Threaded Fittings and Seals in Heating – How to Choose the Right Type and Size

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Threaded connections and seals in heating – how to choose the right type and size

When you install a radiator, replace a pump, or connect an expansion tank, you are usually focused on the bigger picture – the boiler, distributor, manifold. But it is precisely the small details that determine whether the whole system holds and does not leak. Threaded connections and seals are often overlooked or underestimated during installation – and then technicians find them under the radiator in a puddle of water two weeks after installation. This article is intended for plumbers and technically skilled DIY enthusiasts who want to understand what they are installing, choose the right type, size and material of the seal, and avoid the most common mistakes from practice.

Basics of threaded connections – how it works and why details matter

Threaded connections are omnipresent in heating installations. They connect pipes, fittings, radiators, taps, valves, pumps, safety valves, pressure gauges, drain cocks – basically every component that needs to be disconnected, cleaned or replaced at some point. Unlike pressed or welded connections, threaded connections allow disassembly without special tools and reassembly.

The principle of sealing a threaded connection is simple: the thread itself is not perfectly tight, because there is always a small gap between the external (male) and internal (female) thread. This gap must be filled with a sealing material to prevent the penetration of liquid or pressure. The correct choice of seal depends on several factors: the temperature of the medium, the pressure in the system, the type of thread (cylindrical or conical), the material of the fitting and the type of medium (water, gas, ethylene glycol).

In practice, we encounter two basic types of threads according to the ISO 7/1 and DIN 2999 standards:

Conical thread Rp/R (BSP Taper) – the external thread (R) is conical, which means that it seals itself when tightened. It is mainly used for gas and pressure connections.
Cylindrical thread G (BSP Parallel) – both threads are cylindrical (parallel), they do not seal themselves, always require some sealing element – a flat rubber gasket, hemp fiber with paste or Teflon tape.

In heating, the most common are cylindrical threads G (Whitworth), marked according to the nominal diameter in inches: G½", G¾", G1", G1¼", G1½", G2". Smaller sizes (G½" to G¾") can be found on fittings and radiator valves, larger ones (G1" to G2") on distributors, pump units and when connecting boilers.

Types of threaded connections – overview and when to use what

Threaded connections are produced in many configurations. In heating practice, the following are most common:

Straight connectors (couplings)

The simplest type – they connect two pipes or fittings directly in one axis. They can be the same (both ends the same size) or reducing (each end a different size). Straight connectors are used, for example, when extending a pipe, connecting a pump to an existing system or connecting a pressure gauge. When dimensioning, you need to pay attention to whether it is a female-female (FF), female-male (MF) or male-male (MM) connector.

Elbows (bends)

Elbows are used to change the direction of the pipe by 90°. In heating, we encounter them mainly when connecting radiators, where the pipe comes from the wall or floor at a different angle than the valve axis. There are also versions with a 45° angle. Brass is the dominant material, while cast iron or stainless steel is used in less pressure-stressed applications.

An example of a commonly used fitting in this category is a flat brass threaded elbow with sealing 2" – this is a robust piece for larger dimensions, typically for connecting larger fittings or distributors, where a 90° change in direction is also needed.

T-pieces and cross connectors

T-pieces allow a branch off from the main pipe. In heating, they are used when building distributors feeding individual circuits (for example, a branch for radiators vs. a branch for floor heating). Cross connectors are less common and appear in combined systems in technical rooms.

Unions and reducers

A union is a special type of connection that allows easy assembly and disassembly without rotating the connected pipes. It consists of two bodies and a central nut. It is used everywhere where repeated disassembly is expected – at pumps, filters, safety valves. Reducers solve the transition between two different thread sizes.

Materials of threaded fittings – brass, steel, cast iron, plastic

The choice of material for the fitting depends primarily on the environment, pressure, temperature, and compatibility with other materials in the system. Each material has its strengths and limitations.

Brass

Brass is by far the most common material for threaded fittings and valves in heating systems. The reason is simple: excellent machinability, good corrosion resistance, tight thread sealing, and long service life. Brass fittings are standard in hot water heating (central heating, floor heating), sanitary installations, and low-pressure gas. They operate reliably at temperatures from -20 °C to +120 °C, and up to +150 °C in short-term peaks. The working pressure of standard brass fittings is 16 to 25 bar.

Note: Brass contains zinc and lead. When used with potable water, it is necessary to use valves with potable water certification (marked DVGW or W270), where lead content is limited to a minimum. This restriction does not apply to the central heating circuit, where water does not flow into the tap.

Cast iron (GGG, EN-GJS)

Cast iron is used for larger dimensions (from DN50 upwards) and higher pressures. In central heating installations, it is more commonly found in boiler rooms – for main distribution lines, boiler blocks, and larger pump units. Advantage: high strength and durability. Disadvantage: heavy weight, higher cost, and susceptibility to external corrosion if not painted or galvanized.

Carbon and galvanized steel

Steel threaded fittings (galvanized) are used in older installations or in industrial environments. In standard heating systems, they are increasingly replaced by brass or stainless steel today, as galvanization degrades over time and electrochemical corrosion can occur in systems with corrosion inhibitors (glycol mixtures) when brass and steel come into contact. If you are building a new installation, it is better to avoid steel threaded fittings and use brass or stainless steel instead.

Stainless steel

Stainless steel (AISI 304 or 316) is used where there is an increased risk of corrosion – solar systems, geothermal wells, environments with aggressive media. The price is higher, but the service life is correspondingly long. For standard hot water heating, stainless steel is an unnecessary luxury, but it is welcome in condensing systems with low pH condensate.

Synthetic materials (PP, PVDF, PB)

Plastic threaded fittings have limited use in heating – mainly in floor heating with plastic pipes (PEX, PB, PP-R). The threaded connectors on plastic manifolds are usually made of brass or combined (plastic body, brass stop). Pure plastic threaded fittings lose strength at temperatures above 80 °C, so they should not be used in primary circuits with a high temperature difference.

Sealing in threaded connections – types, materials and correct selection

Sealing is just as important as the fitting itself. An incorrect seal is the most common cause of leaks in heating installations. There are several basic types:

Hemp fiber + sealing compound

A classic in heating, well known to older plumbers. Hemp fiber is wound in the direction of the thread (always clockwise when viewed from above the thread) and coated with sealing compound (Fermit, Loctite 55, etc.). The fiber fills in the irregularities of the thread, and the compound ensures lubrication and additional sealing. Advantage: cheap, reliable, easy to repair. Disadvantage: fiber can come out of the gap when the thread moves, the correct amount is important (too little = leakage, too much = cracking of the valve). Suitable for water (cold, hot, heating), not suitable for gas without special certification.

Teflon tape (PTFE)

Teflon tape is probably the most widely used sealing material for threaded connections today. It is wound 3–5 times around the thread in the direction of the clock. Advantages: clean, easy to apply, chemically resistant, no odor. Disadvantages: less reliable than hemp fiber + compound for larger dimensions (from G1" upwards), can be cut off under excessive tightening. PTFE tape is not generally approved for gas pipelines – special tapes or compounds certified for gas must be used.

Flat rubber seals (O-rings, flat washers)

Flat seals are used in connections with a flat seating surface – for example, in fixed nut couplings, unions, filters, and some types of valves. The material is usually EPDM (for hot water and heating, resistant up to +150 °C), NBR (for oils and some chemicals), silicone (up to +200 °C, for solar systems), or flat brass/copper (for high-pressure connections).

For 2" dimensions (such as a flat brass threaded coupling, right angle with sealing 2"), flat EPDM sealing is most commonly used, with an outer diameter matching the seating surface of the nut – always check whether the seal is included in the coupling delivery or needs to be ordered separately.

Anaerobic sealants (liquid sealants, e.g. Loctite 577, Loctite 542)

Liquid anaerobic sealants are suitable for industrial applications and larger dimensions, where hemp fiber is difficult to apply. They harden after the joint is closed without air access. Advantage: excellent sealing even under vibrations, can withstand higher pressures. Disadvantage: the joint is difficult to disassemble (heating above 150 °C is required for disassembly), higher cost. In standard home heating, they are less common, more often found in industrial boiler rooms and pump unit installations.

Thread Dimensions – Imperial System and Its Pitfalls

The imperial system BSP (British Standard Pipe) is omnipresent in European heating practice. The nominal thread size (e.g. G½") does not correspond directly to the outer diameter of the pipe or thread – it is a historically defined dimension referring to the inner diameter of the pipe from which the thread was developed. Practically, this means that a G½" thread has an outer thread diameter of 20.96 mm, not 12.7 mm (which would be half an inch).

The most common dimensions in heating practice and their actual sizes:

G⅜" – outer thread diameter 16.66 mm, pitch 19 threads per inch (1.337 mm/thread) – used for small valves, thermometers, drain cocks
G½" – outer diameter 20.96 mm, 14 threads per inch (1.814 mm/thread) – the most common size for radiator valves and thermostatic heads
G¾" – outer diameter 26.44 mm, 14 threads per inch – boiler connections, safety valves, small circulation pumps
G1" – outer diameter 33.25 mm, 11 threads per inch (2.309 mm/thread) – connections for larger boilers, distributors, pumping units
G1¼" – outer diameter 41.91 mm, 11 threads per inch – larger pipe runs
G1½" – outer diameter 47.80 mm, 11 threads per inch – technical rooms, larger collectors
G2" – outer diameter 59.61 mm, 11 threads per inch – large pipe runs, industrial boiler rooms, as well as some special valves such as the mentioned brass right-angle threaded coupling 2"

A common mistake among beginners is confusing the BSP (G-thread) with the NPT thread (National Pipe Thread, American standard). The NPT thread has a different pitch and a different flank angle (55° for BSP vs. 60° for NPT). They look similar from the outside, but are not mutually compatible – the thread appears to screw in, but will never seal reliably. When purchasing valves from different manufacturers, always check whether it is BSP or NPT.

Practical Procedure for Installing a Threaded Coupling

From my experience in the field, I know that most leaks do not occur due to a bad product, but due to improper installation. Here is a procedure that works reliably:

Visually inspect the thread – burrs, mechanical damage to the thread or an incomplete thread are a sure path to a leak. Do not try to seal a damaged thread with sealing material, but replace the valve or rethread the pipe with a tap/file.
Choose the right sealing material – according to the table above. For most heating connections (G½" to G1½"), the proven combination is hemp fiber + Fermit, or PTFE tape for smaller dimensions.
Wind the sealing material in the correct direction – always clockwise, when viewed from the end of the thread. If wound the opposite way, it will unwind during tightening.
Amount of sealing material – for PTFE tape: 3–5 layers for G½"–G¾", 5–8 layers for G1"–G2". For hemp fiber, it should cover the entire thread in a uniform layer, not in clumps.
Tighten by hand, then with a wrench – first screw in by hand as far as possible, then tighten with a wrench. For smaller dimensions (G½"–G¾") 1–1.5 turns with a wrench after hand tightening is sufficient. For G1" and larger: 1–2 turns with a wrench.
Never overtighten – an overtightened brass valve will crack. This depends on the material and wall thickness – be especially careful with old, rusty valves.
Pressure test always before covering the installation (wall, floor). Standard pressure test for heating: 1.5× operating pressure, minimum 6 bar, for 30 minutes.

Common Mistakes in the Selection and Installation of Threaded Couplings

Over the years of practice I have seen various jobs – from new builds to the reconstruction of 40-year-old installations. These mistakes repeat themselves over and over:

1. Confusing thread dimensions

Very common when purchasing online. The customer sees "1/2" on a radiator valve and orders a G½" coupling. Problem: the radiator valve may have a G½" thread on the pipe connection (for the pipe), but a G¾" thread or a flat conical seating surface for connecting to the radiator. Always measure the actual outer thread diameter with a caliper and convert according to the BSP table.

2. Using the wrong sealing material for gas

Standard PTFE tape is not certified for gas lines. For gas, special yellow PTFE tapes marked "for gas" or sealing pastes with DVGW certification must be used. This is a safety rule, not just a recommendation.

3. Using too much sealing material

Paradoxically, too much sealing material can crack the valve or prevent the thread from being tightened sufficiently. Fiber or tape works like a wedge – when over-tightened, it creates enormous radial pressure that can crack even a brass valve. An old plumber's wisdom says: "Not sealing? Disassemble, clean and do it right – more sealing material won't help."

4. Re-tightening after system start-up

If a connection leaks after the system is started, many plumbers try to re-tighten it. If it is a small leak in a cylindrical thread without hemp fiber (only PTFE), it may help. If there is soaked and swelling hemp fiber, re-tightening will only crack the valve. The correct procedure: disassemble, dry, new sealing material, re-install.

5. Ignoring the direction of tightening on a right-angle fitting

A right-angle fitting has two axes and you must plan the tightening so that the final position of the right-angle outlet points in the correct direction. A 180° rotation is not possible without disassembly. This needs to be addressed during planning – you can only adjust it correctly by turning slightly less than one full turn before tightening, provided the sealing material allows it.

Threaded fittings in floor heating and their relationship to other components

Floor heating (hydronic and electric) requires fittings primarily at the manifold – where individual floor loop circuits are fed. Here, typical thread sizes are G¾" or G1" on the manifold inlet/outlet and European standard (EK) connections for connecting PEX or other floor loop pipes. The fittings on the manifold itself are usually a fixed part of the manifold, while the connections on the boiler side are standard threaded fittings.

Electric floor heating (heating mats, cables) does not require threaded fittings – it is an electrical installation. However, if you are combining electric floor heating with hydronic radiator heating in a bathroom, threaded fittings will appear when connecting the radiator or towel dryer. If you are interested in electric floor heating alone, you may want to check related topics in the Knowledge Center, such as Heating mat under tile vs. heating cable – which solution is better for your case or Installation of an electric floor heating mat step by step.

For completeness: electric heating mats for floor heating, such as HAKL products available in various lengths, are a separate category without threaded connections – more about them in topics like How to choose an electric floor heating mat – power, size, and type of floor or What power of heating mat do I need per square meter of bathroom or living room.

Pressure and temperature resistance – what the standard and practice say

Each threaded fitting is sized for a maximum working pressure (PN – Pressure Nominal) and maximum working temperature. The combination of both parameters determines the actual usable conditions. For standard hydronic central heating in a family home, the following approximate values apply:

System working pressure: 1.5 to 2.5 bar (the system is protected by a safety valve typically set at 3 bar)
Maximum temperature: 80–90 °C in older systems, 55–65 °C in condensing boilers and floor heating
Common brass fittings in sizes G½"–G2" are sized for PN16 (16 bar) at 20 °C – at 100 °C the allowable pressure drops to approximately PN10

For solar systems, different rules apply – during stagnation (when the solar collector is without flow on a hot summer day), temperatures in the primary circuit can reach 180–200 °C and pressure can rise significantly. In such cases, all fittings and seals must be made of materials resistant to these conditions – silicone or graphite seals, certified valves for solar applications.

How to correctly determine the size of a fitting when you don’t have the original

This is a practical situation that every plumber faces when repairing old installations. Procedure:

Measure the outer diameter of the male thread using a caliper. For example, 26.4 mm → this corresponds to G¾".
If you have only the female thread (internal thread), measure its internal diameter. For G¾", it is approximately 24.1–24.5 mm.
Count the number of threads in a 25 mm length – this gives the pitch. For G½" and G¾", it is 14 threads per inch, for G1" and larger it is 11 threads per inch.
Verify with a thread gauge using a measuring pin (thread gauge) – a cheap tool that every plumber should have.

When identifying the thread of an old cast iron valve (80 years or older), you may also encounter old imperial threads or even inch threads from other standards. In such cases, the fastest solution is to replace the entire valve with a new one with a certified BSP thread.

Most frequently asked questions (FAQ)

What does "G½"" mean on a threaded fitting and what is the actual thread diameter?

The marking G½" is a nominal size according to the BSP (British Standard Pipe) standard. It does not correspond to the actual pipe or thread diameter – it is a historical designation derived from the internal diameter of an old gas pipe. The actual outer diameter of an external thread for G½" is 20.96 mm, and the pitch is 1.814 mm (14 threads per inch). When connecting a component with an unknown thread, always measure it with a caliper and never rely on visual estimates.

Can I use PTFE tape on gas lines?

Standard white PTFE tape is not certified for gas installations. There are special yellow PTFE tapes or sealing pastes marked "for gas" (certified DVGW or EN 751), which are approved for gas. Never use standard PTFE tape on gas connections – during an inspection by a service technician, your installation will not be accepted, and in the worst case, it is a safety risk.

What is the difference between a threaded fitting and a union (unión fitting)?

A threaded fitting (nipple, elbow, T-piece) is tightened by rotating the entire fitting or one pipe. A union has a central nut that pulls two fixed bodies together – this allows you to tighten the joint without rotating the connected pipe or valve. Unions are ideal for installing pumps, filters, and devices that are difficult to rotate or must be precisely oriented. They also allow for easy disassembly without a wrench on the entire pipe.

Why did my brass fitting crack during tightening?

The most common causes are threefold: too much hemp fiber or PTFE tape (creating excessive radial pressure), over-tightening with a wrench (brass has its elastic limit and cracks when exceeded), or an old or low-quality fitting with a thin wall. Recommendation: for G½"–G¾", do not use more than 4 layers of PTFE or thin, even hemp fiber, and tighten with a wrench no more than 1–1.5 turns after hand tightening.

What sealing should I use for a 2" (G2") fitting?

For larger sizes like G2", such as a flat, right-angle brass threaded fitting with sealing 2", flat EPDM gaskets are most commonly used (if the fitting has a flat seating surface with a nut), or hemp fiber with sealing paste (if it is a threaded seal). At this size, PTFE tape is less reliable – the tape is harder to evenly wind at larger diameters and can break in uneven places. I recommend hemp fiber with Fermit or anaerobic sealant for static joints.

Can I combine brass and steel threaded fittings in one system?

Technically yes, but you need to be careful. When brass and steel come into contact in the presence of water (an electrolyte), galvanic corrosion occurs – steel corrodes faster. In a heating system with a corrosion inhibitor (usually when filled with a glycol mixture), this effect is partially suppressed, but not eliminated. When combining materials, always use a dielectric insert (insulating bushing) between different metals, or construct the entire system from one material – ideally brass.

Conclusion – healthy skepticism and attention to detail

Threaded fittings and seals are one of those categories where saving a few cents can be very costly – for fixing a leak, drying out a wet wall, or for the entire installation to be recalled. Buy quality brass fittings from trusted manufacturers, use the correct sealant for the type of joint and medium, follow the installation technology, and always perform a pressure test before sealing the installation into the structure.

Detailed rules are not bureaucracy – they are the experience of thousands of installers who have gone through the same path and found out what works and what doesn’t. If you are unsure about the size, material, or type of sealant, consult the supplier of the fittings – a good technical salesperson will save you more time and money than any savings on purchase.

Do you have a question about this topic?

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