Frequently asked questions about connecting and mounting systems for bathrooms and sanitary equipment

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Common questions about connecting and fastening systems for bathrooms and plumbing

Connecting and fastening systems are one of those categories that most people underestimate – until something starts leaking, creaking, or breaks off. From experience, I know that this "small detail" is often the cause of unnecessarily expensive failures, repeated visits by plumbers, and sometimes even extensive renovations. In this article, I have summarized answers to the truly most frequently asked questions that our customers ask – from the very basics to specific technical details that determine whether an installation will last twenty years or start causing problems after the first winter.

Whether you are dealing with fixing a sink, replacing an expansion tank, or just looking for the right threaded fitting for a pump – you will find answers supported by real-life experience in the following lines, not just marketing phrases.

Why are connecting elements in the bathroom and plumbing so important?

When we talk about plumbing installations, we tend to focus on the visible elements – a designer faucet, a modern bathtub, or a water-saving toilet. Connecting and fastening elements, however, are what hold the entire system together – literally and figuratively. A bad thread, underestimated tightening torque, an unsuitable material for the fastener, or a missing sealing insert can lead to problems whose repair will cost several times more than the right material at the beginning.

In practice, we see three main groups of problems:

Corrosion and galvanic corrosion – occurs when two different metals are in contact (e.g., a steel screw in an aluminum body), and is accelerated by humidity and the typical temperature in bathrooms.
Undersized thread or load capacity – a fastener that held an empty tank during a test installation may not withstand the full operational weight under pressure fluctuations.
Inappropriate materials for the environment – a standard galvanized screw in a constantly humid environment may rust within a few years; a stainless steel or brass alternative will last ten times longer.

Therefore, paying attention to the correct selection of connecting elements at the beginning of a project is not pedantry – it is an economically sound decision.

What is the difference between a connecting and a fastening element? Terminology you need to know

These terms are often confused, but in a professional context, they have different meanings. Connecting elements ensure a hermetic or mechanical connection between two pipes or devices, for example, threaded couplings, nipples, transitions, or compression fittings. Fastening elements secure and fix a device or pipe to a building structure – these include consoles, fasteners, hanging screws, anchors, and similar items.

In practice, both groups are often combined. For example, when installing an expansion tank, a mounting console (a fastening element) is needed, as well as a connector with the correct thread (a connecting element). Customers sometimes come in saying that "a screw is missing from the tank" – but after a few questions, it turns out they are missing the entire expansion tank mounting set 3/4", not just one connecting element.

Most frequently asked questions about materials: what lasts and what doesn't

Can I use galvanized screws in the bathroom?

Galvanized connecting elements are cost-effective and trouble-free in dry environments. In a bathroom, where there is constantly high humidity, condensation, and temperature fluctuations, this is a risky choice. The zinc layer typically lasts 5–10 years under such conditions, after which it starts to rust. The paradox is that the problem is not immediately visible – the screw looks intact from the outside, but corrosion progresses from the core. When you need to unscrew it (for example, during a renovation), the screw may break off.

We recommend using only stainless steel (A2 or A4), brass, or special plastic fasteners with stainless steel inserts for constantly humid environments. The investment in higher-quality material is minimal, but the result will be noticeable in ten years.

Can I mix brass and stainless steel in one installation?

Theoretically yes, but with caution. Brass (an alloy of copper and zinc) and stainless steel have different electrochemical potentials, which can cause galvanic corrosion in the presence of an electrolyte (e.g., hard water). In practice, this problem occurs more often in industrial applications; in a standard bathroom installation, where connections are dry or water regularly flushes out the electrolyte, the risk is minimal. If you are unsure, use dielectric inserts (Teflon tape, plastic washer) to isolate the contact surfaces.

What do the strength classes on screws mean?

Screws are marked with a two-digit number, for example, 4.6, 8.8, 10.9. The first number (divided by 10) indicates the tensile strength in hundreds of MPa, and the second number (divided by 10) indicates the yield strength ratio to tensile strength. For standard plumbing installations, strength class 4.8 or 8.8 is sufficient. For hanging consoles with higher loads (e.g., a suspended bathtub or a heavy combined faucet), at least class 8.8 is suitable. It should be noted that strength classes are given for steel screws – stainless steel and brass connecting elements have their own markings (A2-70, A4-80, etc.).

Threads in sanitary installations: G, Rp, M – what is what and why it matters

This is an area where customers make the most mistakes. There are three basic types of threads you will encounter in bathroom installations:

G-thread (BSP, Whitworth) – cylindrical (parallel) pipe thread, marked for example as G 3/4". Sealing is achieved exclusively by a sealing ring or gasket, not by the thread itself. Typical for taps, boiler connections, pumps.
Rp-thread (BSP tapered, internal) – tapered pipe thread, where sealing is achieved by the conical shape of the thread. Marked as Rp 1/2", Rp 3/4", etc. The external part (screwed component) is marked as R.
M-thread (metric) – machine thread according to ISO, marked for example as M8, M10, M12. In sanitary installations, it is mainly used for mechanical fastening – screws into brackets, mounting accessories, etc. For hermetic connections, M-thread is not used alone without a sealing element.

Customers sometimes contact us asking whether they can replace G 3/4" with Rp 3/4". The thread profile dimensions are the same (profile and pitch are identical to the BSP standard), but the sealing method is different. If the other side is conical (R), and you fit a parallel one (G), the connection will not seal properly and will leak. More on this topic can be found in the article Which thread and diameter to choose for an expansion tank in our Knowledge Center.

Why is the M8 mounting bracket so common?

The metric thread M8 is de facto the standard for mounting screws and hanging elements in sanitary installations. Its popularity stems from several reasons: it has sufficient strength for most common devices (sinks, brackets, small tanks up to about 30 kg), it is easily available, and most manufacturers of bathroom equipment dimension holes for M8. The M8 mounting bracket thus becomes one of the most frequently purchased items in this category – and despite that, one of the most frequently confused, when customers buy the wrong size or type without consultation.

For heavier devices (boilers, larger expansion tanks over 35 liters, storage heaters), M10 or M12 is recommended. A more detailed analysis can be found in the article M8 thread vs. other sizes: which mounting bracket do I need.

Mounting an expansion tank: most common scenarios from practice

An expansion tank is a mandatory component of every closed heating system and increasingly also in sanitary circuits (pressure expansion in TÚV circuits). Its proper mounting is not just a matter of aesthetics – a poorly mounted tank vibrates, wears out the connection, and in extreme cases can create a dangerous situation under full load.

Scenario 1: Smaller tank (8–18 liters) on the wall in the boiler room

This is the most common case. A tank with a volume of 8 to 18 liters typically weighs 12–22 kg when full of water and membrane. For mounting, a pair of brackets or one combined bracket with a connection is sufficient. The connection is standard 3/4" external thread (R 3/4") from the side of the tank. Into the system (expansion tank pipe connection), you connect via an internal thread. Specifically for these cases, the expansion tank mounting set 3/4" is designed, which includes all necessary components including sealing and anchoring material.

Scenario 2: Larger upright tank (25–80 liters) in a technical room

Larger tanks over 25 liters are usually upright and mounted differently – not on the wall, but via legs or a base on the floor, or with a surrounding bracket around the shell. In this case, the connection is still G 3/4" or G 1", but mechanical wall mounting serves more as a safety against tipping, not as the main load-bearing element. It is important to prevent the connection pipe from carrying the tank's weight – this leads to material fatigue and leaks.

Scenario 3: Pressure expansion in the TÚV circuit

In systems with closed heating of hot water (e.g., a storage heater with a closed circuit), an expansion tank is equally essential. In this case, however, a special type with a potable water membrane (certified for contact with drinking water) is used. The connection is usually also 3/4", but the specific model must be verified. A mistake we repeatedly see: the customer installs a standard membrane tank (intended for heating) into the TÚV circuit – this is hygienically unacceptable and in some countries directly prohibited.

Sealing elements: what belongs where and what definitely doesn't

Sealing elements are closely related to connecting elements – without proper sealing, even the best connection has no sense. In sanitary installations, several types are used:

Teflon tape (PTFE) – universal, cheap, suitable for cylindrical threads G. On conical threads Rp/R it is often combined with thread sealant. Important: wind it in the direction of the thread, otherwise it will unwind during tightening.
Hemp fiber + thread sealant – traditional and reliable method for water and heating pipelines. It requires experience; an inexperienced hand may apply too much fiber and the connection cannot be tightened, or too little and it will leak.
O-rings (OR) and flat seals – for cylindrical threads G and flanged connections. The material depends on the medium and temperature: NBR (nitrile rubber) for water and oil up to 120 °C, EPDM for hot water and steam up to 150 °C, FKM (viton) for chemically aggressive environments.
Compression (olive) seal – for compression fittings on copper, plastic and combination pipes. The olive deforms and creates a hermetic connection. Note: the olive must not be reused – it is replaced during disassembly.

Common mistakes in sealing and their causes are discussed in detail in the article Common mistakes in the installation of connecting elements in the bathroom and how to avoid them, as well as in the article Why the connection leaks at the expansion tank: causes and solutions.

Installation step by step: what to think about before the first tightening

One of the most common problems customers describe is: "I bought everything, but during installation something is missing." That's why we always recommend a process we verify before every major project:

Identify the type of thread on the device – visually check (cylindrical vs. conical) and measure with a caliper or thread template. Don't guess – a mistake of one thread step (e.g. G 1/2" instead of G 3/4") means unnecessary trips and waiting.
Check the load capacity and wall material – hollow wall, solid concrete and drywall require completely different types of anchors. Never install a bracket in drywall without special expansion anchors for SDK.
Prepare all seals in advance – PTFE tape, O-rings, and possibly thread sealant according to the type of connection.
Check the system pressure before final sealing – do the first start-up at lower pressure, check all connections and only then tighten to full operating pressure.
Document the location of hidden pipelines – photograph, draw or mark the location of hidden connections before installing the cladding. It will pay off in five years.

A detailed procedure can be found in the article Installation of fastening elements for expansion tanks and sanitation step by step, where we have expanded this procedure into all details with specific torque values for different thread sizes.

How long do connecting and fastening elements last? Service and inspection

Properly selected and installed connecting elements should last the entire lifetime of the installation – that is 20–30 years for a standard sanitary installation, 15–25 years for heating pipelines. In practice, however, we often encounter the fact that people do not inspect their installations at all until something bursts.

We recommend at least an annual visual inspection (ideally before the heating season):

Check for visible signs of corrosion on bolts and brackets
Check for signs of moisture, water scale or salts under the connections (a white coating around the connection is a warning sign)
Check if the brackets and consoles are firmly seated in the anchoring points – slight movement under load is a warning sign
Condition of the expansion tank – check the gas pressure according to the manufacturer's instructions (typically 0.5–1.5 bar depending on the system height), and whether the membrane is burst (water dripping from the vent = damaged membrane)

More about regular maintenance can be found in the article Maintenance and inspection of fastening elements in sanitary installations.

Comparison of types of fastening systems: consoles, brackets, suspended systems

There are several basic types of fastening systems on the market, which differ in construction, load capacity and suitability for different applications:

Type	Typical load capacity	Application	Note
Simple console	up to 30 kg	Sinks, small tanks	Depends on anchor
Double console (fork)	up to 60 kg	Tanks 18–35 l	Greater stability
Clamping ring	up to 80 kg	Larger tanks, pipes	Load distribution
Suspended system (threaded rods)	up to 150+ kg	Industry, large equipment	Certified calculation
Pipe bracket	depends on size	Pipes ∅15–108 mm	Mandatory spacing

A comparison of types of fixing systems in terms of material, price and durability can be found in more detail in the article Comparison of types of fixing systems for plumbing.

Anchors and wall plugs: the foundation that customers underestimate

Even the best M8 mounting bracket is useless if you install it in an unsuitable anchor in the wrong wall. This is an area where we see the most improvisation – and consequently also the most failures. Basic rules:

Full concrete / brick – a classic plastic wall plug (UX 8, UX 10) is sufficient for a load of up to 40–60 kg per anchor, provided the drilling depth is correct (at least 60 mm for UX 10).
Hollow brick / aerated concrete – requires special wall plugs for hollow materials (e.g., molly wall plugs, chemical anchors). A standard plastic wall plug in a hollow brick has minimal tensile strength – under dynamic loading (pump vibration, water movement) it will loosen.
Drywall – use only SDK anchors with sufficient pulling force. For heavier equipment (basin, bathroom radiator), always anchor into the load-bearing profile in the drywall.
Chemical anchor – for loads over 100 kg, for chemically anchored threaded rods in concrete. Requires proper filling of the hole, observing the curing time, and in critical applications, a static calculation.

Spacing of pipe clamps: standards and practical experience

Pipes in the bathroom and boiler room must be regularly clamped to minimize stress on joints and vibration. The standard STN EN 806 and internal manufacturer recommendations for pipes specify maximum clamp spacing depending on the material and diameter:

Copper pipes ∅15 mm – max. 1.25 m (horizontal), 1.8 m (vertical)
Copper pipes ∅22 mm – max. 1.5 m / 2.1 m
PPR / PP pipes ∅20 mm – max. 1.0 m (due to thermal expansion)
PEX pipes ∅16 mm – max. 0.5–0.7 m (high expansion)
Steel pipes ∅15 mm – max. 1.5 m / 2.5 m

In practice, customers often underestimate the density of clamps for plastic pipes. In heating circuits, where the pipe cyclically passes through temperatures of 20–75 °C, an unsupported section of plastic pipe 1.5 m long will be visibly deformed (bent downward) after one or two years. It is not just an aesthetic flaw – such areas are preferred locations for condensation, sedimentation of sludge, and in extreme cases, cracking at low temperatures.

FAQ: Most frequently asked questions by customers

What is the difference between G 3/4" and 3/4" BSP thread? Are they the same?

Yes, they are practically identical designations. G is the European (ISO 228) designation for cylindrical pipe thread, while BSP (British Standard Pipe) is the British equivalent. The thread dimensions, pitch, and profile are identical. However, it depends on whether it is a cylindrical (G, BSP Parallel) or tapered (R, BSP Taper) type – these are not interchangeable without adjusting the sealing.

Can I use the original wall fitting when replacing the expansion tank?

It depends on the condition and type of the original fitting. If it is in good condition without corrosion, the thread is undamaged, and the length matches the new tank, you can keep it. However, always replace the gasket – never reuse old O-rings or flat rubber, as it will cost you much more later. If you have any doubts about the condition of the thread, it is cheaper to replace the entire expansion tank mounting set than to risk leaks.

How many Nm should I use when tightening an M8 mounting bracket?

For an M8 mounting bracket with a standard strength class 8.8, the recommended tightening torque is 18–22 Nm. For stainless steel fittings A2-70, it is slightly lower – about 14–16 Nm (stainless steel screws are more likely to break when over-tightened). Always follow the manufacturer's recommendations for the device; for example, the boiler body may have a lower limit due to aluminum threads. If you don't have a torque wrench, learn to work with an estimate: M8 is fully tightened when you feel a clear resistance and your hand "won't go further" without leverage – definitely not by the force of your whole body.

Do I need to use anti-corrosion protection when mounting a bracket for an expansion tank?

In a dry environment (a sealed boiler room without condensation), it is not necessary for stainless steel or galvanized brackets. In basement areas with higher humidity, or in outdoor technical rooms, we recommend applying a basic anti-corrosion paint or using only stainless steel components. However, it is more practical to choose the right material from the start rather than later apply a protective layer to galvanized parts.

What to do if the M8 screw "slips" and won't tighten?

The most common cause is stripped threads (so-called stripping). If the screw head is still in place but the wrench turns without resistance, the thread is damaged. Solutions: a) try a larger diameter screw (M10 into the same hole with clearance – usually doesn't work), b) use a thread insert (Helicoil insert) to repair the thread, c) if it is a hole in the device body, consult the manufacturer. Never install a device with a "slipping" screw – it is a direct threat of falling or leakage.

What is the maximum working pressure for standard connecting parts in the bathroom?

Common connecting parts for plumbing (faucets, filling valves, fittings) are dimensioned for PN 10 (maximum working pressure 10 bar) or PN 16. Single-family homes typically have a network pressure of 2–4 bar. Problems arise when the network pressure is higher (some housing estates have 6–7 bar) – in such cases, a pressure-reducing valve is necessary before the building's entrance. Expansion tanks and their fittings must be dimensioned for the maximum working pressure of the entire system, including a safety margin.

Conclusion: Small details, big difference

Connecting and fixing systems seem to be a trivial category. However, as a technician, I can tell you that precisely these "trivial" elements decide whether the bathroom and boiler room function reliably for decades – or whether they will stress you with complaints, leaks, and emergency repairs. An investment in the right material, checking the threads before installation, thorough sealing, and regular inspection are things that take minimal time and money, but their neglect can be very costly.

If you are working on a specific project and are unsure about the selection, check out other articles from our Knowledge Center – for example, How to choose the right connecting and fixing elements for the bathroom and plumbing or How to mount an expansion tank: selecting the set and the correct fitting size. Each project is a bit different, but the basic principles always apply the same.

Do you have a question about this topic?

Can't decide or are dealing with a specific situation in your home? Write to us – we'll be happy to help.