How to choose an expansion tank for heating – volume, pressure and system type

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How to choose an expansion tank for heating – volume, pressure and system type

The expansion tank is one of those components of the heating system that is most often "forgotten" during planning or reconstruction, or only given marginal attention. We have been dealing with the sale and solving of heating problems for years, and it can be said that almost every second complaint or mysterious boiler malfunction described by the customer is due to an incorrectly selected or improperly set expansion tank. Whether it is a low pressure in the system, repeated water topping up, a hissing safety valve or a jammed pump – very often the culprit is precisely this relatively cheap, yet key component.

This article is intended for anyone who is deciding which expansion tank to buy, or who wants to understand why the old one is not working properly. We will go through the physical basis, calculation methods, system types, specific volume categories and finally also practical recommendations from practice.

Why an expansion tank is needed in heating at all

Water is almost an incompressible liquid. When the boiler heats it from 10 °C to the operating temperature of 70–80 °C, its volume increases by about 2.8–3.5 %. In a closed heating system, this increase would cause an extreme pressure rise – the safety valve would open, water would flow out and the system would lose its medium. The expansion tank is therefore a "pressure damper" – it accepts the expanded volume and captures it using a compressed air (or nitrogen) cushion without the pressure in the system exceeding the acceptable value.

In older open (gravity) systems, this function was fulfilled by an open expansion tank located at the highest point (typically in the attic). In modern closed systems with a circulation pump, a closed membrane expansion tank is used, which is pressure-isolated, hygienically sealed and does not require any access or maintenance except occasional pre-pressure checks.

Types of expansion tanks – membrane vs. classic open

For today's practice, three categories are relevant: open tanks (historical, still in operation in older houses), closed membrane tanks (standard for closed systems) and special tanks for DHW. A detailed look at the differences can be found in the article Membrane vs. classic expansion tank – differences and when to use which.

Open expansion tank

A simple steel tank without a membrane, connected to the highest point of the system. The advantage is zero dependence on pre-pressure, the disadvantage is contact of water with air (corrosion, water loss by evaporation), the need for access and dependence on the height of installation. In practice, we encounter them in houses from the 80s and 90s, where the owner simply did not want to change the entire system. If you are buying or renovating such a house, one of the first changes should be precisely the replacement with a closed membrane tank.

Closed membrane expansion tank

Today's standard. A steel tank divided by a flexible membrane (or bladder) into two chambers – the water side and the air side. The air side is pre-charged to a certain pre-pressure (typically 1.0–1.5 bar) at the factory. Water from the heating circuit enters the water side via a connection with a ¾" or 1" thread. When heated, the membrane is compressed towards the air chamber, thus compensating for the expansion. When cooling down, the membrane returns to its original position. The entire process takes place without water or air loss – the system is hermetically sealed.

Expansion tank for DHW (domestic hot water)

A special category – these tanks must have a membrane certified for use with potable water (approved according to food contact standards), resistant to higher temperatures and with antibacterial treatment of the inner surface. They must not be confused with heating tanks, even if they have the same volume. We will deal with this topic in more detail in the article Expansion tank for DHW systems – what it must meet and how to choose it.

Expansion tank volume – how to calculate it correctly

This is the most important question when choosing one. A too small tank will cause the safety valve to blow off, the system will lose water and the boiler will have outages. A too large tank won't damage anything – you will just pay a bit more for the extra space. It is much better, however, to know how to calculate a realistic volume and buy the right one.

A detailed calculation procedure including tables for different types of systems can be found in a separate article What expansion tank volume do I need for my heating system. Here we will explain the basic logic and shortened practical formulas.

Step 1: Total water volume in the system

This is the sum of the water volume in all radiators, underfloor heating, in the boiler, in the tank, and in the pipes. For an approximate estimate, the following applies:

Steel panel radiator: about 7–12 liters per 1 kW of power (depends on the type)
Cast iron radiator: 12–18 liters per 1 kW
Underfloor heating: 5–8 liters per 1 kW
Cu/plastic pipes DN15–22: 0.2–0.4 liters per standard meter
Condensing boiler: 2–5 liters depending on the model
Boiler tank: 5–20 liters of heating medium on the heat exchanger side

For a typical family house with central heating (boiler 15–25 kW, 8–12 radiators, area 120–180 m²), the total system volume is approximately 80–160 liters.

Step 2: Water expansion coefficient

Water expands when heated according to temperature. For standard heating systems, the following coefficients e apply:

Tmax = 70 °C → e ≈ 0.023 (2.3 %)
Tmax = 80 °C → e ≈ 0.029 (2.9 %)
Tmax = 90 °C → e ≈ 0.036 (3.6 %)

Expanded volume ΔV = total system volume × e. For 100 liters and Tmax = 80 °C: ΔV = 100 × 0.029 = 2.9 liters.

Step 3: Efficiency factor of the expansion tank

The expansion tank cannot use its entire geometric volume – a part of compressed air must always remain. The useful volume depends on the tank's pre-charge pressure and the maximum operating pressure (set on the safety valve). A simplified formula for the required tank volume:

V_tank = ΔV × (P_max + 1) / (P_max – P_0)

where P_max is the pressure of the safety valve in bars (absolute) and P_0 is the tank's pre-charge pressure in bars (absolute). For a typical system with a safety valve at 3 bar and a tank pre-charge pressure of 1.5 bar:

V_tank = 2.9 × (3+1) / (3–1.5) = 2.9 × 4/1.5 = 7.7 liters

This means that for a system with 100 liters of water, a tank with a geometric volume of 8 liters is sufficient – for example, Expansion tank IBO 8l for heating and hot water systems.

Practical table of approximate values

Boiler power	System type	Estimated water volume	Recommended tank volume
up to 12 kW	apartment, small house, radiators	40–70 l	8–12 l
12–20 kW	family house, radiators	70–120 l	12–19 l
20–30 kW	larger house, mix of radiators + floor	120–200 l	19–24 l
30–50 kW	large house, underfloor heating	200–350 l	36 l and more

For a family house with a medium-sized boiler (18–22 kW) and a combined system of radiators and hydronic underfloor heating, the most common choice in practice is Expansion tank IBO 19l for heating and hot water systems or Expansion tank IBO 24l for heating and TÚV systems.

Expansion tank pre-charge pressure – what it is and why it is key

Every closed membrane expansion tank is delivered with a factory pre-charge pressure in the air chamber – usually 1.0 or 1.5 bar. The pre-charge pressure must be set so that it corresponds to the static height of the heating system. Simplified: pre-charge pressure = 0.1 bar × system height in meters, minimum 1.0 bar.

Example: A two-story house, the highest radiator is 6 meters above the boiler room → pre-charge pressure = 6 × 0.1 = 0.6 bar. Here the minimum is 1.0 bar, so we set 1.0 bar.

The precharge pressure is checked using a standard tire pressure gauge through the valve (Schrader valve), which is on the air side of the tank. The tank must be disconnected from the system before checking, and the water side must be pressure-free – otherwise, you will get a distorted result. A detailed procedure for checking and adjusting is described in the article Correct precharge pressure of an expansion tank – how to set and check.

An incorrectly set precharge pressure is one of the most common causes of premature membrane failure. If the precharge pressure is too low, the membrane will be completely flooded with water even during a cold system – water mechanically damages the membrane with pressure shocks. If the precharge pressure is too high, the tank does not perform its function at all, and the water has nowhere to expand.

Type of system and its influence on tank selection

Closed system with a condensing boiler

The most common case in new constructions and renovations since 2010. A condensing boiler operates at lower temperatures (supply water 55–70 °C, return 30–45 °C), which reduces water expansion and thus the requirements for tank volume. Most condensing boilers have an integrated expansion tank with a volume of 8–10 liters – which may be sufficient for smaller apartments, but is not enough for houses with a larger system volume. A very typical mistake: the customer buys a condensing boiler, the installer connects it without an external tank, and after a year, problems start – the safety valve vents, the boiler reports a pressure error. The solution is to add an external expansion tank – typically 12 to 19 liters depending on the system size.

Low-temperature system with underfloor heating

Underfloor heating operates at temperatures of 30–45 °C, but the water volume in the circuit is usually larger than with radiators (long pipe loops in concrete). The expansion is lower (e ≈ 0.013–0.017), but the total ΔV can be comparable. In a house with 150 m² of purely underfloor heating, there may be 120–180 liters of water in the circuit. A reasonable choice here is Expansion tank IBO 36l for heating and underfloor heating.

Old system with cast iron radiators

Cast iron radiators have a large internal volume – typically 1.3–1.8 liters per section. An old house with 8 radiators of 10 sections each has about 100–140 liters of water in the radiators alone. Add the boiler, piping, and storage tank – the total volume easily exceeds 200 liters. Such a system can only be handled by a tank of at least 24–36 liters. Unfortunately, many customers replace only the boiler with a condensing one and close the old open tank, while using the integrated 8-liter tank of the new boiler. This is a recipe for permanent problems.

System with a TÚV storage heater (combination of heating and hot water)

If the system also has an indirect storage water heater, the volume of the heating medium on the heat exchanger side must be included in the calculation. For a 200-liter tank, this can be an additional 10–20 liters of water in the circuit. At the same time, in the summer period, the boiler heats only the storage tank – and thus the system may have significantly lower active water volume, which also affects pressure dynamics. In such combinations, it is good to slightly overdimension the tank.

Where in the system the expansion tank is located

The correct placement of the expansion tank is not arbitrary – it has direct consequences for the functionality and lifespan of the system. The standard is to connect it to the return (cold branch) before the boiler, which reduces thermal stress on the membrane. The tank must never be placed after the pump in the flow direction – this would cause a vacuum on the pump's suction side and cavitation.

Further practical guidelines for placement and installation are described in the article Installation of an expansion tank for heating – procedure, placement, and connection. Briefly: the tank is suspended or placed on a stand in the boiler room, connected with a hose or rigid pipe to a ¾" thread, a shut-off valve is installed before the tank (for the possibility of replacement without draining the system), and a pressure gauge for checking the operating pressure.

Selection of a specific model – parameters of IBO tanks

The range of IBO tanks we offer in the heating category belongs to reliable Central European products with a good price-to-quality ratio. Steel body, EPDM membrane approved for heating systems, connection ¾" internal thread, factory precharge pressure 1.5 bar (adjustable), maximum operating pressure 3 bar, maximum medium temperature 99 °C. An overview of the entire range and comparison of parameters can be found in the article Expansion tanks IBO – overview of models and parameter comparison.

IBO 8l – For small apartments, studios, and garden houses. Boiler up to 10 kW, system with water volume up to 50–60 liters. Also suitable as an additional tank to the integrated tank of a condensing boiler. → Expansion tank IBO 8l
IBO 12l – Standard for larger apartments and smaller houses, boilers 10–16 kW. → Expansion tank IBO 12l
IBO 19l – The most frequently sold size. Family houses 100–160 m², boiler 15–22 kW, mix of radiators and underfloor heating. → Expansion tank IBO 19l
IBO 24l – Larger houses, combination of heating and TÚV storage. Boiler 18–28 kW. → Expansion tank IBO 24l
IBO 36l – Large family houses with extensive underfloor heating, older houses with cast iron radiators, boilers 25–45 kW. → Expansion tank IBO 36l

Lifespan of an expansion tank and when to replace it

A properly dimensioned and correctly set expansion tank can last 10–15 years without major maintenance. The diaphragm is the most common point of failure – it ages over time, may crack, or stop sealing properly against the tank wall. The main signs of a worn or faulty expansion tank are:

The safety valve vents air with every heating cycle of the system
Pressure gauges show 0 bar when the system is cold – the entire tank is filled with water
Frequent need to refill water into the system (even if no visible leak is present)
Water instead of air is released when the air valve is pressed – the diaphragm is torn
The pressure in the system fluctuates between 0 and 3 bar during a single heating cycle

A detailed description of symptoms and diagnostics can be found in the article Common expansion tank faults – symptoms, causes and solutions, and on the topic of lifespan in the article How long does an expansion tank last and when to replace it.

If the issue is only a loss of pre-charge pressure (air has escaped through the valve), you do not need to replace the tank – it is sufficient to refill the air to the correct pressure. If the diaphragm is mechanically damaged, replacing the entire tank is usually more economical than repairing it.

Common mistakes in selection and installation – from practice

From dozens of customer cases where we dealt with problematic systems, several recurring mistakes emerge:

Undersized tank volume. A customer buys an 8-liter tank "because it is cheap" for a house with 200 liters of water. Result: the safety valve vents every morning and the boiler triggers a fault.
Forgotten pre-charge pressure. The tank is installed without checking the pre-charge pressure. The factory setting of 1.5 bar may not be suitable for the system – for example, in a three-story house you need 1.8–2.0 bar.
Incorrect placement. The tank is installed behind the pump in the direction of flow – this causes cavitation, noise, and reduces the pump's lifespan.
Using a heating tank for hot water and vice versa. A heating tank has a diaphragm that is not certified for potable water. A customer uses it on a DHW tank and technically violates the standards.
Closed valve before the tank. The installer fits a ball valve (correctly), but forgets to open it. The tank is not connected to the system – and the customer is puzzled why problems persist even with a new tank.
Ignoring the built-in tank in the boiler. A condensing boiler has an integrated 8-liter tank. A customer adds an external 12-liter tank – the total of 20 liters is acceptable. But if no one checks the pre-charge pressure in the built-in tank, the system operates chaotically.

Most frequently asked questions (FAQ)

Can I use the same expansion tank for heating and for DHW?

No. Heating expansion tanks have a diaphragm made of a material (EPDM) suitable for heating medium, but not certified for potable water. For DHW tanks, you must use special tanks with a diaphragm approved for food contact (usually SBR or special EPDM with a certificate). Using the wrong one is not only technically unsuitable, but also in violation of potable water installation standards.

I have a condensing boiler with an integrated 8-liter tank – do I need an external one as well?

It depends on the size of your system. If the total water volume in the circuit is up to 60–70 liters (small apartment, new building with a minimal number of radiators), the integrated tank may be sufficient. For a standard family house with 80 or more liters of water in the system, an additional external tank is necessary. Check the operating pressure in the system during heating – if it exceeds 2.5 bar, the system needs a larger expansion capacity.

How can I find out if the diaphragm in my tank is torn?

The simplest test: disconnect the system and release the pressure on the water side (open the drain valve). Then press the air valve on the air side of the tank (same as on a tire). If water flows out – the diaphragm is damaged and the tank must be replaced. If air comes out, check its pressure with a manometer and add air if necessary.

What is the maximum pressure I can set on the safety valve?

For standard closed residential heating systems, the standard setting for the safety valve is 3 bar. The maximum operating pressure of most boilers and radiators is 3 bar, so the safety valve must not be set higher. Some systems (e.g., with plastic piping or soldered copper radiators) may have a lower limit – always check the boiler and other components' documentation.

The tank is new, but the safety valve still vents. What am I doing wrong?

The most common cause: the pre-charge pressure of the new tank is not set correctly, or you bought a tank that is too small. Check the pre-charge pressure (it must be 0.2–0.3 bar lower than the static pressure of the system when cold). If the pre-charge pressure is correct, recalculate the required tank volume – you may need a larger or additional tank. Another possible cause is a faulty (stuck) safety valve that drips at a lower pressure than set.

Can I install an expansion tank horizontally?

Most diaphragm-type expansion tanks are primarily designed for vertical installation with a downward connection. Some models (with a bladder system instead of a flat diaphragm) can also be installed horizontally. Check the technical data sheet of the specific product. A horizontal position with a flat diaphragm can worsen its even loading and shorten its lifespan. If you have no other option, consult the manufacturer or seller.

Conclusion – summary of expansion tank selection

Selecting an expansion tank is not rocket science, but it does require at least a basic calculation and understanding of what the tank does. To summarize, it can be reduced to a few points:

Always calculate the total water volume in the system – do not rely on estimates.
Use the correct type of tank: heating for heating, potable water for DHW.
Set the pre-charge pressure according to the system height, not just the factory setting.
If you have a condensing boiler with an integrated tank, you will almost always need an additional external tank for a standard family house.
Choose a slightly larger volume if possible – paying 5–10 euros extra for a larger tank is much cheaper than dealing with problems season after season.
Do not forget regular pressure checks – at least once a year before the heating season.

If you are unsure about the calculation or have an atypical system (a mixed circuit with cast iron, a combination of underfloor heating and radiators, a large DHW tank), see other articles in the Knowledge Center – specifically Common questions about heating expansion tanks or IBO expansion tanks – model overview and parameter comparison. We have described most practical situations there with specific numbers and solutions.

Do you have a question about this topic?

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