Flexible pipes during heating renovation – what you need to know in advance

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Flexible hoses in heating system reconstruction – what you need to know in advance

Every reconstruction of a heating system – whether it's a boiler replacement, installation of a new manifold, or complete disconnection and reconnection of radiators – brings with it a whole range of small, seemingly trivial decisions. One of them is the selection of flexible hoses. An experienced plumber usually handles this routinely, but a property owner who is doing part of the work themselves or at least wants to understand what the tradesperson is doing can easily get lost in a flood of terms: G 1/2, liquid pressure class PN 16, stainless steel braiding, EPDM sealing, length 30 cm vs. 50 cm... And that's before we even start talking about the difference between hoses for potable and utility media.

This article will give you a complete overview of what you need to know before you start the reconstruction – so that you don't end up buying hoses twice, don't have to deal with leaks after the first pressure test, and aren't surprised by facts that experienced plumbers consider obvious.

Why flexible hoses and not rigid pipes?

The first question many people ask when reconstructing a heating system is: why flexible hoses at all? After all, rigid copper or steel pipes can last for decades. That's true, but flexible hoses have irreplaceable advantages at certain key points in the system:

Compensation for thermal expansion – the boiler, pump, and TÜV tank expand during operation. A rigid pipe connection to these devices will transfer mechanical stress directly into the body of the appliance or into the thread of the valve. A flexible hose will absorb this stress.
Vibration damping – the circulation pump produces vibrations that spread through rigid piping into the entire system. A flexible hose will significantly dampen this frequency.
Easier installation – in limited space (a boiler room in a hallway, a cabinet under the stairs), adjusting the position of the hose in several axes is much easier than bending copper pipe.
Quick replacement – if the system needs to be opened again (boiler replacement, pump servicing), you can unscrew and reattach flexible hoses in a fraction of the time it would take to cut and solder rigid piping.
Cost reduction for smaller jobs – when connecting a radiator or valve with a flexible hose, you avoid the investment in compression fittings, special tools, or a soldering kit.

Of course, flexible hoses are not suitable for long pipe runs. They serve as end / connecting elements between valves and equipment, or between two valves in close proximity. Long runs in walls, floors, and ceilings are handled with rigid pipes.

Where exactly are flexible hoses used in heating system reconstruction?

When going through a typical heating system reconstruction project, you'll encounter flexible hoses almost everywhere where rigid piping meets a device or valve. Specifically, it looks like this:

1. Boiler and its connection

Every gas, oil, or heat pump boiler has an outlet and an inlet – usually G 3/4 or G 1 external or internal thread. Flexible hoses form a flexible connection between the boiler ports and the first valves (ball valves, check valves, expansion tank). The length depends on the distance, but typically ranges between 30 and 50 cm. It's crucial that the hoses can withstand the boiler's operating temperature – at the output of condensing boilers, you typically have 65–80 °C, less in low-temperature systems, but it's safest to have hoses certified for at least 90 °C under the system's operating pressure.

2. Pump group, distributor-collector

Pump groups are connected to distributors or the main piping either directly with fittings or via short flexible hoses (20–30 cm). Vibration from the pump is the biggest argument for a flexible connection here. In a reconstruction where the distributor remains in its original location but a new pump with different dimensional parameters is added, a flexible hose is practically a lifesaver – it eliminates the need to cut and re-solder rigid piping.

3. Expansion tank

The expansion tank is connected to the system with a hose that must be able to withstand the pressure of the entire system (typically 1.5–3 bar operating pressure, up to 6 bar during testing). If the tank is mounted on the wall and not directly welded to the piping, a flexible hose is the standard solution.

4. Radiators

When replacing radiators, it's very common for the new radiator to have connections at slightly different positions than the old one. A flexible hose can compensate for this difference (2–5 cm) without any need to move the piping. For radiators, G 1/2 hoses with a length of 30–50 cm, in either right-angle or straight orientation, are typically used.

5. TÜV tank and solar system

The hot water storage tank (boiler) is interesting in that it is touched by two different circuits – the heating circuit (secondary side) and the potable water circuit (primary side from the consumer). For each of these sides, you must use a different type of hose: on the potable water side, a hose with certification for potable medium; on the heating side, a hose for utility medium (this doesn't mean they are worse – just differently certified).

Parameters of flex hoses that matter when renovating heating systems

Selecting a flex hose is not just about "hitting the thread". For heating systems, you have specific requirements regarding material, pressure, temperature, and length. Let's look at them systematically.

Thread size and type

In the Slovak and Central European installation practice, pipe threads according to the ISO 228 standard (G-threads) and British BSPT threads dominate. Flex hoses for heating most commonly have:

G 1/2 (DN 15) – radiators, small valves, measuring instruments
G 3/4 (DN 20) – boilers up to approx. 25 kW, smaller pump units
G 1 (DN 25) – boilers 25–50 kW, TÜV storage tanks, distributors
G 1 1/4 and G 1 1/2 – larger boilers, heat pumps, industrial equipment

Important: distinguish between external (M – male) and internal (F – female) threads on both ends of the hose. A typical hose has an external thread on one end and an internal thread on the other, or both internal. Always verify what thread the valve and boiler have before purchasing – otherwise you will end up with a hose that lacks a transition.

Hose length

This is one of the most common sources of error when ordering. The length of a flex hose is given as total length including fittings (sometimes only the braided length – always check what the manufacturer means). Practical rules:

Do not measure just from center to center. Add the length of the threaded connections on both sides (typically 2–3 cm on each side).
Leave a reserve of 3–5 cm for boilers – a hose in a fully straightened position is more stressed than one that is slightly bent.
A hose that is too long cannot be easily "bent" into the wall without the risk of damaging the braid – choose a length to measure rather than significantly longer.
Common lengths on the market: 20, 30, 40, 50, 60, 80, 100, 150 cm – some manufacturers offer atypical lengths as well.

Pressure and temperature

For single-family homes with a gas condensing boiler, the following is standard: operating pressure 1.5–2.5 bar, maximum temperature at the outlet 80 °C. Hoses must be certified with sufficient reserve – at least PN 10 (10 bar) for regular heating. More on this topic can be found in the article Pressure and temperature – what to pay attention to when choosing a flex hose in our Knowledge Center.

Material of the inner hose and braid

For heating, there are three dominant combinations:

EPDM inner hose + stainless steel braid (AISI 304) – the gold standard for heating systems. EPDM withstands temperatures up to 150 °C, is resistant to acids and alkalis, and does not release oxygen (which is critically important for heating – see below).
PTFE (teflon) inner hose + steel braid – extreme chemical resistance, low friction, suitable for aggressive media and high temperatures. The price is higher, and flexibility is slightly worse than EPDM.
EPDM with corrugated stainless steel tube (annular corrugation) – so-called "corrugated stainless steel hoses", very durable, long service life, suitable as a replacement for rigid piping on shorter runs.

A more detailed comparison of materials can be found in the article Braided vs. smooth flex hoses for heating – comparison.

Oxygen diffusion – a problem most people don't know about

This is a topic that is often neglected when selecting flex hoses for heating, even by plumbers. Heating systems with steel pipes, cast iron valves, or steel radiators are extremely sensitive to the presence of oxygen in the water. Oxygen causes corrosion of metal components – it quickly damages the circulation pump, the heat exchanger of the boiler, and radiators.

Common rubber materials (e.g., NBR or cheap PVC) allow oxygen to diffuse through – oxygen penetrates through the hose wall from the outside air into the medium. EPDM has significantly lower oxygen diffusion, but for open heating systems or systems with long rubber sections, even EPDM may be insufficient. In such cases, hoses with a barrier layer (EVOH or aluminum foil) or full-metal stainless steel hoses are recommended. Warning: some manufacturers mention diffusion protection only for pipes, not for hoses – always read the product's technical specifications.

For single-family homes with a modern condensing boiler and a closed system, an EPDM hose is practically always sufficient. The problem arises in older open systems or when mixing materials (aluminum + steel + copper) without an inhibitor.

Sealing of flexible hoses – a detail that determines leakage

Most leaks in flexible hoses do not start with the hose bursting, but with leakage at the threaded connection. And that despite the fact that the hose is of high quality. Why? Improper or low-quality sealing.

With flexible hoses, we encounter two main types of sealing systems:

Flat rubber washer (gasket) – the most common type, inserted into the hose nut. It seals by contact with the flat seating surface of the fitting. Advantage: simple replacement, easy installation. Disadvantage: if the seating surface of the fitting is damaged (scratches, dirt), the seal may not be perfect even after tightening.
Conical seat (60°) – the hose seals against the conical seat of the fitting. A more robust system, less sensitive to surface defects of the seating surface.

Important rule for installation: never add Teflon tape or sealant to the thread where a flat washer is used. The rubber washer is the primary sealing element – the thread is used only to press it. Teflon may also cause you to tighten the nut more than necessary and crack the brittle brass fitting. For more detailed installation instructions, see the article Installation of flexible water hose step by step.

Specifics in renovation vs. new construction

Renovating heating systems brings challenges that simply do not exist in new construction. Here is an overview of situations we most commonly encounter:

Old threads and corrosion

In older houses (before 1990), it was common to use steel threaded piping. After 30–40 years of operation, the external threads on the piping may be damaged by corrosion, scratches, or simply "eaten away" – that is, covered with a layer of oxide that changes the actual thread range. Before installing flexible hoses, always try to clean the old thread with a brass brush and check whether the hose nut offers resistance evenly or only at certain points (which indicates thread damage). If necessary, have the thread re-tapped or use an adapter.

Different thread standards in old equipment

Old boilers and fittings from abroad (German, Austrian, Czech products) may have threads according to DIN or ČSN standards, which slightly differ from the current ISO 228. In practice, this usually does not cause problems – the threads are compatible for most connections, but in non-standard situations, verify compatibility before embedding the hose into the wall.

Limited space and awkward angles

Renovation means that the piping and equipment are located where they were historically placed – not always in an ideal location. Flexible hoses can be bent, but be careful: the minimum bending radius is specified by the manufacturer and typically ranges between 5 and 15 cm (depending on the diameter and braiding structure). If you exceed it, the braiding may break and you will lose the reinforcing element of the hose – the hose then becomes only a rubber tube under pressure.

Combination of old and new materials

Galvanic corrosion is a real issue when mixing materials – copper, steel, galvanized steel, aluminum. In this context, flexible hoses can also serve as an electrical barrier – the EPDM inside and plastic or brass fittings break the galvanic circuit between two metal pipes made of different materials. This is one of the arguments for using flexible hoses even where a rigid fitting would technically suffice.

Plan with future service in mind

One of the most valuable pieces of advice a skilled plumber can give you: design flexible hoses so that they can be replaced even after 15 years. What does this mean in practice?

Never embed a flexible hose into a wall or floor. Flexible hoses are service elements – they must always be accessible. If the route requires passing through a wall, place the flexible hose in a protective sleeve or use rigid piping through the wall and flexible hoses only on both sides of the passage.
Leave enough space for working with a wrench. A G 3/4 hose nut requires a spanner or a 27 mm box wrench. If the hose is pressed against the wall by 2 cm, you will have to break the wall open during replacement.
Label the hoses or document them. At a TÜV tank where both heating and potable water hoses are present, the mystery of "what is what" is a time bomb for the next service technician.
Plan for replacement every 10–15 years. Flexible hoses are consumable materials with a limited lifespan. EPDM ages, rubber cracks – especially in areas with higher temperatures (above 70 °C). Tips for extending the lifespan can be found in the article How to extend the lifespan of flexible hoses in heating and plumbing systems.

Flexible hoses and standards – what to look for on the packaging

A lot of cheap products without valid certification circulate on the Slovak and Czech markets. For flexible hoses used in heating, look for:

EN ISO 6945 or the appropriate standard for braided hoses
KTW / DVGW – for hoses used with potable water (German certification, recognized throughout the EU)
WRAS – British certification for potable water, also accepted
CE marking – mandatory for pressure equipment above certain limits according to PED directive 2014/68/EU
Stated operating parameters – pressure (bar or MPa), temperature (°C), medium – must be directly on the hose or on a label, not just in the catalog

If you don't have these data on the packaging or directly on the hose, it's better to give up on the purchase. A cheap, non-certified hose can cost you much more – one water leak in the boiler room, where no one is around on the weekend, can cause damage in the thousands of euros.

Typical mistakes during reconstructions – examples from practice

Over the years of contract practice, the same mistakes keep repeating themselves. Here are the most common ones:

Mistake No. 1: Hose too short

The customer measured the distance from center to center (e.g., 40 cm) and ordered a 40 cm hose. After installation, they find out that the hose is stretched like a string. Due to thermal expansion of the boiler or pump vibrations, this will eventually cause a leak at the threaded connection. The correct procedure: always add 5–10 cm of reserve to the measured distance and consider the length of the threads themselves.

Mistake No. 2: Using a potable water hose in heating and vice versa

Hoses certified for potable water are often made from materials that do not have a barrier against oxygen diffusion – this does not matter in heating due to certification, but the material may not be optimized for long-term exposure at 80 °C. Customer story: the customer bought hygienic hoses (cheaper, on sale) and used them on the boiler. After two years, the rubber cracked at the bend – the material simply could not withstand the temperature. Always use hoses intended for the specific medium and parameters.

Mistake No. 3: Tightening without a sealing element

A flat rubber washer fell out or was missing (e.g., during reinstallation after service). The thread was tightened "hard", without a sealing element. Result: a micro-leak that will only become apparent under pressure. Always check the presence of a sealing element before installation.

Mistake No. 4: Stainless steel hose in an aggressive atmosphere

AISI 304 stainless steel braiding is not resistant to all environments. In boiler rooms near the sea (salt atmosphere) or in the presence of chlorides (brine, disinfectants), corrosion cracking of the braiding may occur. In such cases, choose AISI 316L or hoses with a rubber outer jacket.

How to correctly determine how many and which hoses you need for a reconstruction

Before you start ordering, do a simple node audit. Go through the entire system and for each node where you plan to use a flexible hose, note down:

Type of thread on the device (G 1/2, G 3/4, G 1...) and whether it is external or internal
Type of thread on the valve/piping (same procedure for identification)
Center-to-center distance + reserve of 5–10 cm
Working medium and parameters (temperature, pressure – usually from the boiler's technical sheet)
Whether it is a potable water or heating circuit

Based on this list, you can precisely determine which hoses you need. If you are unsure about any parameter, contact us – our technical team of flexible water hoses can advise you based on photos from the installation.

A wide selection of flexible hoses for heating systems can be found directly in the category flexible water hoses on atria.sk, where the hoses are categorized by diameter, length, and material, so filtering according to your parameters is simple.

Most frequently asked questions (FAQ)

Can I use the same flexible hoses for heating reconstruction as for sanitation (water taps)?

In most cases, no – sanitary flexible hoses (for cold/warm water up to 60 °C, pressure up to 10 bar) are certified for potable medium and typically do not have optimized thermal resistance for long-term operation at 70–80 °C in heating. In addition, they do not have a barrier layer against oxygen diffusion. Always choose hoses specifically intended for heating systems, ideally with EPDM inside and certification for operating temperatures of at least 90 °C.

What is the lifespan of flexible hoses in heating and when should they be replaced preventively?

Quality EPDM hoses with stainless steel braiding should last 15–20 years without problems when installed and operated within technical parameters. In practice, we recommend a preventive inspection every 5 years (visual inspection of the braiding, threads, signs of moisture) and replacement after 10–15 years or at the first sign of visible damage. Hoses near the boiler (higher temperature) age faster – a 10-year interval is recommended there.

Is it necessary to drain the system before replacing flexible hoses during a heating reconstruction?

Yes, always. Before replacing the flexible hose, close the ball valves on both sides (which is why there are ball valves on each device), drain the system using a bleed valve or filling/drain valve, and only then loosen the hose nuts. Even with a "closed" ball valve, there may still be residual water under pressure in the hose – loosening without draining can cause hot water to spray out under pressure. This is particularly dangerous at temperatures above 60 °C.

Can I extend a flexible hose by connecting two shorter hoses?

Technically yes, but it is not recommended. Each connection is a potential leak point. In addition, if you connect two hoses using an intermediate piece (e.g., a straight pipe or extension), the whole loses its flexibility and compensating function. If you need a non-standard length, order a custom-length hose or use a longer hose and slightly bend it – this way you do not gain an additional risky connection. Choosing the correct lengths is discussed in the article What diameter and length of flexible hose suits me.

What wrench should I use to install a flexible hose to avoid damaging the braiding?

Use a French adjustable wrench or a properly sized box wrench for the threaded part of the flexible hose (G 1/2 → 22–24 mm, G 3/4 → 27–30 mm, G 1 → 33–36 mm). Never use pipe pliers (so-called "pigeon beaks") directly on the braiding – the pliers' teeth will damage the braiding and may cause hidden corrosion at the damaged site. Always apply force to the solid valve part, not the braiding itself.

What if the old boiler has a different thread than the new flexible hose?

Use an adapter – a shortened brass piece with threads on both sides in different sizes (e.g., G 3/4 to G 1/2). Adapters are commonly available and inexpensive. Never use brute force – if the thread does not easily fit, look for compatibility issues, not a higher torque. Damaged threads on boilers are expensive to repair.

Conclusion: planning is the foundation of a successful reconstruction

Flexible hoses may look like a trivial detail in a heating reconstruction. But it's precisely these details that determine whether the system will work without problems for the next 15 years, or whether an installer will call you after the first pressure test because three connections are leaking. The right choice starts at home – measure, verify the threads, determine the system parameters, and check the certifications. Don't save on material quality where high temperatures are present or where service access is limited.

If you are planning a larger reconstruction and are unsure which hoses to use for specific nodes, read other articles in our Knowledge Center – for example, How to choose a flexible water hose for a heating system, Common leaks and failures of flexible hoses – causes and solutions, or Which materials of flexible hoses are suitable for potable and utility water. The complete range of flexible water hoses can be found on atria.sk, where you can filter by diameter, length, material, and pressure class.

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're happy to help.