Expansion tank for TÚV systems – what it must meet and how to choose it

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Expansion tank for TWW systems – what it must meet and how to choose it

An expansion tank is an essential component of every closed hydraulic system, protecting the entire circuit from dangerous pressure increases. While its presence is almost a given in heating systems and every plumber automatically includes it in the design, it is surprisingly often overlooked in hot water supply (TWW) systems. Yet, in this case, a missing or improperly selected expansion tank can cause serious problems – from continuous dripping of the safety valve to bursting of the storage heater or damage to the plumbing network in the house.

This article focuses exclusively on expansion tanks for TWW systems – not for heating. Although they may look almost identical from the outside, they are technically different devices with different requirements for material, certification and sizing. If you are looking for information specifically for heating circuits, I recommend also reading the article How to choose an expansion tank for heating – volume, pressure and system type, where this issue is treated separately.

Why we need an expansion tank in a TWW system at all

Water is an almost incompressible liquid. When heated, it increases its volume – quite significantly. Cold water at a temperature of 10 °C has a density of approximately 999.7 kg/m³, at 60 °C (typical temperature of a TWW tank) the density drops to about 983.2 kg/m³. This means that one liter of water expands by about 1.7% when heated from 10 to 60 °C. At first glance, a small number, but in a closed system without the possibility of expansion, it causes a dramatic increase in pressure.

In practice, it works like this: the water supply bringing cold water into the house has a pressure in the range of 2.5 to 6 bar. Behind the pressure-reducing valve (if installed), the pressure is usually reduced to 3–4 bar. When the water in the tank is heated, its volume increases, but since the check valve (or the pressure-reducing valve with an integrated check valve) prevents the return of water to the public network, the expanded water has nowhere to go. The pressure rises, the safety valve opens, drips, which is not only annoying, but also costly (unnecessary water consumption) and in the long run damages the valve itself.

The solution is an expansion tank, which absorbs this "excess" amount of water, balances pressure peaks and protects the system from overloading the safety valve.

Fundamental difference: expansion tank for TWW vs. for heating

This is where the most common mistake occurs, which I encounter with customers and in DIY installations. Expansion tanks for heating and expansion tanks for TWW look almost identical – both are red (or blue), diaphragm, pressurized. But they cannot be interchanged, and this is due to one fundamental reason: contact with potable water.

An expansion tank for heating has a membrane made of standard EPDM rubber, which is not certified for contact with potable water. It contains softeners and other chemicals that can migrate into the drinking water and cause contamination. Therefore, every expansion tank intended for TWW systems must have:

A membrane made of a material certified for contact with potable water – most often a special EPDM with WRAS or KTW certification, or butyl rubber or SBR with the appropriate certification.
Interior surface made of stainless steel or food-grade epoxy – standard heating tanks have an interior made of galvanized or black steel, which is unsuitable for potable water.
Certification ACS, WRAS or NSF (depending on the country of sale) confirming hygienic safety.

If you are unsure, always check the label or the technical data sheet of the product – it must explicitly state "suitable for potable water" or "for TWW systems". For example, Expansion tank IBO 24l for heating and TWW systems clearly indicates in the name and description for which type of application it is suitable.

Technical parameters to consider when choosing

Maximum working pressure

Expansion vessels for DHW are usually supplied with a maximum working pressure of 10 bar. This is sufficient for the vast majority of apartment and house installations, where pressure-reducing valves maintain pressure within the range of 3–5 bar and the safety valve of the storage tank is typically set to 6 or 8 bar. In industrial applications or high-rise buildings, a higher pressure may be required – this is, however, the responsibility of the HVAC designer.

Pre-charge pressure in the vessel

The expansion vessel is filled with air or nitrogen in the pressure chamber behind the diaphragm. This pre-charge pressure must be set correctly – ideally to the static pressure at the installation location. For domestic DHW systems, this usually means a pre-charge pressure of 3.0 to 4.0 bar. How to set and check it is described in detail in the article Correct pre-charge pressure of an expansion vessel – how to set and check.

Attention – vessels are shipped with a factory-set pre-charge pressure, usually at 1.5 bar or 4 bar. Before installation, it is always necessary to verify that the pre-charge pressure corresponds to the specific conditions of your installation and, if necessary, adjust it using a standard valve for inflating (Schraeder valve).

Expansion vessel volume

This is the most important parameter. A too small vessel will not absorb the entire volume of expanding water and the safety valve will continue to drip. A too large vessel is not technically problematic, but it is economically inefficient. The exact calculation depends on several factors:

Volume of the DHW tank (in liters)
Maximum temperature of heated water (typically 55–65 °C)
Temperature of cold water (10–15 °C)
Maximum system pressure
Pre-charge pressure in the expansion vessel

There is a simplified formula commonly used in practice for preliminary sizing (i.e., with a safety margin):

V_EN = (V_tank × e) / ((P_max – P₀) / P_max)

where e is the coefficient of thermal expansion of water for the given temperature range (when heating from 10 to 60 °C, it is approximately 0.017), P_max is the maximum system pressure, and P₀ is the pre-charge pressure of the vessel (both in absolute bars).

Practical example: The tank has a volume of 120 liters, heated from 10 to 60 °C (e = 0.017), the maximum system pressure is 6 bar abs., and the pre-charge pressure of the vessel is 3.5 bar abs.

V_EN = (120 × 0.017) / ((6 – 3.5) / 6) = 2.04 / 0.417 = 4.9 liters

For a 120-liter tank, the calculation gives a vessel of about 5 liters, but in practice, most plumbers choose a vessel of a larger size category – both for a safety margin and because a larger vessel reduces the frequency of pressure cycles and extends the life of the diaphragm. For a 120-liter tank, the suitable choice would be Expansion vessel IBO 12l – it provides double the reserve and the diaphragm will not operate at the limit of capacity.

Reference table: tank volume vs. recommended expansion vessel

DHW tank volume	Minimum EN volume (calculation)	Recommended EN (with reserve)	Suitable product
30–50 l	1.5–2.5 l	8 l	IBO 8l
60–100 l	3–5 l	12 l	IBO 12l
100–150 l	5–7.5 l	18–19 l	IBO 19l
150–250 l	7.5–12 l	24 l	IBO 24l
over 250 l	over 12 l	36 l and more	IBO 36l

Note: The table is valid for typical conditions – heating temperature 60 °C, cold water temperature 10–15 °C, max. system pressure 6 bar, supply pressure EN 3.5 bar. In case of different parameters, an individual calculation is required.

Where and how to connect an expansion vessel for TÚV

Correct placement of an expansion vessel for TÚV is significantly simpler than in heating systems, but there are still rules that must be followed.

Placement in the system

The expansion vessel for TÚV is connected to the cold branch of the distribution line, before the storage heater. The logic is simple: the water entering the tank is cold and pressurized – it is precisely here that we need to capture the expansion caused by heating. The connection is usually implemented using a branch with a shut-off valve, which allows the vessel to be disconnected from the system for maintenance without the need to drain the entire circuit.

Several important rules:

The vessel must be connected to the cold side of the tank, not the hot side.
There must be no shut-off valve between the vessel and the system that is normally closed – if it is forgotten to open, the vessel does not perform its function.
The vessel must not be connected after the safety valve or between the safety valve and the tank.
Installation direction: the vessel is usually mounted with the connecting neck pointing downward (vertical position), but most standard membrane vessels can also be installed horizontally – refer to the technical data sheet.
The vessel must be mechanically supported – a hanger on the pipe without a bracket is not a suitable solution, as the weight of the filled vessel can be several kilograms.

Relationship with mandatory accessories

The expansion vessel for TÚV always works in tandem with the pressure relief valve of the tank. The safety valve opens when the pressure exceeds the safety limit – usually 6 or 8 bar. The expansion vessel ensures that these limits are not reached during normal heating. Therefore, it is important that the pre-charge pressure of the expansion vessel is set lower than the opening pressure of the safety valve (with a sufficient margin of at least 1–2 bar).

Installation specifics – what is often forgotten

Incorrect installation without a check valve

In older houses without a pressure-reducing valve and a check valve on the cold water inlet, the expanded water freely returns back into the public water supply, so no pressure increase occurs and the safety valve does not drip. However, the moment someone installs a pressure-reducing valve in the system (for example, during a renovation), the situation changes – suddenly an open system becomes closed and without an expansion vessel, the safety valve starts dripping. This is one of the most common reasons why customers come in saying that the safety valve "always worked normally, but after the renovation it started". The solution is simple: install an expansion vessel.

Tanks with an integrated anode and contact with salt

In houses with water softeners or inhibitor dosing systems, it is necessary to verify whether the membrane of the expansion vessel is resistant to the given chemical environment. Most standard EPDM membranes certified for potable water can withstand common salt, but with some special additives, it is worth checking.

Stagnant water in the vessel and hygiene risks

Inside the TÚV expansion vessel, water stagnates – this is the part that is "separated" from the main circuit and does not flow directly through the valves. In an improperly sized vessel (too large), the large volume of this water can remain motionless for a long time, which under certain conditions (temperature 25–50 °C) creates a favorable environment for the growth of Legionella bacteria. Therefore, there is a rule: the TÚV expansion vessel should not be oversized by more than double the calculated minimum volume. It is therefore not advisable to install a 36-liter vessel for a 30-liter tank – from a hygiene perspective, this is counterproductive.

Certification and standards – what the product must meet

Expansion vessels for potable water systems are subject to several standards and directives within the European Union. The basis is the Pressure Equipment Directive PED 2014/68/EU, but for contact with potable water, national and European hygiene certifications are decisive:

EN 13831 – European standard for closed expansion vessels with a membrane. This standard defines minimum construction and testing requirements. The product should carry a conformity certificate to this standard.
KTW (Kunststoff-Trinkwasser) – German certification for plastics and rubbers in contact with drinking water, recognized throughout Europe.
WRAS – British equivalent (Water Regulations Advisory Scheme), less common in Central Europe, but a good indicator of quality.
ACS – French certification for materials in contact with water intended for human consumption.

In practice, it is sufficient if the manufacturer states in the technical documentation or on the label that the vessel is approved for drinking water (Trinkwasser, drinking water, eau potable) and complies with EN 13831. Most reputable manufacturers, including IBO, provide this information both on the product and in the documentation.

Typical problems from practice and how to recognize them

Over the years of practice, I have seen several recurring situations where a missing or non-functional expansion vessel caused noticeable problems:

Pressure relief valve constantly drips

This is the most common symptom. A customer calls to say that the pressure relief valve of the tank (usually a red 6-bar valve with a lever) regularly – especially during heating – drips, or even sprays directly into the drain. First question: Is there an expansion vessel at all? If not, it needs to be added. If yes, is the membrane still functional? A worn or cracked membrane no longer performs its function – the vessel is full of water on both sides and expansion has nowhere to go. For signs of a non-functional vessel and when to replace it, see the separate article Common expansion vessel faults – symptoms, causes and solutions.

Boiler "calibrates" with a bang

Sometimes customers describe hearing knocks or bangs in the piping when turning on the heating. This may indicate a water hammer caused by too rapid pressure increase, which the expansion vessel does not compensate for – either it is missing or it is too small.

Short lifespan of the pressure relief valve

The pressure relief valve is not designed to open regularly with every heating cycle. Each cycle slightly wears it out – deposits prevent a perfect seal and the valve starts to leak continuously. If a customer replaces the pressure relief valve every two years, the cause is almost always the absence or malfunction of the expansion vessel.

Selecting an IBO expansion vessel for TÚV – practical notes

The IBO expansion vessel series available at atria.sk covers common needs from small single-pipe heaters up to larger tank systems. The vessels are equipped with a membrane certified for drinking water and the inner surface of the vessel is protected by an appropriate treatment. The connection is made using a standard 3/4" thread (some larger models 1"), which allows direct connection to standard plumbing without special reductions.

For small flow heaters or boilers up to 50 liters, the IBO 8L expansion vessel is sufficient. It has a compact size and can be mounted directly on the wall next to the tank. For medium-sized tanks of 80–120 liters, the IBO 19L expansion vessel is a good choice, offering sufficient capacity even at higher heating temperatures. Larger tanks of 150–200 liters in family homes with two bathrooms are covered by the IBO 24L expansion vessel for heating and TÚV systems, which is explicitly intended for TÚV applications.

A detailed comparison of the parameters of individual models can be found in the article IBO expansion vessels – model overview and parameter comparison.

Maintenance and inspection of the expansion vessel in a TÚV system

An expansion vessel is not a "install and forget" device. For reliable operation, regular inspection is necessary, at least once a year during the annual tank inspection:

Pre-pressure check: Close the inlet valve, drain the system on the water side (open a hot water tap), and measure the pressure on the Schraeder valve of the vessel using a manometer. The value should correspond to the set pre-pressure (usually 3–4 bar). If it is lower, air has escaped and needs to be refilled using a standard pump (Schraeder = car valve).
Membrane check: If water instead of air comes out when opening the Schraeder valve, the membrane is broken and the vessel needs to be replaced.
Visual inspection of the vessel body: Rust, mechanical damage, water leakage from the vessel body – reason for replacement.

The lifespan of a membrane expansion vessel for TÚV is 8–12 years under normal conditions. Factors that shorten it: excessively high pressure, too low pre-pressure (membrane works with a large amplitude), aggressive water with low pH, or overdimensioning of the vessel (long water stagnation). More on this topic can be found in the article How long does an expansion vessel last and when to replace it.

Expansion vessel TÚV in apartment buildings and shared systems

In apartment buildings, the situation is more complicated. If the TÚV tank is central (shared for the whole building), it is considered as one system and the expansion vessel must be sized for the total tank volume plus the volume of water in the hot water distribution system of the entire building. In practice, this can be several hundred liters and one device is not enough – multiple vessels are installed in parallel. Such projects always require a professional HVAC design and static calculation; it cannot be solved "on the fly".

In apartments with individual tanks (e.g., electric boiler in the apartment), the situation is the same as in a family home – the expansion vessel is sized according to the volume of the specific tank.

Most frequently asked questions (FAQ)

Can I use a standard red expansion vessel for heating in a TÚV system?

No. A red expansion vessel for heating is not certified for contact with drinking water – its membrane may release chemicals into the water. For TÚV systems, it is essential to use a vessel with a membrane approved for drinking water, ideally with a certificate according to EN 13831 and KTW or WRAS. These vessels are usually blue or have the label explicitly stating "drinking water" / "Trinkwasser" on the label.

The pressure relief valve of the tank occasionally drips, but not always. Does this mean the expansion vessel is too small?

Not necessarily too small – it may also mean that the membrane is partially damaged and the vessel is only partially performing its function, or that the pre-pressure in the vessel is too low. I recommend first checking the pre-pressure and membrane condition according to the procedure described in the Maintenance section. If the pre-pressure is correct and the membrane is functional, it may really be an insufficient vessel volume.

Where exactly in the system should the expansion vessel be connected?

The expansion vessel for TÚV is connected to the cold inlet of the tank, before the cold water enters the tank. The reason is that cold water is under pressure from the water supply and the vessel must be in a part of the circuit where pressure increases occur during heating. It should never be connected after the pressure relief valve or on the hot water outlet.

What pre-pressure should be set in the TÚV expansion vessel?

The pre-pressure should be the same or 0.2–0.5 bar lower than the minimum operating pressure of the system at the point of connection. In practice, for domestic TÚV systems, 3.0 to 4.0 bar is most commonly set. A detailed procedure for measuring and setting is available in the article Correct pre-pressure of the expansion vessel – how to set and check.

Is it necessary to install a TÚV expansion vessel if I don't have a pressure-reducing valve?

If your system does not have a pressure-reducing valve or a check valve on the cold water inlet, the expanded water can return to the public network and the pressure does not rise significantly – in this case, an expansion vessel is technically not necessary. However, if there is a pressure-reducing valve or a check valve in the house (which is standard nowadays), a closed system is created automatically and an expansion vessel is necessary. I recommend confirming this with an installer.

What if the TÜV tank is combined with a solar collector system?

In combined systems (solar tank with electric or gas booster heating), the situation is more complex – the solar circuit has its own expansion vessel (for heating, not for drinking water), but the TÜV side of the tank still requires its own expansion vessel certified for drinking water. Both vessels serve different circuits and must not be interchanged. I recommend consulting the overall dimensioning of such a system with a designer.

Conclusion

An expansion vessel for TÜV systems is a cheap and relatively simple component, but its absence or incorrect selection can cause significant problems – from constant dripping of the safety valve through tank damage to hygiene risks due to water stagnation. The key is to remember: it must be explicitly intended for drinking water (certificates KTW, WRAS or ACS, standard EN 13831), properly dimensioned according to the tank volume and heating temperature, and installed on the cold branch before the tank.

For standard family homes with 50–200 litre tanks, the range of IBO vessels available at atria.sk covers the entire spectrum of needs, from the compact 8-litre version up to the larger 36-litre vessel for more extensive systems. If you are unsure about the correct choice, refer to the table in this article or use information from other articles in the Vedomostné centrum – for example What expansion vessel volume do I need for my heating system? or Membrane vs. conventional expansion vessel – differences and when to use which. A careful choice at the beginning will save you trouble for years to come.

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