A dripping safety valve, pressure that needs to be refilled over and over again, a boiler reporting an error - sounds familiar? The expansion vessel is one of the cheapest elements of the heating system, and at the same time one of the ones whose incorrect selection costs the most nerves. It determines whether the pressure in the system remains stable or jumps with each heating cycle. In this guide, we explain step by step how to choose an expansion vessel for central heating and hot water - and why the capacity itself is only half the battle.
Why do you need an expansion vessel in the installation?
Water has one inconvenient feature: when heated, it rises. In an open pot, no one will notice it, but in a tightly closed installation, this excess has nowhere to escape. The pressure climbs with each step until it finally activates the safety valve, which dumps water onto the boiler room floor. Therefore, the installation needs a place where it can "breathe" freely - and exactly this place is the expansion vessel, also called the expansion vessel.
The construction of an expansion vessel is simpler than the name suggests: it is a steel tank separated by a flexible membrane, most often made of EPDM. On one side of the membrane there is water from the installation, on the other side there is a gas cushion, factory-filled with air or nitrogen. How does the diaphragmatic vessel work in practice? As the installation heats up, excess water is forced into the tank and compresses the gas like a spring. When it cools down, the spring releases: the gas pushes the water back into the circulation. The pressure remains stable throughout the cycle and the safety valve has no reason to respond.
Not just liters - what to check before purchasing
Most of the questions we receive only concern capacity. Meanwhile, the selection of an expansion vessel is more than just a matter of liters. Two vessels of identical capacity may behave completely differently in the same boiler room - one will operate maintenance-free for years, the other will end up with a punctured membrane after the first season. The difference lies in parameters that are easy to miss:
- Pre-pressure — adjusted to the pressure in your installation (specific values are provided below).
- Installation height — the higher the radiators reach above the vessel, the higher the filling pressure must be: approximately 1 bar for every 10 m. In a two-story house, this is a real difference.
- Maximum working pressure — cannot be lower than the safety valve setting (typically 3 bar in central heating, 6 bar in hot water).
- Operating temperature — the membrane and the tank must withstand the temperature of the medium in a given installation.
- Type of membrane — for hot water use only a membrane approved for drinking water; the central heating expansion vessel cannot operate on domestic water.
- Connection and place of installation — in central heating, the vessel is installed on the return, in DHW, on cold water before the heater.
A full overview of the types of cookware can be found on our website, in the tab Central heating, hot water and solar expansion vessels.
Selection of a vessel for central heating - how many liters per boiler power?
The selection rule is simple: the capacity of the expansion vessel should be approximately 10% of the water, i.e. all the water circulating in the installation - in pipes, radiators and the boiler. This is a value with a margin, but it is worth being wrong in this direction: excess capacity does not threaten anything, but a shortage does. Don't you know the order? For radiator installations, you can use approximately 13.5 liters of water per 1 kW of boiler power. The answer to the question of which expansion vessel to choose for WHAT results directly from the table:
| Boiler power | Estimated order | Central heating vessel |
|---|---|---|
| up to 10 kW | approx. 135 years old | expansion vessel 12l |
| 15 kW | approx. 200 l | 18–24 years old |
| 20 kW | approx. 270 years | 24–25 years old |
| 25 kW | approx. 340 years | 35 years old |
| 30 kW | approx. 400 l | 40 years old |
But before you buy anything, two caveats. First of all, if you have a wall-mounted gas boiler, check its documentation: most such boilers have a factory vessel with a capacity of about 8 liters, which is quite sufficient for a small boiler. An additional vessel becomes necessary only when the volume exceeds the limit specified by the manufacturer (often approximately 100 liters) or the installation is high. Secondly, in the case of underfloor heating and installations with a buffer, the consumption may be much greater than suggested by the boiler power - and it must be calculated carefully. We write more about the specificity of such systems in the article about circulation pump for underfloor heating.
The expansion vessel works in one system with the pump, so if you are modernizing the installation, also refer to the guide on energy-saving central heating circulation pumps. And if you prefer quick calculations, you will find them on our website dedicated calculator, designed to support you in selecting the right vessel. Once you have completed your details, you can easily send your results to our office for comprehensive decision-making assistance.
DHW expansion vessel - which one to choose for the boiler?
The utility water heater also needs its own protection - and a separate one at that, because the vessel from the heating circuit cannot operate on drinking water. The expansion vessel for the boiler takes over the increase in volume with each heating and ends the problem of the safety valve that drips when the water is heated. How many liters should an expansion vessel in this role have? The industry standard is 5-10% of the heater capacity - plus a drinking water-approved membrane and a maximum operating pressure of at least 8-10 bar:
| Boiler capacity | DHW expansion vessel |
|---|---|
| 100 years | 8 years old |
| 150 years | 8–12 years old |
| 200 years | 12–18 years old |
| 250 years | 18–24 years old |
| 300 l | 24 years old |
The ranges in the table are not accidental: the higher the pressure in the water supply network and the hotter you heat the water, the closer to the upper limit you should aim. Our assortment includes a wide range of tanks with capacities ranging from: 8 years old, by 50 years old up to even 80 years old - so you can choose a vessel for both a small boiler under the sink and a large tank in the boiler room.
An expansion vessel that is too large or too small - which is worse?
A vessel that is too small will not be able to absorb the entire increase in volume: the pressure jumps, the safety valve releases water, and the installation regularly "loses" pressure and needs to be checked. Expansion vessel too large? It's just a higher purchase cost and a larger size - it doesn't matter hydraulically. Therefore, when limiting the capacity of the expansion vessel, always round it up. And if your installation is unusual or you have doubts about the order - get in touch with our technical department, we will help you choose.
FAQ - most frequently asked questions about the selection of an expansion vessel
Which expansion vessel for a 200l boiler?
According to the rule of 5-10% of the heater's capacity, a 12-18 l vessel with a membrane approved for drinking water will be optimal. With higher network pressure and frequent heating to high temperatures, choose 18 l.
Which expansion vessel for a 300l boiler?
For a 300 l tank, the standard is a 24 l vessel. If the installation operates close to the safety valve setting, a larger capacity will be safer.
Which expansion vessel for a 500l/1000l buffer?
We calculate the heating buffer as a CO2 supply: the vessel should have approximately 10% of the capacity of the entire system, i.e. approximately 50 l for a 500 l buffer and 100 l for a 1000 l buffer - plus the volume of the rest of the installation. Such large systems should be calculated carefully with the installer.
What is the difference between an expansion tank and an expansion vessel?
An expansion tank is a colloquial, broader name - historically it meant an open tank in gravity installations. A diaphragm expansion vessel is its closed equivalent with a membrane, used in modern pressure installations.
Where to buy an expansion vessel for central heating or hot water?
You can buy expansion vessels, pumps and accessories of our brands from distributors throughout Poland - the nearest point can be found in Dambat business partner search engine.
