Frequently asked questions about expansion tanks for heating

We are still preparing products.

But you can look at other categories.

Common questions about expansion tanks for heating – a complete overview

The expansion tank is one of those components in the heating system that most homeowners don't even notice – until something stops working properly. The boiler starts releasing water through the safety valve, the pressure in the system fluctuates, or air appears in the radiators. Suddenly, everyone starts wondering what that red tank in the boiler room actually does and why it is important.

In this article, we have gathered dozens of questions that customers actually ask – during phone calls, in stores, and during technical consultations on site. The answers are practical, technically accurate, and based on real experience with hundreds of installations. If you are looking for a more detailed analysis of specific topics, we also refer you to other articles in the Knowledge Center – for example, How to choose an expansion tank for heating, Correct pre-pressure of the expansion tank, or Common expansion tank faults.

What does the expansion tank actually do and why does the system need it?

This is the basic question from which everything else follows. Water expands when heated – it increases in volume. When heated from cold (about 10 °C) to operating temperature (for example, 70–80 °C), the volume of water increases by about 2.5–3%. In a closed central heating system, where there is nowhere to expand, this would mean an increase in pressure to levels that would open the safety valve or even damage the boiler, radiators, or connections.

The expansion tank elegantly solves this problem: it has a membrane (or bladder) that separates the air (nitrogen) chamber from the water in the system. When the water expands, it pushes the membrane and compresses the air. The pressure increases only slightly – within safe limits for the system. When the system cools down and the water contracts, the air pushes the membrane back and the water returns to the circuit. The entire cycle takes place automatically, without the need for user intervention.

Without a functioning expansion tank, the system loses water through the safety valve with every heating cycle, and when it cools down, it draws in air or the pressure drops below the minimum. The boiler then reports a low-pressure fault, the system needs to be refilled, and air has to be bled from it. This is not only annoying – it is also economically disadvantageous and in the long run damages the circuit and the boiler.

What is the difference between a closed and an open expansion tank?

The open expansion tank is an old technology – a simple tank (most often galvanized sheet metal) placed at the highest point in the system, open to the atmosphere. Water could freely expand upwards. This technology worked for decades, but it has serious drawbacks: the water is in contact with air, which accelerates corrosion, and the tank must be higher than the highest radiator, which complicates installation in modern homes.

The membrane (closed) expansion tank works on the principle described above. The system is hermetically sealed, with no contact with atmospheric air. It can be placed anywhere in the circuit, most often in the boiler room right next to the boiler. Today, it is the standard for all modern boiler systems. A detailed comparison of both types can be found in the article Membrane vs. classic expansion tank in the Knowledge Center.

How do I know if the expansion tank is correctly sized?

A correctly sized expansion tank easily handles normal system operation – the pressure when the system is cold (static pressure) and the pressure at full power (dynamic pressure) remain within reasonable ranges, the safety valve remains silent, water does not need to be added, and the boiler operates smoothly.

Problems arise with an undersized tank. The system functions properly for the first few years, until the membrane loses part of its pre-pressure or until the boiler is replaced with a more powerful one. At that point, the safety valve starts releasing water with every heating cycle – not because the safety valve is faulty, but because the tank cannot accommodate the volume of the expanded water.

A rough rule of thumb for residential houses: the volume of the expansion tank should be about 10% of the total water volume in the system (circuit + boiler + TÜV tank, if connected). For a system with a water content of 100–150 liters, you therefore need a tank with a volume of 15–24 liters. For small apartment units with one boiler and a few radiators, even the IBO 8l expansion tank or a model with a volume of 12 liters may be sufficient. A detailed calculation procedure is available in the article What expansion tank volume do I need for my heating system.

Why is the boiler constantly reporting low pressure, even though I keep topping up the system?

This is by far the most common issue customers call about. The system works fine in the morning, but after a few days the pressure is low again. You top it up, and a week later it is again. In such a case, the first thing to check is the expansion tank – specifically its pre-charge pressure.

The pre-charge is an air (or nitrogen) cushion on the air side of the membrane. The correct pre-charge pressure should correspond to the static pressure of the system – that is, the height of the water column from the tank to the highest radiator plus approximately 0.2–0.3 bar. For a typical single-story family house, this is usually 0.8–1.2 bar.

If the pre-charge pressure drops (the membrane leaks air or it escapes through the air valve), the tank loses the ability to accommodate the expanded water. When the water is heated, it pushes the membrane to its limit, the pressure rises sharply, and the safety valve releases water. When the water cools down, the pressure drops below the minimum. Result: the system constantly loses water and air.

Solution: check the pre-charge pressure via the Schrader valve (the same as on a bicycle or car tire) on the air side of the tank – using a standard pressure gauge. If the pressure is low, top it up with compressed air or nitrogen. If the pressure cannot be maintained, the membrane is likely perforated and the tank needs to be replaced. More about this procedure can be found in the article Correct pre-charge pressure of an expansion tank – how to set and check it.

Can an expansion tank be too large? Is it worth going with a reserve?

Yes, an expansion tank can be oversized – although this problem is less serious than underdimensioning. A too large tank is not dangerous, but it can cause the pre-charge pressure to be harder to set, and in certain situations (for example, in a very small system), it may be harder to maintain a stable operating pressure.

In practice: a reserve of 20–30% over the calculated volume is reasonable and recommended. For a system where the calculation comes out to 17 liters, it is therefore appropriate to choose an IBO 19L expansion tank or even a 24-liter model, not the other way around. A tank with triple the volume would unnecessarily take up space and, in addition, sedimentation of sludge could occur inside due to slow water flow.

Where is the expansion tank installed and what should be considered when placing it?

A diaphragm expansion tank can in principle be placed anywhere in the circuit where a connection is available. In practice, there are several recommendations:

Suction circuit of the pump – the ideal location is on the cold branch (return) before the circulation pump, at the so-called neutral point. Here the pressure is most stable and the tank functions most efficiently.
Vertical position with the connection pointing downward – the tank should be suspended with the connection pointing downward to prevent air inside the tank body from entering the circuit. Some models can also be mounted horizontally, but always check the manufacturer's instructions.
Secure mounting – the tank must be fastened in a bracket or hanger, not just hanging on the connecting pipe. IBO expansion tanks have a welded eye or bracket on the outer shell for this purpose.
Accessibility – don't forget that the tank needs to be checked and possibly serviced. Mounting it behind the boiler in such a way that you can't even reach the air valve is a common mistake.
Installation location temperature – the tank should not be placed in a location where freezing is a risk, nor directly at the boiler outlet where temperatures reach 80–90 °C (the membrane would degrade). The recommended maximum temperature for most diaphragm tanks is 70 °C, for some models 99 °C – always check the specifications.

A complete installation procedure including step-by-step instructions and tips for different types of boilers can be found in the article Installation of an expansion tank for heating – procedure, placement and connection.

Can an expansion tank for heating be used for DHW (domestic hot water) as well?

No, it is not possible – at least not without special certification. Standard expansion tanks for heating have an inner shell and membrane made of materials that are not certified for contact with drinking water. The membrane may leach harmful substances into the water.

For DHW systems – for example, as expansion protection in a combined storage tank, in a solar water heating system, or in systems with a flow heater – special tanks made from food-safe materials must be used (marked WRAS, ACS or EN 13831). The IBO model range also covers these requirements. The issue is discussed in detail in the article Expansion tank for DHW systems – what it must meet and how to choose it.

How long does an expansion tank last? When should it be replaced?

A quality diaphragm expansion tank should last 10–15 years without problems under proper operation. In practice, however, the lifespan depends mainly on three factors:

Water quality in the system – hard water with a high calcium content or acidic water accelerates membrane degradation. Proper water treatment (softening, corrosion inhibitors) extends the lifespan.
Operating pressure – systems with variable pressure (e.g., poor dimensioning, repeated topping up) mechanically fatigue the membrane more quickly.
Water temperature – prolonged operation above 70 °C significantly accelerates membrane degradation. Therefore, placing the tank on the return (cold branch) is correct.

Signs that the tank is reaching the end of its life: water leaks through the air valve (the membrane is perforated), the pre-charge pressure cannot be maintained, the system repeatedly goes through a pressure loss - refill - release cycle through the safety valve. A detailed guide on diagnostics and the decision to repair or replace can be found in the article How long does an expansion tank last and when to replace it.

What pre-charge pressure should I set in a new expansion tank?

New IBO expansion tanks are factory-filled with a pre-charge pressure of 1.5 bar. However, this pressure is pre-set as a general compromise for various systems - for most specific installations, it needs to be adjusted.

The correct pre-charge pressure depends on the height of the system: for every 10 meters of water column height, a pressure of approximately 1 bar is required. For a single-story residential house (system height 3–5 m), the correct pre-charge pressure is 0.5–0.8 bar. For a two-story house (height 5–8 m), it will be 0.8–1.0 bar. In three-story or higher buildings, the pressure is further increased.

The procedure is simple: first, drain the water from the system (or at least isolate the tank by closing the ball valve and draining it on the water side). Then, check the current pre-charge pressure using a pressure gauge via the Schrader valve. If it is higher than required, release air; if lower, inflate using a regular bicycle or car pump. After setting it, return the water to the system and check whether the cold pressure in the system is 0.2–0.3 bar higher than the tank's pre-charge pressure.

Can I install an expansion tank myself or must a professional do it?

Replacing an existing expansion tank with one of the same dimensions is a mechanically simple task - unscrewing the old tank, setting the pre-charge pressure on the new one, and screwing it back in with Teflon or hemp sealing. This task can be handled by a skilled DIY enthusiast. However, there are a few things to keep in mind:

Before work, drain the water from the system or at least close and drain the branch with the tank.
Check and set the pre-charge pressure before refilling the system, not after.
After refilling, check the tightness of the connection and set the correct operating pressure of the system.
In the case of a new installation (originally there is no tank or you are changing the system), it is recommended to have the work done by a qualified person who will design the correct volume and connection point.

Regarding legally required qualifications: the replacement of an expansion tank in a pressure circuit up to 0.5 MPa in a residential house is not a regulated activity requiring authorization. However, any interventions in a gas boiler, its gas supply, or pressure circuits above 0.5 MPa must be carried out by a certified technician.

Why is the safety valve dripping after every heating cycle?

This is one of the most common symptoms reported by customers - and most people think the safety valve is faulty. In reality, this is almost never the case. The safety valve is simply doing its job: when the pressure in the system exceeds its setting (typically 3 bar), the valve opens and releases water.

The causes of this condition are almost always related to the expansion tank:

The tank's pre-charge pressure is too low - the membrane reaches its limit during expansion and the system has nowhere to "breathe".
The membrane is perforated - the tank is full of water, the air cushion is missing.
The tank is undersized - the volume is insufficient to accommodate the full expansion of the boiler.
A closed valve on the tank's connection - the tank is physically disconnected from the system (this can happen after service, when a technician forgets to open the valve).

If the safety valve is truly worn out and drips even when the pressure is low, it needs to be replaced - but only after you have ruled out the problem with the expansion tank.

How much does an expansion tank cost and is it worth saving on it?

An expansion tank is a relatively affordable component compared to a boiler, storage tank, or even a circulation pump. IBO models in the range of 8–36 liters range in price from a few dozen to a few hundred euros - depending on the volume and model. Specific prices can be found directly on the products, for example, on the IBO 36L expansion tank.

It does not make sense to save money on an expansion tank. An undersized or low-quality tank can cause corrosion in the boiler body (repeated oxygen intake), wear out the safety valve, or in extreme cases, rupture connections due to overpressure. The resulting damage is many times more expensive than the cost of a properly chosen tank.

Do I need to service the expansion tank in any way on a regular basis?

Yes, although the service is simple and quick. The recommended interval for checking the pre-charge pressure is once a year, ideally at the beginning of the heating season. Procedure:

Turn off the boiler and let the system cool down (at least to a temperature below 40 °C).
If you don't have a shut-off valve on the tank connection, you must drain the water from the system. If you have a valve, close it and drain the tank via the drain valve.
Measure the pre-charge pressure using a pressure gauge via the Schrader valve. The value should correspond to the set pre-charge pressure (typically 0.8–1.2 bar for a standard residential house).
If the pre-charge pressure is low, inflate it. If it is high, release air.
Open the valve or refill the system, and check the operating pressure when cold.

This check takes about 15 minutes and can extend the life of the tank by years. If during the annual check you notice that the pre-charge pressure drops despite the previous setting, the membrane or air valve is likely faulty – it is time to consider replacing the tank. Details about typical faults and their solutions are covered in the article Common faults of expansion tanks – symptoms, causes and solutions.

What are the differences between models IBO 8l, 12l, 19l, 24l and 36l?

IBO models share the same construction philosophy: a membrane tank with an internal airbag (or replaceable membrane depending on the model), a steel shell with an epoxy coating, a 3/4" or 1" connection, a Schrader air valve, and a factory pre-charge pressure of 1.5 bar. The differences are mainly in:

Total volume – directly determines how much water the tank can hold during expansion.
Maximum operating pressure – standard 3 bar (10 bar for top models used in larger systems).
Physical dimensions and weight – important for installation in a boiler room with limited space.
Connection – smaller models have a 3/4" thread, while larger models (from 24 l and up) usually have a 1" thread.

An overview and technical comparison of all models can be found in the article IBO expansion tanks – model overview and parameter comparison. For installation in an apartment or small boiler room with a single heat source, models 8l and 12l are suitable. For a family house with a larger system, models 19l and 24l are appropriate. For heating a larger building or systems with TÜV, it is appropriate to choose an expansion tank with a volume of 36 liters or larger.

Can one expansion tank be used for multiple boilers or circuits?

Technically yes – if the circuits are interconnected and form one hydraulic system, one properly dimensioned tank is sufficient. In practice, however, you need to carefully calculate the total water volume in the system (the sum of all circuits including the storage tank) and choose the tank volume accordingly. Very large systems (e.g., apartment buildings or commercial buildings) may require multiple tanks connected in parallel.

If the circuits are hydraulically separated (e.g., a solar circuit and a heating circuit are separated by a heat exchanger without direct water connection), each circuit requires its own expansion tank.

Most frequently asked questions (FAQ)

How can I tell that my expansion tank is faulty without draining the system?

The most reliable indicator without draining: press the Schrader valve (air valve on the tank) with your finger or a short wrench. If water comes out, the membrane is perforated and the tank is full of water – it needs to be replaced. If air comes out at the correct pressure (measurable with a gauge), the membrane is in good condition. If there is no pressure and only air comes out without resistance, the pre-charge is depleted – try to re-pressurize. Another sign: the boiler requires repeated water refills every day to week, even though there is no visible leak.

Can I fill the expansion tank with nitrogen instead of air? Does it make sense?

Yes, professional service technicians use nitrogen, especially in larger or professional systems. Nitrogen (N₂) is inert – it does not participate in chemical reactions, unlike air, which contains oxygen. Oxygen from the air cushion can diffuse through the membrane into the water over the years and contribute to corrosion processes inside the system. In typical family homes, this amount is negligible and air is fully sufficient. Nitrogen makes sense in stainless steel systems, solar installations, or where corrosion is a long-term risk factor.

I have a new condensing boiler and the dealer says that the built-in expansion tank in the boiler is sufficient. Is that true?

Most modern condensing boilers have a built-in expansion tank with a volume of 6–10 liters. For small apartments with a limited number of radiators, this may be sufficient – but only if the system is truly small and the water volume does not exceed about 60–80 liters. For a family house with three rooms and five radiators, the built-in tank is almost always undersized. The correct procedure is to calculate the actual system volume, compare it with the capacity of the built-in tank, and decide whether an external tank is needed. Many installers install the system without an external tank – and customers start to deal with the issue when after a few years the safety valve starts dripping.

Why does the new expansion tank from the factory have a pre-charge of 1.5 bar, when I am told I need only 0.8 bar?

The manufacturer sets the pre-charge to 1.5 bar as a safe and commercially acceptable compromise – to keep the membrane centered during transport and storage and to meet the most common systems in taller buildings (apartment buildings, where pressure is higher). Before installation in a specific system, always check and adjust the pre-charge according to the actual parameters of your system. Installing a tank with an inappropriate pre-charge – even if it is new – can lead to the same problems as with a faulty tank.

I have a boiler that is only a year old, but the safety valve is dripping. Can the expansion tank be faulty after only one year?

Yes, even a new tank can have a problem. The most common cause after a short period of operation: the pre-charge was not adjusted before installation (it remained at the factory setting of 1.5 bar), while the system required, for example, 0.8 bar. As a result, the membrane does not work properly and does not allow full expansion. Another possibility: the shut-off valve on the tank connection was left closed – the tank is physically disconnected from the system. In rare cases, the membrane may be faulty right from the manufacturer. Check step by step: whether the valve is open, what the pre-charge is, and whether water is leaking from the Schrader valve.

Do I need to prepare the system in any way (chemically or mechanically) before installing a new tank?

If you are only replacing the expansion tank and the system has been operating without major problems so far, no special preparation is necessary. We recommend, however, to check and possibly replenish the corrosion inhibitor in the system and check the water pH (it should be in the range of 7–9 for steel systems) at this opportunity. If the system has suffered from repeated corrosion or sedimentation, it is appropriate to flush it with a chemical cleaning agent before installation. A new tank in a contaminated and chemically aggressive system will significantly shorten the membrane's lifespan.

Conclusion: the expansion tank is a small part with a big impact

The expansion tank is affordable, inconspicuous and can function for decades without any attention – if it is properly selected, installed and adjusted. Problems arise precisely when these three conditions are underestimated. The result is repeated dripping of the safety valve, constant water refilling, air in radiators and premature boiler wear.

If you are unsure about the correct choice, the IBO model range from 8 liters up to 36 liters covers the vast majority of standard installations in family homes and apartments. With proper sizing – which you can verify in the article What expansion tank volume do I need for my heating system – the choice is simple and the investment pays off in trouble-free operation of the heating system for many years.

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.