How to choose an expansion vessel (heating and hot water)

In short: an expansion vessel absorbs the increase in water volume as the water heats up. To choose the right one, you need to know what it is for (heating or domestic hot water), how much water the system holds and at what pressure it operates. A vessel that is too small or wrongly pre-charged makes the pressure rise and the safety valve drip.

What does an expansion vessel do?

Water expands when it heats up: between 10 °C and 70 °C its volume increases by about 2.2%, and by about 3.5% up to 90 °C. In a closed circuit, this extra water has to go somewhere. The expansion vessel takes it in: it contains a membrane, with water on one side and pressurised gas (nitrogen or air) on the other. When the water heats up, it pushes the membrane back; when it cools down, the gas pushes it back into the circuit.

Without a vessel, or with a faulty one, the pressure rises with every heating cycle until the safety valve opens, then drops again as the system cools down. The result: lost water, frequent refilling and a system that wears out faster.

Heating or drinking-water vessel: don't mix them up

• Heating vessel: for the closed circuit of radiators or underfloor heating. Examples: Reflex N, Zilmet Zilflex H, Varem Extravarem LR.
• Drinking-water vessel: for drinking water heated in a water heater or boiler. It must be approved for drinking water and withstand the mains pressure. Flow-through models, where the water passes through the vessel, avoid stagnant water and are preferable for hygiene. Examples: Reflex Refix DD, Flamco Airfix A, Zilmet Hydroflex.

Good to know: heating vessels are often red or grey, but this is not a rule (some are white). Always check the intended use on the product page. A heating vessel must never be used for drinking water.

What size for heating?

The size depends on three things: the amount of water in the system, the maximum water temperature and the set pressure of the safety valve.

1. Estimate the water volume of the system

If you don't know it, here are the usual rules of thumb per kW of heating output:

Add the content of any buffer tank: with a heat pump, it often makes the biggest difference.

2. Set the pre-charge pressure

The gas pressure in the vessel (the pre-charge pressure) depends on the height of water above the vessel:

Pre-charge pressure (bar) = height between the vessel and the highest point of the system (m) ÷ 10 + 0.2 to 0.3 bar, and at least 0.7 bar.

Example: vessel in the basement, highest radiator 7 m above → 0.7 + 0.3 = 1.0 bar. Vessels are often supplied pre-charged at 1.5 bar: remember to adjust the pressure before filling the vessel with water.

The cold fill pressure of the system must then be at least 0.3 bar above the pre-charge pressure (in our example: 1.3 bar or more).

3. Calculate the vessel volume

Installers use the following formula:

Vessel volume = (expansion volume + water reserve) × (final pressure + 1) ÷ (final pressure − pre-charge pressure)

• Expansion volume = system volume × 2.2% (water at 70 °C maximum) or × 3.5% (90 °C).
• Water reserve = 0.5% of the system volume, at least 3 litres.
• Final pressure = safety valve set pressure − 0.5 bar, i.e. 2.5 bar for a 3 bar valve.

Worked example

House with modern radiators, 12 kW, water at 70 °C maximum, 3 bar safety valve, highest radiator 7 m above the vessel:

• System volume: 12 kW × 11 l/kW ≈ 130 litres
• Expansion volume: 130 × 2.2% ≈ 2.9 litres, plus the 3-litre reserve = 5.9 litres
• Pre-charge pressure: 1.0 bar; final pressure: 2.5 bar
• Vessel volume: 5.9 × (2.5 + 1) ÷ (2.5 − 1.0) ≈ 14 litres

Choose the next standard size up: an 18-litre vessel. If the boiler heats up to 90 °C, the same calculation gives about 17.5 litres: an 18-litre vessel only just fits, a 25-litre vessel leaves a margin.

A slightly larger vessel is not a problem, whereas one that is too small makes the safety valve drip. If in doubt, take the next size up. For a large system, an apartment building or a heat pump with a big buffer tank, have the calculation checked by an installer.

What size for domestic hot water?

Here the vessel absorbs the expansion of the water in the water heater (about 1.7% between 10 °C and 60 °C). Its size depends on the water heater volume, the mains pressure after the pressure reducing valve and the safety valve (often 6 bar on water heaters).

• Pre-charge pressure = pressure reducing valve setting − 0.2 bar. Drinking-water vessels are often supplied pre-charged at 3 or 4 bar: check and adjust before commissioning.
• Final pressure: at most 80% of the safety valve set pressure, i.e. 4.8 bar for a 6 bar valve.

Calculated minimum volume for water at 60 °C and a 6 bar safety valve:

Choose the next commercially available size above the calculated volume. As you can see, the closer the mains pressure is to the safety valve pressure, the larger the vessel must be. Setting the pressure reducing valve to 3 bar instead of 4 bar allows a vessel about half the size.

Signs of a faulty expansion vessel

• The pressure on the gauge rises sharply when the heating starts, up to nearly 3 bar.
• The safety valve drips or opens regularly.
• The pressure drops when the system is cold and you often have to top up with water.
• Water comes out when you press the gas valve: the membrane is damaged and the vessel must be replaced.

Often the vessel has simply lost its pre-charge: re-inflating it is enough. If the membrane is damaged, it must be replaced.

Is your safety valve dripping? See also our guide Heating safety valve dripping: causes and solutions.

Check your vessel once a year

The pre-charge pressure slowly drops over time. Manufacturers recommend an annual check, ideally before the heating season:

1. Isolate the vessel from the system with its shut-off valve, then drain it on the water side.
2. Measure the pressure at the valve with a pressure gauge.
3. Re-inflate if necessary with a pump or a refill aerosol, then reopen the valve.

If you are not comfortable with these steps, leave them to a professional.

Installing your vessel correctly

• Fit a capped shut-off valve with drain between the vessel and the system, for example a CALEFFI valve. It lets you check or replace the vessel without draining the whole circuit, and the cap prevents accidental closing.
• For heating, connect the vessel on the return, close to the boiler or heat pump, ideally on the suction side of the circulator pump.
• To save time, a connection group combines the valve, safety valve and fittings in a single unit.

Frequently asked questions

What size of vessel for a single-family house?

For a house with radiators and an output of 10 to 15 kW, an 18 to 25-litre heating vessel is usually suitable. With a heat pump and a buffer tank, the water volume is larger: often count on 25 to 50 litres. Use the calculation above to confirm.

Can a heating vessel be used for drinking water?

No. A drinking-water vessel has a membrane approved for drinking water and corrosion protection. A heating vessel is not designed for this use.

Isn't the vessel built into my boiler enough?

Many wall-hung boilers have a small built-in vessel, often around ten litres. It is enough for the system planned by the manufacturer, but not always if the system has been extended or a buffer tank has been added. In that case, an additional vessel is fitted.

How often should the vessel pressure be checked?

Once a year, ideally before the heating season.

Find the right expansion vessel

Browse all our expansion vessels and accessories (valves, brackets, connection groups), as well as our safety valves and pressure gauges.

The calculations in this guide follow the methods of the EN 12828 (heating) and DIN 4807-5 (drinking water) standards. They give a reliable estimate for a house, but do not replace a professional calculation for a large system.