Pressure and temperature – what to watch out for when choosing a flexible hose

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Pressure and temperature – what to pay attention to when choosing a flexible hose

When renovating heating systems or changing boiler, radiator, or pump connections, flexible hoses are often the last thing someone gives serious attention to. Usually, people reach for whatever is at hand or the cheapest option that "looks good". A few months – or even a few years – later, water may appear from the basement, or the hose may fall apart during a service intervention when unscrewing. In practice, these are far more common problems than one might expect.

This article focuses on the two most important technical parameters of flexible hoses – working pressure and working temperature – and explains what these numbers actually mean, why they cannot be ignored, and how to read them correctly when comparing products. If you are also interested in choosing by diameter, length, or material, see related topics in the Knowledge Center, such as What diameter and length of flexible hose suits me or Which materials of flexible hoses are suitable for potable and service water.

Why pressure and temperature are key – not just numbers in a catalog

Every flexible hose has two basic operational limits listed: maximum working pressure (in bars) and maximum working temperature (in °C). These values are not random – they result from the hose construction, the material of the inner lining, the braiding method, the wall thickness, and the quality of the seals at the fittings.

In everyday practice, I often see customers comparing two hoses by price and diameter, without noticing that one has a maximum pressure of 10 bar and the other only 6 bar. Or that one is certified up to 90 °C, while the other is only up to 70 °C. This difference can be decisive – and in a heating system with a condensing boiler or a direct connection to a floor heating circuit, it is not an academic question.

It is important to understand that catalog values are maximums under steady-state conditions, not values you can freely operate with. In other words: a hose labeled "10 bar / 90 °C" is not designed for continuous operation at the edge of both limits simultaneously. Manufacturers usually list the so-called PN (Pressure Nominal) at room temperature, and the allowable pressure decreases as the temperature rises. This is a technical detail most plumbers know, but customers are usually unaware of it.

What does "working pressure" mean and how to read it correctly

The working pressure of a flexible hose (in English: Maximum Working Pressure, MWP or abbreviation PN) indicates the maximum overpressure the hose can withstand long-term without deforming or allowing the medium to leak. It is expressed in bars, less commonly in MPa (1 MPa = 10 bar).

Flexible hoses on the market fall into several typical ranges:

Up to 6 bar – cheaper hoses intended for cold water supply lines, drain valves, and fill valves. Not suitable for heating.
Up to 10 bar – standard category for most installations in apartment buildings and single-family homes. Covers typical heating systems and plumbing.
Up to 16 bar – heavily braided hoses for industrial applications, boiler rooms, pump stations, and boiler connections with higher pressure. Also suitable for multi-story buildings with higher static pressure.
Above 16 bar – special industrial hoses, not usually needed in residential or apartment heating systems.

For orientation: in a typical apartment heating system, the working pressure is 1.5 to 2.5 bar. In a single-family home with a boiler and a closed expansion tank, it is similar. The static pressure of cold water from the municipal supply can be 3–6 bar. Pressure testing of installations is usually done at 1.5 times the working pressure, so at 3 bar operation, this is a pressure test at 4.5 bar. A hose with PN 6 bar would meet the conditions in this case, but has little margin. Therefore, experts recommend always reaching at least one category higher than the current need.

From the graph, it is clear why it is important to consider pressure and temperature together. A hose labeled PN10 (10 bar at 20 °C) may be able to withstand only 4 bar at 95 °C. If you install such a hose on the boiler outlet with a temperature of 85–90 °C at 2.5 bar, you are still within the limits – but if the pressure expansion circuit is not properly set and there is a sudden pressure increase, you may be at the edge.

Medium temperature: boiler outlet, return, floor heating circuit – each is different

The temperature at which the hose will operate depends on where exactly in the system you install it. This is the second big mistake – people treat the system as a whole and do not check where the actual highest temperature is.

In practice, we encounter these typical placements and corresponding temperatures:

Outlet (flow) from a gas condensing boiler: usually 55–75 °C, occasionally up to 80–85 °C at full load. Here, hoses certified at least up to 90 °C, ideally up to 110 °C, are needed.
Return: usually 45–60 °C. Requirements are lower, but it is still a heating medium.
Floor heating circuit: maximum supply temperature usually 40–45 °C. Temperature requirements for hoses are lower here, but pressure requirements are not.
Radiator connection: supply temperature 60–75 °C, depending on the boiler regulation. Hoses up to 90 °C are commonly used here.
Domestic hot water storage heater: maximum hot water outlet temperature 60 °C (legally limited to 55 °C for hygiene reasons during normal operation). For sanitary connections, hoses up to 70 °C are sufficient, ideally 90 °C for a reserve.
Cold water (supply, drain): maximum 20–25 °C under normal conditions. Temperature requirements are minimal, pressure is decisive.

Practical example: A plumber is replacing hoses on an old cast iron radiator in a panel apartment. The radiator is connected to a central heating system with supply temperatures up to 90 °C and a pressure of 4–5 bar during peak times. The plumber chose cheap hoses rated for 70 °C and PN6. Result: within a year, the hose started to "weep" at the connections – micro-leaks caused by repeated thermal stress exceeding the declared limit. The correct choice in this case would have been PN10 / 110 °C.

Material of the inner lining and its influence on temperature limits

Flexible hoses are not a monolithic product – they consist of multiple layers, each serving a different function. The material of the inner lining (corrugated tube or smooth lining) has a decisive influence on temperature and pressure limits.

In practice, we encounter the following basic materials:

Stainless steel (AISI 304, AISI 316): Corrugated hoses made of stainless steel are the most temperature-resistant type commonly available in heating. They can withstand 150 °C and more over the long term and are very robust in terms of pressure. Material AISI 316 is more suitable for aggressive media and environments with chlorides. These hoses are also resistant to sudden temperature shocks. For heating, they are the gold standard.
EPDM rubber (inner lining in braided hoses): Resistant to temperatures up to 120–130 °C, commonly rated for 90 °C at working pressure. Suitable for heating and DHW. Note: some cheaper hoses have linings made of NBR or PVC, which have lower temperature resistance.
PVC and composite materials: Usually limited to cold water or up to 60 °C. Identifiable by their flexibility and lower price. Not suitable for heating at normal boiler operating temperatures.
PTFE (teflon): Exceptional temperature and chemical resistance, but not used in standard heating – it is a specialized and expensive variant for laboratories and industry.

A more detailed overview of the properties of individual materials, including their suitability for potable water, can be found in the article Which materials of flexible hoses are suitable for potable and utility water in the Knowledge Center.

Influence of braiding on pressure resistance

In addition to the inner lining, the method and material of braiding significantly affect the maximum working pressure. This is an area where there are large differences between various products on the market, which may look similar at first glance.

Basic types of braiding:

Single stainless steel braid: Common in PN10 hoses for heating and water supply. The steel braid protects the inner lining mechanically and prevents radial expansion under pressure.
Double (dual) braid: Significantly higher pressure resistance – usually PN16 to PN25. Visibly thicker and heavier. Used for boiler rooms, industrial circuits, and multi-story apartment buildings.
Textile braid (polyester, nylon): A cheaper alternative with lower resistance. Not suitable for heating – rather for garden hoses and sanitary applications at low pressures.
No braid (smooth hoses): Corrugated or smooth stainless steel hoses may also be without a textile or braided surface. Their pressure resistance depends on the thickness and shape of the corrugation wall.

A comparison of braided and smooth (non-braided) hoses in terms of advantages and disadvantages is covered in a separate article Braided vs. smooth flexible hoses for heating – comparison.

Pressure surge and safety factor – what manufacturers don’t say out loud

In every water and heating distribution system, so-called pressure surges (water hammer, hydraulic shock) occur. These are short-term sudden increases in pressure that happen when valves are closed quickly, when a pump is started, or in the case of a failure in the expansion tank. These impulses can be several times higher than the normal operating pressure – in extreme cases even 3–5 times higher.

That is why quality flexible hoses have a defined safety factor – the ratio between the burst pressure and the maximum operating pressure. A typical safety factor is 3:1 to 4:1, which means that a hose with PN10 should have a burst pressure of at least 30–40 bar. Cheaper hoses of unknown origin may have a significantly lower safety factor.

In practice, this means: do not buy hoses where the manufacturer does not specify the burst pressure or burst pressure, or where the safety factor is lower than 3:1. These information should be available in the product technical data sheet.

Besides surges, it is also important to consider thermal expansion. Flexible hoses expand and stretch when heated. While steel corrugated hoses can cope with this thanks to their corrugated geometry, hoses with a stiffer insert can transfer forces to the connections and fittings. Therefore, flexible hoses should be installed with a certain bend radius, not stretched straight – more on this in the article Flexible hose installation for water step by step.

Certifications and standards – what to look for on the packaging or in the documentation

Flexible hoses for heating systems and water supply are subject to several European standards and certifications. Their presence is a sign that the manufacturer tested the product according to established procedures and not just stated values from a table.

Key standards and certifications for flexible hoses:

EN 13618 – European standard for flexible connecting hoses for heating systems. Includes requirements for pressure, temperature, durability and testing methods.
EN 15780 / ISO 9001 – general quality production standards; relevant for assessing the reliability of the manufacturer.
ACS (Attestation de Conformité Sanitaire) – French certification for contact with drinking water; recognized also on the Slovak market.
DVGW (Deutsche Vereinigung des Gas- und Wasserfaches) – German certification for gas and water supply. Very strict and recognized throughout the EU.
KIWA – Dutch certification for drinking water and installation components.
CE marking – basic requirement for placing on the EU market, but on its own it is not sufficient.

Note: CE marking does not replace specific product certifications. There are hoses with CE that have only formal documentation without serious testing. Always ask for the specific certificate relevant to the type of application – for heating it is EN 13618, for drinking water ACS or DVGW.

Temperature limits according to the installation location – practical overview

Based on practical experience, I have compiled a brief overview of recommended minimum hose parameters for different locations in the system:

Installation location	Media temperature	Recommended PN	Recommended max. hose temperature
Boiler outlet (flow)	75–90 °C	PN10	min. 110 °C
Boiler return	45–60 °C	PN10	min. 90 °C
Radiator connection	60–75 °C	PN10	min. 90 °C
Floor heating loop (after mixer)	35–45 °C	PN6–10	min. 70 °C
TÚV tank	55–60 °C	PN10	min. 90 °C
Cold water (water supply)	up to 25 °C	PN10	min. 60 °C
Central heating supply (CZT)	80–110 °C	PN16	min. 120 °C

How hoses behave at the limits of parameters – what actually threatens

Failure of flexible hoses is also discussed in the article Common leaks and failures of flexible hoses – causes and solutions, but here we will look specifically at the damage mechanisms caused by exceeding temperature and pressure limits.

Thermal overload manifests in several ways depending on the material. A rubber insert (EPDM or NBR) when exposed to temperatures above the declared limit for a long time becomes hard, cracks and loses elasticity. At first glance, nothing is visible – the hose appears undamaged. Problems appear when unscrewing: the sealing becomes disintegrated, or the hose loosens during a pressure test of a new loop. Corrugated hoses made of stainless steel are more resistant to thermal overload, but even here long-term operation above the limit accelerates material fatigue in the corrugation folds.

Pressure overload without a thermal component usually manifests more quickly – at sufficient pressure, the hose bulges at the weakest point (usually at the connection to the fitting), or directly bursts. With gradual overload (e.g., slightly exceeding pressure over a long period), fatigue of the braid occurs – individual wires begin to break, which can be recognized by visual inspection.

Combination of heat and pressure is the most risky scenario. Here, both degradation mechanisms accumulate, with each reducing resistance to the other. A hose that easily handles pressure at room temperature may operate at the edge of its capacity at 85 °C. That is why it is essential to consider both parameters simultaneously in every dimensioning process.

Aging and lifespan: why even a suitable hose eventually reaches the end

Flexible hoses are not eternal. Even if properly dimensioned and installed, materials age due to cyclic thermal stress, pressure shocks, water quality, and chemical content. Manufacturers usually specify a lifespan of 10–15 years under operating conditions. In practice, I have seen hoses functioning after 20 years – and also in a poor condition after just 5 years. The difference was always whether the operating conditions were within the norm.

Risk factors shortening the lifespan:

Frequent temperature cycling (boiler turned on and off multiple times a day at full temperature)
Aggressive water with high chlorine or acid content (corrosion of internal surfaces)
Mechanical stress – the hose is stretched or bent sharply
Improperly tightened connections – uneven loading of the sealing
Closeness to heat sources (the hose runs close to a hot pipe without insulation)

More detailed recommendations for extending the lifespan can be found in the article How to extend the lifespan of flexible hoses in heating and plumbing systems.

Most common mistakes in selection from practice – specific scenarios

Over the years of practice, the same mistakes repeat. Here are five of the most common ones, with an explanation of why they are problematic:

1. "I bought a hose for water, it will be sufficient."
Hoses for cold water (sanitary) are dimensioned for lower temperatures. If installed at the boiler or on a radiator inlet, they may have an insufficient temperature limit. Result: degradation of the inner lining, leaks at the connections.

2. "I have PN10, that is more than enough."
PN10 applies at 20 °C. At 80 °C, the allowable pressure may be only 5–6 bar. If the system is operating at the edge, you have no reserve for pressure shocks or service pressure tests.

3. "I took the longest hose, it can be bent."
A hose that is too long can be bent without control, sometimes with sharp radii, causing local mechanical stress on the corrugation or lining. Related info: What diameter and length of flexible hose is suitable for me.

4. "The hose looks fine, it doesn't bulge, nothing is wrong."
Thermal damage to the inner lining is not visible from the outside. The hose may look undamaged but have internal micro-cracks. Leaks appear later – typically when pressure or temperature increases.

5. "Certifications are not necessary, it's just a hose."
Certification is not just paper. It ensures that the product was tested under defined conditions and the results match the declared values. With low-quality hoses without certification, you have no certainty that the pressure and temperature values on the label are valid at all.

Proper dimensioning – step-by-step procedure

If you are unsure which hose to choose, the procedure is quite simple:

Find out exactly where in the system the hose will be placed – boiler outlet, return, radiator, floor, water supply.
Find out the maximum temperature of the medium at that location – from the technical documentation of the boiler or from experience with similar systems.
Find out the maximum working pressure of the system – from the setting of the safety valve or from the pressure gauge.
Add a safety margin of at least 20–30 °C to the maximum temperature. Add a margin of at least 50 % to the pressure (for pressure shocks).
Select a hose that covers both – and check the certification for the given medium.

Example: Output of a gas boiler, max. temperature 85 °C, max. system pressure 2.5 bar (pressure relief valve set to 3 bar). With a safety margin, you are looking for a hose for min. 110 °C and PN6 (after applying the safety margin). In practice, you reach for PN10 / 110 °C – a standard braided hose for heating.

Flexible hoses in the context of a complete renovation

If you are renovating the entire heating system, flexible hoses are part of a more complex decision. In such a case, I recommend reading Flexible hoses during heating renovation – what to know in advance first, where there is more context about planning the entire circuit and choosing individual components including hoses.

Important practical point: always buy flexible hoses in the context of the entire system, not in isolation. If you are replacing the boiler with a condensing one with lower return temperatures, the requirements for the hoses may change compared to the previous state. If you are adding a TÜV tank or a floor heating circuit, new locations with different parameters arise. Always check whether the original hoses meet the new conditions – and if not, replace them preventively.

Common questions (FAQ)

Can I use a cold water hose (PN6 / 60 °C) to connect a radiator?

No, that is unsuitable. The temperature of the supply water to the radiator usually reaches 60–75 °C and at certification up to 60 °C, the hose is operating at the absolute limit or beyond it. The risk is degradation of the inner lining and leakage. Always choose a hose certified for at least 90 °C for radiators.

Is a hose marked PN10 safe for a standard city water supply with pressure 4–5 bar?

Yes, PN10 at cold water temperature (20–25 °C) has sufficient margin for a city water supply. A safety factor of 3:1 means a burst pressure of around 30 bar. For plumbing systems, PN10 is standard and easily covers even higher static pressure in lower floors of apartment buildings.

What does it mean if individual wires of the braid on the hose crack?

Cracked braid wires are a serious sign – the hose must be replaced immediately. The braid is the primary pressure reinforcement. Each cracked wire reduces pressure resistance and transfers higher load to the rest. With cracked wires, the question is not whether the hose will hold, but when it will burst – and that could happen during the next pressure surge or during a pressure test.

What is the difference between PN and MOP (Maximum Operating Pressure)?

PN (Pressure Nominal) is a nominal value – a standardized resistance level at a reference temperature (usually 20 °C). MOP is the actual maximum operating value declared for a specific product under specific conditions. In practice, they are often identical or close, but for temperature-sensitive products, MOP at higher temperatures can be lower than PN. It is always important to read the technical specifications and not just the label.

Do I have to replace flexible hoses when replacing the boiler?

It depends on the age and condition of the existing hoses and the change in operating parameters. If the hoses are more than 10 years old, I recommend replacing them preventively. If the new boiler changes the temperatures or pressures in the system (e.g., a condensing boiler vs. an old boiler), check whether the original hoses meet the new conditions. During every boiler replacement, it is appropriate to at least visually and manually check the condition of all hoses in the circuit.

Do the temperature limits of the hoses apply to flow in both directions (supply and return)?

Yes, the limits apply to the medium flowing through the hose, regardless of the direction. However, in practice, the return has a lower temperature than the supply, so the temperature requirements for the return are milder. If you are unsure about the temperatures, install the same type of hose on both circuits – it simplifies maintenance and increases the safety margin.

Conclusion

Pressure and temperature are parameters that cannot be underestimated or simplified to "the higher the number, the better" when choosing flexible hoses. It is crucial to understand exactly where in the system the hose is working and what real conditions it is exposed to.

Do you have a question about this topic?

Not sure or dealing with a specific situation in your household? Write to us – we are happy to help.