How do you inflate an expansion vessel? This is most often asked by people dealing with regularly dropping boiler pressure, dripping safety valves, and a fluctuating pressure-gauge needle. All three symptoms usually have one cause: loss of pressure in the expansion vessel’s air cushion. An expansion vessel is a tank divided by a diaphragm into a water section and an air cushion, which absorbs the increase in water volume as it heats up. If the cushion loses pressure, every time the system heats up, water is discharged through the safety valve. In this article, we explain how to calculate the correct pressure for your system, how to measure it properly, and what to remember when topping up the vessel.
What pressure should there be in the expansion vessel?
In a typical single-family home, the correct pressure in the C.H. expansion vessel is 0.7–1.2 bar, depending on the system height. Calculate the exact value as follows: divide the height in metres by 10 and add a 0.2-bar reserve — for a seven-metre-high house, this gives 0.9 bar. Remember that the pre-charge pressure in the expansion vessel is the gas-side value, not the reading on the boiler pressure gauge — you can obtain a reliable measurement only after draining the water from the vessel.
| System height above the vessel | Pre-charge pressure (air) | Filling pressure (system cold) |
|---|---|---|
| up to 5 m | 0.7 bar | 0.9–1.0 bar |
| 5–7 m | 0.9 bar | 1.1–1.2 bar |
| 7–10 m | 1.2 bar | 1.4–1.5 bar |
| 10–12 m | 1.4 bar | 1.6–1.7 bar |
| 12–15 m | 1.7 bar | 1.9–2.0 bar |
| above 15 m | height ÷ 10 + 0.2 bar | pre-charge + 0.2–0.3 bar |
Measure the height from the vessel connection, usually located near the boiler, to the highest point of the system — not from the boiler-room floor or ground level. A 0.2-bar reserve maintains positive pressure at the highest radiator and follows the principle used in Polish design practice under PN-B-02414:1999.
Two limits worth remembering. The pre-charge pressure must not be raised above 4 bar — this is the limit common to all our tanks, specified in the operating instructions. The system filling pressure, in turn, should be at least 0.5 bar lower than the safety valve setting. Especially in older systems, the setting may be unknown to the user and impossible to verify. In such situations, installing a complete safety group matched to the system’s requirements is an excellent solution. For a standard single-family home, the appropriate choice is the SGB 2.5 bar safety group. Its installation provides a reliable reference point: the setting is known and marked, so the filling pressure should not exceed 2.0 bar.
The correct pressure in a domestic hot-water diaphragm tank is set differently — it does not depend on the height of the building, but on the supply water pressure. Set it 0.3 bar below the pressure-reducing valve setting: with this difference, the tank always has a small water reserve and responds to every increase in volume instead of activating only after a noticeable rise in pressure.
| Pressure-reducing valve setting | Pressure in the domestic hot-water diaphragm tank |
|---|---|
| 3.0 bar | 2.7 bar |
| 3.5 bar | 3.2 bar |
| 4.0 bar | 3.7 bar |
| Series | Application | Factory precharge pressure |
|---|---|---|
| IBO HEAT C.O. BASIC | central heating | 1.5 bar |
| IBO ITALY HEATS | central heating | 1.5 bar |
| IBO ITALY SOLAR | solar systems | 2.5 bar |
| IBO ITALY C.W.U. | domestic hot water | 3.5 bar |
| IPRO FIX | central heating and domestic hot water | 2.5 bar |
| IBO Fiberglass | composite tanks | 0.2–2.0 bar |
None of these values may be increased above 4 bar.
If the system does not have a pressure-reducing valve, the supply pressure changes along with the pressure in the water supply network, so a fixed setting cannot be derived from it. In that case, it is worth installing one — we described how to select and adjust it in a separate guide about the pressure-reducing valve in the system. The safety valve in domestic hot-water systems is usually set to 6 bar.
Choosing the pressure is not the only factor that affects the proper operation of a diaphragm vessel. Choosing the right capacity is equally important — with a tank that is too small, even correctly set pressure will not stabilize the system. You can calculate the capacity using our sizing calculator, and we explained the differences between the individual series in the article about IBO expansion tanks for central heating and domestic hot water. Important: a diaphragm tank for central heating is not suitable for domestic hot water — it has a different factory pressure and a different operating temperature range.
How to check the pressure — and the mistake most people make
Measuring when the system is filled does not show the air pressure, but the water pressure exerted on the diaphragm. This is where the most common error comes from: the inflated reading leads you to assume that the diaphragm tank is pressurized, even though the air cushion has long since deflated.
How to check the pressure in a diaphragm expansion vessel — four steps
- Turn off the boiler and wait for the system to cool down.
- Close the valve isolating the vessel from the system.
- Drain the water from the water side until the pressure drops to zero.
- Only now place the pressure gauge on the valve.
You can read the correct pressure only in a cold system, using an instrument with the appropriate range. 1/4" 0–4 bar radial pressure gauge shows tenths of a bar — on a 0–10 bar scale, this difference disappears.
How to inflate a diaphragm expansion vessel step by step
You can inflate the vessel through the valve with the tank isolated from the system and emptied of water, to a value determined by the height of the building. With a quick connector, this takes about fifteen minutes; without one, you need to drain the system, extending the work to an hour. You need a pressure gauge with a range of up to 4 bar and a source of compressed air. The valve is a Schrader type, the same as on a bicycle wheel, so a hand pump, a 12 V car pump, or a compressor will work. A 24-litre vessel requires several dozen pump strokes, while a 100-litre vessel requires several hundred — in that case, a compressor is the only sensible option.
The limitation of a pump is not its pressure, but its gauge. A typical bicycle pump reaches 6–8 bar, many times more than the required 1–3 bar. However, its built-in scale may be approximate and, at values around 1 bar, downright misleading. Treat the pump merely as a source of air, and read the pressure using a separate instrument. This is the difference between inflating by guesswork and setting up the system correctly.
The pump delivers moist air, while our instructions specify dry air or nitrogen. This does not matter when correcting the pressure by a few tenths of a bar; for the initial filling, however, it is better to use a source with a moisture separator, because moisture and oxygen accelerate diaphragm ageing.
The pre-charge pressure of a new vessel is set before it is screwed into the system. IBO C.O. BASIC and IBO ITALY HEATS models leave the factory set to 1.5 bar, which corresponds to a 13-metre installation. In a typical 7-metre-high house, the air therefore needs to be released, not added. C.W.U. and IPRO FIX versions operate in different systems and have higher settings — you can find the value for your series in the factory pressure table.
How to inflate a central-heating expansion vessel
- Turn off the boiler and wait for the system to cool down.
- Close the shut-off valve beneath the vessel.
- Drain the water to zero.
- Measure the pressure at the Schrader valve.
- Bring it to the value shown in the table — inflate it with a pump or compressor, or release air by pressing the valve stem.
- Open the shut-off valve and fill the system to a pressure 0.2–0.3 bar higher than the pre-charge pressure just set in the vessel.
- Bleed the radiators and adjust the filling pressure.
How to inflate a DHW expansion vessel
- Turn off the water heater’s power supply and wait until the water cools down.
- Shut off the cold-water supply to the water heater.
- Open the nearest tap to release the pressure in the system.
- Close the shut-off valve and drain the water from the tank.
- Set the value 0.3 bar lower than the pressure-reducer setting.
- Open the valves and restore the power supply.
Warning: The system operates under pressure and contains hot water. Do not exceed 4 bar on the gas side — exceeding this value may void the warranty. The range is narrower in hydrophore tanks: operation below 1 bar or above 3 bar damages the diaphragm, and its replacement is then chargeable only. You can inflate a tank installed in the system yourself. Work on a gas boiler, including the vessel built into the boiler, requires G3 gas qualifications and service authorization.
How to drain water from an expansion vessel and bleed the system
Drain the water from the vessel itself through the shut-off valve under the expansion tank, without interfering with the rest of the system.
How to drain water from an expansion vessel — four steps
- Turn off the boiler and wait until the system cools down — the water is hot and remains under pressure.
- Close the shut-off valve under the tank.
- Unscrew the union connecting the vessel to the system.
- Wait until the pressure in the water section drops to zero.
This procedure is often confused with two others. Draining water from the entire central-heating system is necessary only when the vessel does not have its own valve — in that case, the operation takes hours and ends with refilling the system. Bleeding involves removing air from radiators and pipes, which is a completely different issue discussed in our guide to bleeding central-heating and booster pumps.
Water in an expansion vessel — when it is normal and when it indicates a failure
Water flowing from the Schrader valve indicates a damaged diaphragm. The diaphragm has stopped separating the water section from the air cushion, and the device no longer serves its purpose — the expansion vessel must be replaced or, in models with a replaceable diaphragm, the diaphragm itself.
The mere presence of water in the water section is normal — in a cold system, the water section may be almost empty and only fills up once the system heats up. A failure is indicated by water on the air side, not on the water side.
Before replacing the diaphragm tank, check which series you have, as the scope of the repair varies. IBO C.O. BASIC tanks and IBO ITALY tanks have replaceable diaphragms. The IPRO FIX series has a non-replaceable butyl membrane covered by a 5-year warranty — for these tanks, replacing the diaphragm tank means replacing the entire device. You can find the full range in the diaphragm tanks category.
After replacing the diaphragm tank and filling the system, it is worth performing a leak test — the PRO 50 manual pressure-testing pump is designed for this purpose. Hydrofor systems, on the other hand, use different values and a different adjustment logic, which we described in the article about why properly setting the diaphragm in a hydrofor tank is crucial.
How often to check the pressure and what a service quick coupling does
For central-heating tanks, we recommend checking the pressure at least once every three months — the correct pressure can drop unnoticed, without causing any symptoms in the system. Thanks to their diaphragm design, IPRO FIX tanks only need to be checked once a year, making them practically maintenance-free. In both cases, these are minimum intervals.
To make regular checks easier, it is worth considering this already at the installation stage. A tank connected without its own shut-off valve requires the entire system to be drained, refilled, and the radiators bled each time it is checked. The ZSV 3/4" service quick coupling reduces this to about a dozen minutes — isolate the tank itself, drain the water from it, and take the measurement. The best time to install it is when replacing the tank, when the system is empty anyway.
FAQ — frequently asked questions about pressure in a diaphragm tank
How do you add air to a diaphragm tank?
Isolate the device from the system using the shut-off valve and drain all the water from it. Place the gauge on the Schrader valve and check the pressure on the gas side. If it is too low, add air until it reaches the level corresponding to the system’s height. Perform this procedure only when the system is cold.
How do you top up the pressure in a diaphragm expansion tank?
The pressure should be topped up to a value determined by the height of the installation: divide the number of metres by 10 and add a 0.2-bar reserve. The tank must be isolated and drained of water; otherwise, the instrument will show the water pressure instead of the air pressure. The manufacturer recommends dry air or nitrogen.
What pressure should a domestic hot-water diaphragm expansion vessel have?
Set the value 0.3 bar below the pressure-reducer setting. With a 3.0-bar reducer, this gives 2.7 bar; with a 4.0-bar reducer, 3.7 bar. The building height is irrelevant here; only the incoming water pressure matters. In a system without a pressure reducer, it is worth installing one, because otherwise there is no fixed reference point.
Can you inflate a diaphragm expansion vessel with a bicycle pump?
Yes. The valve on the vessel is a Schrader valve, identical to the one on a bicycle wheel, and a pump reaches 6–8 bar compared with the required 1–3 bar. However, do not rely on its built-in gauge — at 1 bar, it is only approximate. Read the pressure with a separate instrument and treat the pump solely as a source of air. For tanks of 50 litres or more, a car pump is more convenient, while for 100-litre tanks, use a compressor.
What does excessive pressure in a diaphragm expansion vessel mean?
Excessive pre-charge pressure causes the device to accept water too late — the system quickly reaches the safety valve setting, and the valve starts releasing water. The most common cause is factory-set: new central-heating tanks are set to 1.5 bar, which corresponds to a thirteen-metre installation, while a typical home has seven metres. To remedy this, isolate the expansion vessel from the system, drain the water from it, and release excess air through the Schrader valve by pressing the pin until the pressure gauge shows the value from the table.
What should you do if the pressure drops after a week?
A weekly pressure drop indicates three possible causes: a leak in the system, a leaking valve, or a damaged diaphragm. Check in order whether the safety valve is dripping, whether water is coming out of the valve, and whether the fittings are damp. Water at the valve means that the diaphragm expansion vessel must be replaced.
How do you top up the pressure in a diaphragm expansion vessel?
Topping up involves pumping air into the gas side with the tank isolated and drained — do not confuse this with adding water to the system, which raises the reading on the boiler but does not change the setting in the expansion vessel. If you are unsure which action to take, contact our technical department.
