Frequently asked questions about gas installation products – valves, hoses, fittings

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Frequently asked questions about gas installation products – valves, hoses, fittings

The gas installation product range is an area where both laymen and experienced technicians encounter many doubts. What is the correct thread? Can I use this hose for natural gas and propane? Why does the seller recommend a different valve than I expected? We deal with these questions every day – and that is precisely why we have compiled this comprehensive overview that answers what our customers ask most often. We are not looking for short answers – we want you to truly understand what you are buying, why you are buying it, and how to use it correctly.

Basic orientation in the gas installation product range

The gas installation product range includes all components used for gas distribution in buildings and for connecting appliances. These include ball valves, various types of hoses (rigid and flexible), fittings, couplings, reducers, and appropriate seals. At first glance, this area may seem simple – after all, what is complicated about a valve? However, practical experience shows that the most errors occur precisely here, and errors in gas installation can have serious consequences.

The key principle is one: every component in a gas installation must be certified for use with a gaseous medium, must be correctly dimensioned, and must be installed according to current standards. This applies to an inexpensive fitting for one euro as well as to a more expensive ball valve. More about certificates and standards can be found in the related article Certificates and standards for gas installation components – what every part must have.

Gas ball valves – most frequently asked questions

What does the marking FF, MF, and MM mean on ball valves?

This is one of the most popular questions. The marking FF, MF, and MM describes the type of thread on both ends of the valve. F stands for female – meaning an internal (nut) thread, M stands for male – an external (screw) thread. So:

FF – both ends have an internal thread (a coupling with an internal thread on both sides)
MF – one end has an external thread, the other an internal thread
MM – both ends have an external thread

Practical example: if you have a pipe outlet with an external thread (typically at a wall gas point) and you want to screw a ball valve onto it, you need a valve with an internal thread on the pipe side – that is, F on that side. Gas ball valve FF 2" with internal/internal thread and lever is a typical example – the valve has an internal thread on both sides, so it can be screwed onto two pipes with external threads.

What is the correct valve size – inches or DN?

The size of the valve is indicated in inches (") or in the nominal diameter DN. These values are related but not identical to the physical outer diameter of the thread. The most commonly used sizes in domestic installations are:

1/2" (DN 15) – common appliance connections (gas stove, boiler)
3/4" (DN 20) – distribution in an apartment, supply to larger boilers
1" (DN 25) – apartment and commercial distribution
1 1/4" (DN 32) – larger distribution, commercial buildings
1 1/2" (DN 40) – industry, boiler rooms
2" (DN 50) – main house shutoffs, boiler rooms, industry

A 2" (two-inch) valve is a relatively large component – typically installed as the main gas shutoff in an apartment building or in a boiler room, where gas flow is significant. A typical household appliance – stove, gas boiler – operates with a 1/2" connection.

What is the sealing class and why does it matter?

Sealing class A is the minimum standard for gas ball valves according to the EN 331 standard. This standard defines requirements for manually operated ball shutoffs for residential and industrial applications. The sealing class indicates how well the valve seals the flow when in the "closed" position – how much gas can leak at maximum. For gas installation, a certified valve according to EN 331 with sealing class A or better is always required. Cheap valves without certification are not suitable for gas – even though they may look the same. Details can be found in the article How to choose the right gas ball valve – thread, material, and sealing class.

Lever or knob – which is better?

Gas ball valves come with a lever or (less commonly) with a knob. For gas applications, the lever is clearly preferred. The reason is simple and practical: with a lever, you can immediately see whether the valve is open or closed. When the lever is parallel to the pipe, the valve is open; when it is perpendicular, it is closed. With a knob, this is not as clear, and in a stressful situation (gas leak, emergency shutoff), the quick operation of a lever is much safer. Therefore, most gas technicians install valves with a lever exclusively.

What material should a gas ball valve be made of?

Standard gas ball valves are made of yellow brass (Ms 58 or Ms 63), or alternatively red brass, nickel-plated brass, or stainless steel. For domestic gas supply, yellow brass is absolutely standard. Stainless steel is used in more aggressive environments or in special industrial applications. It is important that the valve is not made of grey cast iron or ordinary steel without protection – these materials are unsuitable for gas supply systems.

Flexible gas hose – what you need to know before purchasing

What is the difference between a flexible hose for gas and for water?

This is a mistake that is still being made despite everything. Flexible hoses for water and gas may look very similar at first glance – stainless steel braiding, similar fittings – but they are not interchangeable. A gas hose must be certified for use with gaseous media (natural gas, propane-butane) and must meet the standards EN 14800 or EN 15266. The inner hose is made of a material resistant to hydrocarbons. A water hose does not have these properties – it can degrade when in contact with gas and lead to a leak. A hose intended for gas must have clear marking, for example “GAS” or the appropriate certification mark. Always check the label and the accompanying documentation.

What is the correct length of a flexible gas hose?

Practical answer: the hose should be as short as possible but long enough to avoid pulling stress and to allow the appliance to be moved slightly for cleaning or maintenance. An unnecessarily long hose tends to twist, bend, and reduces its lifespan – and in extreme cases, excessive bending can cause mechanical damage to the braiding.

Common lengths of flexible gas hoses on the market are 75 cm, 100 cm, 150 cm, and 200 cm. For connecting a gas stove, a 100–150 cm hose is most often needed. Flexible gas hose for connection WS-CE R1/2(T)-G1/2 MF 150 cm is a typical solution for a standard gas stove – 150 cm provides enough reserve for normal appliance movement, while not being unnecessarily long. For greater distances or special situations, longer versions are also available – for example, Flexira xConnect Gas Standard R1/2(T)-G1/2 with a length of 2000 mm, which is suitable for atypical kitchen layouts or when the appliance is further away from the outlet.

The exact procedure for measuring and choosing the correct length can be found in a separate article What diameter and length of flexible gas hose – how to measure and choose correctly.

What does the marking R1/2(T) on the hose mean?

The marking R1/2 is a standard designation for an external (male) thread with a size of 1/2 inch according to the ISO 7/1 standard. The letter T in parentheses indicates that it is a tapered thread (from the English “tapered”). Such a thread is sealed with Teflon, hemp with sealing compound, or another suitable sealing material for gas – depending on whether it is a tapered (NPT/BSPT) or parallel (BSP) type.

G1/2, on the other hand, denotes a parallel (cylindrical) internal thread of 1/2 inch – type BSP. When combined with R1/2(T) on one end and G1/2 on the other end, you get a hose that connects to an external tapered thread on one side (typically an outlet from the wall or a gas meter valve) and to an internal parallel thread on the other side (typically the inlet to the appliance). This is exactly the combination that corresponds to a standard gas meter outlet and gas stove inlet in Slovak and Czech installations.

Can I use a flexible hose for propane-butane and natural gas?

It depends on the certification of the specific hose. Most commonly sold flexible gas hoses are certified for both media – natural gas (G20) and propane (G31), or butane and their mixtures. Always read the technical specifications or product description – it should explicitly state which types of gas the hose is suitable for. If you have propane-butane from a cylinder and the hose is certified only for natural gas, you should not use it – and vice versa.

How long does a flexible gas hose last and when should it be replaced?

The lifespan of a flexible gas hose is stated by the manufacturer – usually 5 to 10 years with normal use. In practice, it is reasonable to inspect the hose annually. Signs that it is time to replace the hose:

Visible corrosion, discoloration, or mechanical damage to the stainless steel braiding
Cracks or hardening of the inner rubber/plastic hose (visible at the ends)
The expiration date stamped on the hose has been exceeded
The hose has been bent below the minimum bending radius or has been compressed by furniture
A smell of gas near the hose or a positive result from a soapy water test

A detailed guide on how to recognize when it is time to replace the hose can be found in the article Replacement of a flexible gas hose – how to recognize when it is time to replace it.

Fittings, reducers and transition pieces for gas

Can I use standard water fittings for gas piping?

Absolutely not. This is one of the most dangerous substitutions made by the general public. Water fittings – even brass ones – may not be resistant to hydrocarbon media, do not have the correct surface treatment, and above all are not certified for gas. At first glance, a brass water fitting looks the same as a gas fitting, but their lifespan, sealing and resistance to gaseous media differ. Always use fittings with clear marking for gas use and the appropriate certification (e.g. EN 331 or manufacturer-declared conformity).

How to seal threaded joints for gas?

Sealing materials for gas threaded joints are three common options:

Hemp fiber (flax) with gas sealant – a traditional method, still valid and commonly used. Hemp is wound in the direction of the thread and impregnated with gas sealant (not standard masonry sealant or paint). It is a reliable and repairable method.
PTFE tape (Teflon) – a quick and clean method, but for gas, use a tape of at least 0.1 mm thickness and apply at least 3–4 layers. For larger diameters (3/4" and above), Teflon is not recommended as the only sealing material – it is better to combine it with hemp.
Anaerobic sealants (Loctite, Tangit) – modern adhesives for threaded joints, certified for gas. Very reliable, but the joint is permanent after curing and repairability is limited. They are mainly used in professional installations.

What you must not use: silicone (not suitable for gas, degrades and loses sealing), standard PVC-based tape or any other material not explicitly marked for gas supply.

What are transitions, bridges and elbows for gas?

As well as direct connections, you will also need shaped parts in the installation:

Elbow (angle fitting) – a 90° or 45° change in pipe direction. It must again be certified for pressure and medium for gas.
T-piece – a branch, allows supply to two appliances from one line.
Reducer – a transition from a larger to a smaller diameter (e.g. from 3/4" to 1/2"). Used when transitioning from the main line to the appliance connection.
Straight transition (nipple) – a flat transition piece between two different types of threads.
Nipple – a short piece of pipe with external thread on both ends, used to extend or bridge.

Gas pressure and installation sizing

Working pressure in a household gas installation is usually low – from 18 to 25 mbar for natural gas and from 25 to 37 mbar for propane-butane. You must choose all components so that their nominal pressure (PN – Pressure Nominal) significantly exceeds these values. Standard household components (ball valves, hoses) have PN from 5 bar to 16 bar – which is a large safety margin above working pressures.

For industrial or commercial applications (distribution networks, boiler rooms, production), pressures may be higher – in such cases, special components with the appropriate PN are required. In such cases, cooperation with a certified gas technician is always necessary.

Who is allowed to install gas installation components?

This is a question with a clear legal answer that must be respected. According to current Slovak legislation (Decree of the Ministry of Transport, Construction and Regional Development of the SR on safety and health protection at work) and gas company regulations, the following applies:

Any permanent installation, change or repair of a gas installation may be carried out only by a person with a valid authorization to perform work on gas equipment (a certified gas technician according to § 15 of the BOZP Act and the relevant decree).
A layperson may replace an appliance with a new one of the same type, provided all connections are preserved, and this must be done in accordance with strict rules including a tightness test after connection.
Replacement of a flexible hose is a borderline area – technically, it is a replacement of a connecting element, not a structural change. Nevertheless, we recommend that such a replacement be carried out by a professional, or at least checked by a professional after replacement.

In any case: after any manipulation with the gas installation, a tightness test must be performed. The procedure for this check is described in the article Tightness of gas connections – how to check for gas leaks yourself.

Installation, assembly and common mistakes

What tightening torque should be used for threaded joints?

Many people tighten gas threaded joints too much – in the belief that the tighter, the better. The opposite is true. Over-tightening can damage the thread, break the sealing, or deform the fitting or valve. Approximate tightening torque values for brass threads:

1/2": 20–30 Nm
3/4": 30–40 Nm
1": 50–65 Nm
1 1/4": 65–80 Nm
1 1/2": 80–100 Nm
2": 100–130 Nm

In practice, an experienced gas technician "feels" these values by hand – tightening the thread with a wrench until resistance is met and adding another 1–1.5 turns. For a layperson, it is better to use a torque wrench if possible. A more detailed installation procedure can be found in the article Installation of a gas ball valve – procedure, tools and safety rules.

Most common mistakes when installing gas installation components

Over the years of practice, we have observed the same mistakes repeatedly:

Use of non-qualified sealing material (silicone, water sealing materials)
Bending the flexible hose at a sharp angle or compressing it by furniture
Use of a hose, fitting or valve without certification for gas
Use of the wrong diameter – excessive reduction of flow reduces appliance performance
Forgetting to check tightness after installation
Installing a valve in an area without sufficient ventilation and without access for operation
Using extension fittings where a longer hose would be appropriate

For a complete overview of errors and their solutions, see the article Common mistakes when installing gas installation components and how to avoid them.

Where to buy gas installation components and what to watch out for

It is important to buy from a seller who can prove the certificates and origin of the goods. An e-shop or a physical store should be able to provide a Declaration of Conformity (DoC) for each gas installation component sold. For the Slovak market, EU standards apply – EN 331 for shut-off valves, EN 14800 and EN 15266 for hoses. Products without certification from unverified sources can be dangerous – and worse, damage caused by unqualified installation or non-approved materials may not be covered by property insurance.

We categorically do not recommend buying gas installation components from flea markets, from unknown Asian e-shops without EU certification, or as "surplus" goods of unknown origin. This material is literally a matter of life safety and health.

Frequently asked questions (FAQ)

Can I install a ball valve for gas myself, without a specialist?

The legal answer is no – any interventions into the fixed gas installation (including installation and replacement of shut-off valves) must be carried out by a qualified person. There is an exception for replacing a flexible hose while keeping the existing connections, but even in this case, the result must be checked for tightness. If you have any doubts, contact your local certified gas technician – it is a matter of safety.

How can I find out if my ball valve is suitable for gas, or only for water?

Look for the CE marking with the appropriate standard (EN 331 for gas valves) on the valve or its packaging, or the explicit mention of "GAS" in the technical description. If such marking is missing, the valve is likely not certified for gas and should not be used in a gas installation. Always request a Declaration of Conformity when purchasing.

Why does my flexible hose have a flat end on one side and not on the other – is this a mistake?

No, it is not a mistake – it is intentional. Most gas flexible hoses have a sealing flat (a flat metal or rubber gasket at the flange) on one end and a conical thread on the other end, which is sealed with thread compound. Never add hemp or Teflon to the sealing flat (flat face) – a clean contact of the surfaces is sufficient. The direction of installation matters: look at the hose instructions, which refer to the "M" end (male = external thread) and the "F" end (female = internal thread).

What should I do if I smell gas near the hose or valve?

Immediately close the main gas shut-off valve, do not switch on or off any electrical appliances or lights, do not use a mobile phone in the room, open the windows and leave the area. Call the emergency number of the gas distribution company (0850 111 727 for SPP distribution) or the emergency services. Never try to find the leak with a burning match or open flame. A soap test (soap water) is only allowed if you are sure it is not a major leak – and only with the main valve closed and the room ventilated.

Can I extend a flexible hose using a coupling?

The technical and safety answer is no. A flexible hose must always be a single piece – without an intermediate coupling. If the hose is too short, buy a longer one. An intermediate coupling is an additional potential leak point and is not an approved installation method. If the distance is really large, the solution is to extend the fixed pipe closer to the appliance and then connect the hose.

Why are some gas ball valves sold with a rotation limiter – what is it?

A rotation limiter (or position lock) is a mechanism that prevents the lever from being accidentally turned to a position other than the allowed one. In some applications (e.g., in common areas of apartment buildings where children or unauthorised persons may have access to the valve), valves with the option to lock the lever are installed. This feature increases the safety of the installation – the valve can be locked in the "closed" or "open" position using a seal or lock. For standard home use, this feature is not usually necessary.

Conclusion – why all this is worth knowing

The gas installation range – valves, hoses, fittings – may seem simple at first glance. But as you have seen, behind each seemingly trivial question lie important technical connections, safety requirements and legislative rules. Choosing and installing the right components is not only a matter of functionality – it is a matter of the safety of your household and family.

If you are unsure about the choice of a specific component, consult a specialist. If you need more information on individual topics, see related articles in the Knowledge Centre: How to choose the right ball valve for gas – thread, material and sealing class, Connecting an appliance via a flexible gas hose – step by step, or Ball valve vs. other types of gas shut-off valves – when to use which. These materials are written to give you all the information you need to make the right decision – whether you are buying for yourself or preparing an order for a specialist.

Do you have a question about this topic?

Having trouble deciding or dealing with a specific situation in your home? Write to us – we are happy to help.