choice of nuts and seals in relation to the pipe diameter (DN12 vs. DN16), temperature conditions, and the chemical composition of the medium. Adhering to the installation procedure step by step, using the correct materials, and performing a final pressure test are the three pillars of a reliable solar system that will last 15–20 years without problems. As I have seen in many projects – systems where connections were handled responsibly during installation never required assistance. Those where materials or time were saved on, we visited repeatedly.
If you are looking for suitable stainless steel pipe for your installation, take a look at our category solar double pipes and stainless steel corrugated pipes, where you will find a complete range for various diameters and lengths of runs. Further topics on installation, insulation, and material selection can be found in other articles in this Knowledge Center.
Do you have a question about this topic?
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