Sealing threaded connections in heating: Teflon or hemp?
When a small wet spot appears under a threaded connection in the heating system, most people reach for the first sealing material they find in the drawer. Sometimes it works, sometimes it doesn't – and it's precisely that "sometimes it doesn't" that causes unnecessary damage to the floor, wall, or your neighbor below. After twenty years of working with heating systems, I've seen dozens of cases where the cause of the leak wasn't a bad connection, but an incorrectly chosen or improperly applied sealing medium. This article is therefore devoted to one seemingly simple, yet often underestimated topic: how to correctly seal threaded connections on heating fittings, why there are two main approaches (Teflon and hemp), and which one to choose in a specific situation.
This topic is directly related to products from the threaded connection series – and even though the choice of sealing material may seem like a marginal detail, it is precisely this that determines whether the connection will last for decades without problems, or whether it will start dripping the day after installation.
Why do we need sealing material at all?
Threaded connections are not airtight by themselves. A metal thread (whether on copper, brass, or cast iron parts) is never perfectly smooth – each thread groove has microscopic irregularities that occur during machining. Under a magnifying glass, the thread would look more like a spiral path with tiny notches and protrusions. When you screw two parts together without any sealing, water under pressure will simply find these microchannels and leak through them.
Sealing material – whether Teflon, hemp, or liquid sealant – fills these microchannels and creates a physical barrier. It is important that the material is:
- chemically resistant to the heat transfer fluid (water, glycol mixture)
- thermally stable within the range of operating temperatures of the system (typically 20–95 °C, higher under condensing boilers)
- mechanically strong enough not to be squeezed out during tightening
- easy to apply even for less experienced installers
Both basic materials – PTFE tape (Teflon) and hemp fiber – meet these requirements, but each in a different way and with different limitations.
PTFE tape (Teflon): properties, advantages, disadvantages
PTFE is an abbreviation for polytetrafluoroethylene – a substance we commonly know under the trade name Teflon. It is sold in the form of a thin white tape wound on a plastic spool. It is one of the most widely used sealing materials in the entire plumbing industry, and not by accident.
Technical properties of PTFE tape
- Temperature resistance: PTFE is stable up to about 260 °C at atmospheric pressure – in heating, where operating temperatures rarely exceed 90 °C, it is therefore well within its limits
- Chemical inertness: it reacts only with fluorine and alkali metals – common additives in heating systems (corrosion inhibitors, glycol mixtures) have no effect on it
- Tape thickness: standard plumbing tape has a thickness of 0.075–0.1 mm, width 12 mm; there are also thicker tapes (0.2 mm) for larger threads or poorly machined threads
- Friction: PTFE has a very low coefficient of friction – when tightening, the parts may "slip" if you are not careful (this is a disadvantage)
- Does not support bacteria: unlike organic hemp, it does not create an environment for biological growth
How to correctly wrap PTFE tape
This is where most amateurs make a mistake. PTFE tape must be wrapped in the direction of the thread – that is, so that when tightening the fitting, the tape is pulled in and not unwound. On the right hand (for a right-hand thread, which is standard), this means: start the tape at the tip of the thread, wrap it backward (away from you), with each layer slightly overlapping the previous one by a third of the tape width.
The number of wraps depends on the quality of the thread and the pipe size:
- 1/2" thread (R1/2): 3–5 layers for new parts with a good thread, 5–8 layers for older or mechanically damaged threads
- 3/4" thread (R3/4): 4–6 layers standard
- 1" and larger: 5–8 layers, possibly combined with liquid sealant
After wrapping, press the tape with your finger along the entire length of the thread – the tape will partially sink into the grooves and hold better during installation. Then screw the connected parts together with a normal motion – the first few turns are "dry", then the tape begins to act as both a lubricant and a seal at the same time.
When PTFE tape is not enough or fails
Despite its popularity, PTFE tape has clear limitations. The most common mistake I see in practice: the customer gets advice from a neighbor to "wind that white tape" and winds it only once or twice. The result – a wet spot after two days of heating. Another typical mistake: winding in the wrong direction, so that the tape "unwinds" during tightening instead of firmly pressing in.
PTFE tape is also not ideal for:
- Strongly damaged or corroded threads – the tape is too thin to fill larger metal gaps
- Large diameters over 1.5" – hemp is a more reliable choice here
- Connections that will be disassembled again – PTFE tears during disassembly and the remnants must be thoroughly removed before remounting (which people often overlook)
Hemp fiber: a traditional material with a long history
Hemp fiber (technical hemp, in practice also called "flax wick" although this is not botanically accurate) belongs to the oldest seals in piping. It has been used in this form for centuries – and not without reason, because when applied correctly with paste, it gives an excellent and durable seal.
Why hemp alone is not enough
The key detail that many overlook: hemp fiber is always combined with sealing paste (traditionally linseed oil was used, today there are special synthetic pastes available). Hemp alone would stop sealing after drying and hardening of the fiber – the paste maintains the flexibility of the fiber and ensures that it remains compressed and plastically within the joint.
Common pastes available on the market:
- Loctite 55 / Bostik / Unipak – synthetic pastes based on glycerol or silicone, resistant to most media including glycol
- Classic linseed oil – a traditional choice, still functional, but hardens more slowly and may be less suitable in modern systems with glycol
- Fermit / Würth Teflon paste – combined products, some of which also increase the elasticity of hemp during thermal expansion
Correct application technique for hemp
The application process for hemp fiber is a bit more laborious than PTFE tape, but the result is more robust, especially for larger diameters and older, less perfect threads:
- The thread must be clean and dry – remove old paste, rust, oil residue, and remnants of previous sealing
- Select an appropriate amount of fiber – an experienced installer estimates the amount by sight, a beginner should start with a smaller amount (excess fiber will cause the joint to not tighten sufficiently or the fiber to "extrude")
- Wind the fiber on the thread in the direction of the thread (same logic as with PTFE) – start at the tip, wind backward
- Completely moisten the wrapped fiber with paste – the paste should evenly impregnate the fiber, not run down the thread
- Join the parts with a wrench – the hemp will "compact" during tightening and fill all the gaps
Important: hemp requires a few hours (sometimes 24–48 hours for larger diameters) after the system is started to fully harden and stabilize the fiber. Therefore, after a repair using hemp fiber, it is recommended to fill the system, check for leaks, and let it run for at least a day before making a final assessment of the sealing success.
Specific situations: what to choose and why
Scenario 1: Replacing a radiator valve (1/2" or 3/4" thread)
This is by far the most common situation in households. You disconnect the old valve and install a new one, or connect a Threaded connection 1/2" x 15 for connecting a 15 mm copper pipe. The threads are usually in good condition on both sides (new parts or an old radiator with an unused thread).
Recommendation: PTFE tape, 4–5 layers. Reason: speed, cleanliness, and in case of an error, easy correction. If you find after starting the system (when the temperature rises to the working value) that the joint is slightly dripping, it is not necessary to completely disassemble it – sometimes a slight tightening is enough. Hepworth joints for these dimensions are designed for standard British BSP threads, which are well compatible with domestic plumbing tools.
Scenario 2: Connecting to existing old threads from cast iron (boiler room, old distributors)
An old cast iron distribution system from the years 1960–1990 often has slightly corroded, mechanically worn, or even with small metal gaps. Here, PTFE tape usually does not suffice – it does not have enough volume to fill larger gaps.
Recommendation: hemp fiber with paste, or a combined approach. Some experienced installers combine both methods: first wrap PTFE tape (2–3 layers) and then apply a layer of hemp with paste on top. This combination gives a very good result for problematic threads.
Scenario 3: Installation of a direct transition on a new build
On a new build or complete system reconstruction, you have new clean parts with precise threads. For example, you install a Direct transition 3/4" x 22 with internal thread to connect 22 mm copper pipes. The threads are sharp, clean, and free of corrosion.
Recommendation: PTFE tape, 4–6 layers for a 3/4" thread. In a new build, speed and predictability are key – dozens of joints, all the same, installers can work systematically. Hemp would add time without real benefit on new parts.
Scenario 4: Repeated failure – a joint where Teflon or hemp failed
If you have already repaired a joint twice and it still leaks, the problem is likely not the sealing material, but the thread itself (damage, corrosion, wrong type of thread). In this case, I recommend visually inspecting the thread thoroughly before the third repair, or cutting it off and replacing the corresponding part of the pipe. More on this topic can be found in the topic Why the threaded connection is leaking and how to fix it in this Knowledge Center.
Special situations and what to do in them
Glycol-based non-freezing mixtures in the system
If you have a non-freezing mixture (glycol water, typically 30–40% concentration) in the heating system, it is important to choose sealing materials compatible with glycol. Standard PTFE tape is inert to glycol – no problem. Hemp fiber is also resistant, but the paste used with it must be certified for glycol systems – most modern synthetic pastes meet this requirement, but always check the technical sheet from the paste manufacturer. Old linseed oil combined with hemp in a glycol system may degrade faster over time.
Solar collectors and high temperatures
In solar thermal systems, temperatures in stagnation (when the collectors are not supplied by the pump) can rise above 150 °C. Here, classic PTFE tape is approaching the limits of its capabilities – special high-temperature seals or more durable PTFE variants with additives are recommended. Hemp fiber with standard paste usually cannot withstand these temperatures without damage. For solar systems, seals made of elastomers (EPDM) or special silicone-based pastes are more suitable.
Difference between conical and cylindrical threads
In practice, you will encounter two basic types of threads: cylindrical (BSP parallel, designation G) and conical (BSP taper, designation R). With a cylindrical thread, the sealing is provided exclusively by the sealing material – the thread itself does not seal. With a conical thread, the two cones press against each other during tightening and the sealing is partially mechanical – but even here, sealing material is necessary, as metal alone is never enough. Most heating joints, including Threaded connection 3/4" x 22 and Direct transition 1/2" x 15 with internal thread, work with BSP threads – that is, the British cylindrical standard, where sealing depends on the sealing materials.
The topic of thread types is discussed in more detail in the topic Internal vs. external thread: which type of transition do you need in this Knowledge Center.
Practical tips from field experience
Over the years of working with heating systems, a few insights have accumulated that are not found in technical manuals but are crucial in practice:
- Dry thread before installation: Residues of old sealing material, rust, oil from manufacturing – all of this must be removed. For copper pipe, a fine sandpaper (gr. 120) is enough, for brass and cast iron threads, a cloth or special spiral thread cleaner.
- Do not over-tighten!: Teflon has no "stop" – excessive tightening can cause the thread to extrude in brass or cast iron and damage the thread. For a 1/2" BSP thread, the optimal tightening value is 15–25 Nm with a wrench, for 3/4" about 25–40 Nm. In practice: tighten by hand, then with a wrench another 1.5–2 turns.
- Temperature during installation: Do not make threaded connections in a cold room (below 5 °C) – the paste for hemp polymerizes incorrectly and PTFE becomes more brittle and less elastic.
- Check after the first start: First heating – always visually check all new connections after 24 hours. Temperature causes expansion and consolidation of the seals. Sometimes a slight drip from new hemp joints can "self-seal" after the paste hardens – but not always, so check.
- Never reuse: A disassembled joint (even if it looks clean) must not go back without new sealing. Old paste and fiber may have a damaged structure you can't see.
- Combination with O-rings: Some modern joints (flow meters, balancing valves) have their own O-ring as the primary seal. Do NOT add hemp fiber or PTFE to the thread in these joints – you could prevent the O-ring from seating properly. Carefully read the manufacturer's installation instructions.
Selection of sealing materials according to standards and regulations
For heating in apartments and family homes in Slovakia, regulations based on STN EN 14336 (installation of warm air heating systems) and related standards apply. Standards do not directly dictate "PTFE or hemp" – this is a technological freedom of the installer. But they require that the joints are durable, tight, and suitable for the operating temperature and pressure of the system. For low-temperature heating (floor heating, 40/30 °C), standard panel (75/65 °C), and for maximum boiler values (90 °C, pressure usually 1.5–2 bar), both materials meet the requirements when applied correctly.
In cases where threaded joints are part of systems with pressure above 6 bar (industrial, some solar systems) or temperatures above 120 °C, it is necessary to use specially certified seals and this situation goes beyond standard residential heating installation.
If you plan to install a large number of joints at once, for example when connecting Threaded connection 3/4" x 15 in combination with Direct transition 3/4" x 22 during a heating distribution system reconstruction, it is practical to uniformly decide on one sealing system and consistently apply it throughout the project – this ensures consistency and easier inspection.
What to do if the joint still leaks despite sealing
This is a topic in itself, we cover it in detail in the topic Why the threaded connection is leaking and how to fix it, but here is a quick algorithm for first aid:
- Close the supply or air vent – reduce the pressure in the system if possible
- Determine exactly where it is leaking – is it from the thread body, or from the sealing surface? (Different diagnosis, different solution)
- If it leaks from the thread body: disassemble the joint, remove the old sealing material, repeat with new material (possibly a larger amount)
- If it leaks from the sealing surface (flat metal contact): the problem is in the contact surface or in the deformed seal – here neither hemp nor PTFE on the thread will help
- If it depends on temperature – leaks only when hot and not when cold: classic expansion leak, usually solved by slightly tightening (be careful – not over-tightening) or complete disassembly and reassembly
More on the procedure for annual inspection and preventive tightening can be found in the topic Inspection and tightening of threaded joints during annual heating service.
Most frequently asked questions (FAQ)
How many layers of PTFE tape to wrap on a 1/2" thread for a heating joint?
Standard is 4 to 5 layers for a 1/2" (BSP R1/2) thread on new parts. If the thread is older, slightly corroded, or of questionable origin, increase to 6–8 layers, or combine with hemp. More important than the number is the direction of winding (in the direction of the thread, from the tip backward) and evenly filling the entire thread.
Can I wrap PTFE tape on hemp fiber, or vice versa – combine both materials?
Yes, combining is a common practice for problematic threads. The most common combination: 2–3 layers of PTFE tape directly on the thread, then a layer of hemp fiber with paste on top. PTFE provides primary sealing of microchannels, hemp fills larger irregularities and gives the joint mechanical robustness. The reverse combination (hemp directly, PTFE on top) is less common and less effective.
Is PTFE tape suitable for threads smaller than 1/2", for example for air vents?
Yes, PTFE tape works for very small threads (M10, 3/8" BSP). 2–3 layers are enough. For such small threads, it is even more important not to over-tighten – the thread in plastic or aluminum of a small valve can easily be damaged by excessive torque.
How long do I have to wait after installing hemp fiber before starting the heating?
Technically, you can fill the system immediately – hemp fiber starts sealing right away, the paste does not need to fully harden for basic sealing. But final assessment of the seal should be done no sooner than 24 hours after operation, ideally after two to three heating cycles. The paste polymerizes at operating temperature and the fiber stabilizes definitively only after a few temperature fluctuations.
Is PTFE tape suitable for gas threads?
For gas lines (natural gas, propane-butane), PTFE tape is only acceptable if the manufacturer of the threaded fitting and the relevant regulation explicitly permit it. In many cases, special yellow tape (thicker, different composition) or approved liquid sealants are prescribed for gas. For heating (hot water), standard white PTFE tape is without restriction suitable.
What to do if I wrapped the PTFE tape in the wrong direction and the tape is unwinding slightly during tightening?
Disassemble, remove the remaining PTFE (with a clean cloth or soft brush – fibers must not remain in the pipe or thread) and wrap it again in the correct direction. Do not use the tape that unwound during installation – it is deformed and will not provide a reliable seal. Wrapping takes a few seconds and a spool of PTFE tape costs less than a euro – saving is not a reason to risk a leaky joint.
Conclusion: practice beats theory, but theory saves work
Choosing between PTFE and hemp is not a matter of belief or tradition – it is a matter of the situation, dimensions, and quality of the available parts. For a home DIYer with a standard job (valve replacement, connecting a new radiator, installing 1/2" or 3/4" joints), I recommend starting with PTFE tape – it is fast, clean, and absolutely reliable when wrapped correctly. For an experienced installer or when working with larger diameters and older installations, hemp fiber with quality paste is unbeatable.
More detailed information on selecting specific products, dimensions, and compatibility can be found in the topics How to choose the right threaded connection for heating, Dimensions of threaded connections: how to correctly measure the pipe diameter, and Installation of a threaded connection on copper pipe step by step in this Knowledge Center.