A dripping safety valve, pressure that constantly needs topping up, a boiler reporting an error — sound familiar? An expansion vessel is one of the least expensive components of a heating installation, yet it is also one of those whose incorrect selection causes the most frustration. It determines whether the system pressure remains stable or jumps with every heating cycle. In this guide, we explain step by step how to select an expansion vessel for central heating and domestic hot water systems — and why capacity alone is only half the battle.
Why does an installation need an expansion vessel?
Water has one inconvenient characteristic: when heated, it expands. Nobody notices this in an open pot, but in a tightly sealed installation, the excess water has nowhere to go. Pressure rises with every degree until it eventually activates the safety valve, which discharges water onto the boiler-room floor. The installation therefore needs a place where it can freely “breathe” — and that place is precisely the expansion vessel, also known as the diaphragm vessel.
The construction of an expansion vessel is simpler than its name suggests: it is a steel tank divided by a flexible diaphragm, usually made of EPDM. On one side of the diaphragm is water from the installation; on the other is a gas cushion, factory-filled with air or nitrogen. How does an expansion vessel work in practice? When the installation heats up, excess water is pushed into the tank and compresses the gas like a spring. As the system cools, the spring releases its energy: the gas pushes the water back into the circuit. Pressure remains stable throughout the cycle, so there is no reason for the safety valve to open.
Not just litres — what to check before buying
Most of the questions we receive concern capacity alone. However, selecting an expansion vessel involves more than just the number of litres. Two vessels with identical capacities can behave completely differently in the same boiler room — one may operate maintenance-free for years, while the other may end up with a ruptured diaphragm after the first season. The difference lies in parameters that are easy to overlook:
- Pre-charge pressure — matched to the pressure in your installation.
- Installation height — the higher the radiators are above the expansion vessel, the higher the filling pressure must be: approximately 1 bar for every 10 m. In a multi-storey house, that makes a real difference.
- Maximum operating pressure — it must not be lower than the safety valve setting (typically 3 bar in heating systems and 6 bar in domestic hot-water systems).
- Operating temperature — the membrane and tank must withstand the temperature of the medium in the given installation.
- Membrane type — for domestic hot water, use only a membrane certified for potable water; a heating system diaphragm expansion vessel must not be used with domestic water.
- Connection and installation location — in a heating system, the vessel is installed on the return line; for domestic hot water, it is installed on the cold-water line before the heater.
You can find a complete overview of vessel types on our website, under Diaphragm expansion vessels for heating, domestic hot water and solar systems.
Selecting a heating system vessel — how many litres for the boiler output?
The selection rule is simple: the capacity of the diaphragm expansion vessel should be approximately 10% of the system volume, meaning all the water circulating in the installation—in the pipes, radiators and boiler. This value includes a margin, but it is better to err on this side: excess capacity poses no risk, whereas insufficient capacity does. Don't know the system volume? For radiator systems, you can assume approximately 13.5 l of water per 1 kW of boiler output. The answer to the question of which diaphragm expansion vessel to choose for a heating system then follows directly from the table:
| Boiler output | Estimated system volume | Heating system vessel |
|---|---|---|
| up to 10 kW | approx. 135 l | 12 l heating system expansion vessel |
| 15 kW | approx. 200 l | 18–24 l |
| 20 kW | approx. 270 l | 24–25 l |
| 25 kW | approx. 340 l | 35 l |
| 30 kW | approx. 400 l | 40 l |
Before you buy anything, however, two caveats. First, if you have a wall-hung gas boiler, check its documentation: most such boilers have a factory-fitted vessel with a capacity of around 8 l, which is entirely sufficient for a small system water volume. An additional vessel is only needed when the system volume exceeds the limit specified by the manufacturer (often around 100 litres) or the installation is tall. Second, with underfloor heating and systems with a buffer tank, the system volume may be much greater than the boiler output would suggest—and it must be calculated precisely. We discuss the specifics of such systems in more detail in our article about the underfloor heating circulation pump.
The expansion vessel also operates in a single system with the pump, so if you are modernizing the installation, also take a look at the guide to energy-efficient central heating circulation pumps. And if you prefer quick calculations, you will find a dedicated calculator on our website, designed to help you choose the right vessel. After entering the data, you can easily send your results to our office to receive comprehensive assistance in making your decision.
DHW expansion vessel—which one for a water heater?
A domestic hot water heater also needs its own protection—and a separate one, because a vessel from the heating circuit cannot be used with drinking water. An expansion vessel for a water heater absorbs the increase in volume each time the water is heated and puts an end to the problem of a safety valve that drips after the water is heated. How many litres should an expansion vessel for this purpose have? The industry standard is 5–10% of the heater's capacity—plus a membrane approved for drinking water and a maximum working pressure of at least 8–10 bar:
| Water heater capacity | DHW expansion vessel |
|---|---|
| 100 l | 8 l |
| 150 l | 8–12 l |
| 200 l | 12–18 l |
| 250 l | 18–24 l |
| 300 l | 24 l |
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. In our range, you will find a wide selection of tanks with capacities from 8 l, through 50 l, up to as much as 80 l—so you can choose a vessel for both a small under-sink water heater and a large storage tank in the boiler room.
Expansion vessel too large or too small—which is worse?
A vessel that is too small will not accommodate the entire increase in volume: pressure will fluctuate, the safety valve will discharge water, and the installation will regularly “lose” pressure and require topping up. What about an oversized diaphragm vessel? It only means a higher purchase cost and larger dimensions — it does not cause any hydraulic problems. Therefore, when the capacity is borderline, always round the diaphragm vessel size up. And if your installation is unusual or you are unsure about its system volume — contact our technical department; we will help you choose the right one.
FAQ — frequently asked questions about selecting an expansion vessel
What size diaphragm vessel is suitable for a 200 L water heater?
According to the rule of 5–10% of the heater’s capacity, a 12–18 L vessel with a membrane certified for potable water will be optimal. With higher mains pressure and frequent heating to high temperatures, choose 18 L.
Should there be water in a diaphragm vessel?
Yes — but only on one side of the membrane. The vessel is divided into a water chamber and a gas chamber, so water should be present only in the former. If water comes out instead of air when you press the valve stem (Schrader valve), the membrane is ruptured and the vessel should be replaced. The opposite situation is also normal: in a cold, freshly filled installation, the water side of the vessel may be almost empty — it fills only when the water heats up and expands.
What size diaphragm vessel is suitable for a 300 L water heater?
For a 300 L storage tank, a 24 L vessel is standard. If the installation operates close to the safety valve setting, a vessel one size larger will be safer.
What size diaphragm vessel is suitable for a 500 L / 1,000 L buffer?
Calculate a heating buffer like a central-heating system volume: the vessel should have approximately 10% of the total system capacity, i.e. roughly 50 L for a 500 L buffer and 100 L for a 1,000 L buffer — plus the volume of the rest of the installation. Such large systems should be calculated precisely with an installer.
What is the difference between an expansion tank and a diaphragm vessel?
“Expansion tank” is a colloquial, broader term — historically, it referred to an open tank in gravity-fed systems. A diaphragm expansion vessel is its closed equivalent with a membrane, used in modern pressurized systems.
Where can I buy a diaphragm vessel for central heating or domestic hot water?
You can buy diaphragm vessels, pumps, and accessories from our brands at distributors throughout Poland — find your nearest location using the Dambat sales partner locator.
