What Size Expansion Vessel Do You Need at Home?
A hot-water cylinder can appear to be working perfectly until the expansion vessel is undersized, flat or incorrectly charged. Then the pressure-relief valve may discharge water every time the cylinder heats. If you are asking what size expansion vessel you need, the answer is based on more than the litre capacity printed on the cylinder. System pressure, stored-water temperature and the safety controls all matter.
For an unvented hot-water system, the vessel must be sized to absorb the extra volume created as cold water heats. It is a small component with a vital job: keeping pressure within the intended operating range rather than allowing the relief valve to open unnecessarily.
What does an expansion vessel do?
Water expands when heated. In an open-vented system, that expanded water can rise into a feed and expansion cistern. An unvented cylinder is sealed from the mains by a check valve or combination valve, so it needs an expansion vessel instead.
Inside the vessel is a flexible diaphragm. One side contains air or nitrogen at a set pre-charge pressure; the other accepts expanding potable water from the cylinder. As the water heats, it compresses the gas cushion and limits the pressure rise.
This is not simply a convenience feature. A correctly selected and charged vessel helps prevent repeated discharge from the expansion relief valve, avoids wasted water and protects the cylinder’s safety controls from avoidable stress.
What size expansion vessel is right for an unvented cylinder?
Start with the cylinder’s actual stored-water volume, then allow for water expansion between its cold incoming temperature and its maximum operating temperature. The calculation must also account for the pressure-reducing valve setting, vessel pre-charge and the expansion relief valve setting.
As a broad guide, water heated from around 10°C to 65°C expands by approximately 1.9%. A 150-litre cylinder therefore produces roughly 2.85 litres of expanded water. A 210-litre cylinder produces around 4 litres. That does not mean you can fit a 3-litre or 4-litre vessel, because a vessel cannot use all of its stated capacity for water expansion.
Its usable acceptance volume depends on the pressure range. In a typical domestic unvented arrangement, the cold supply may be regulated to 3 bar and the expansion relief valve may operate at 6 bar. With a correctly matched 3 bar pre-charge, only part of the vessel’s nominal volume is available before the system reaches its relief setting.
For that reason, a 12-litre vessel is often used on larger domestic cylinders, but it is not a universal answer. A compact cylinder, lower stored temperature or different valve settings may need a different size. Larger cylinders, higher temperatures and more demanding pressure conditions can require an 18-litre vessel or more.
The cylinder manufacturer’s installation instructions take priority. They specify the required vessel capacity and safety-valve arrangement for that model, and should be followed rather than substituted with a general rule of thumb.
A simple sizing example
Take a 210-litre cylinder heated from 10°C to 65°C. Its water expansion is approximately 4 litres. With a 3 bar cold pressure and a 6 bar relief setting, a vessel’s theoretical acceptance factor is about 43% of its nominal volume.
Dividing 4 litres by 0.43 gives a minimum calculated vessel volume of roughly 9.3 litres. In practice, the next suitable standard vessel size may be 12 litres, provided this agrees with the cylinder manufacturer’s requirements and the system controls.
The calculation changes if the pre-charge, incoming pressure, relief-valve setting or maximum stored temperature changes. That is why matching a vessel solely to the cylinder’s litre capacity can give the wrong result.
The information you need before choosing a vessel
A reliable selection begins with the data plate and installation manual, not a visual estimate. Confirm the cylinder capacity in litres and its maximum operating temperature. Check the incoming mains pressure and the setting of the pressure-reducing valve, if fitted. Then identify the expansion relief-valve pressure and the vessel’s specified pre-charge.
The vessel must also be suitable for domestic hot water. Do not assume a vessel intended for a sealed central-heating circuit is interchangeable with one for potable water. Heating-system vessels can have different diaphragm materials, connections, pressure ratings and fluid compatibility requirements.
For a UK unvented cylinder, also make sure the vessel is compatible with the manufacturer’s control group and approved for the installation. The cold-water expansion connection is normally made on the correct side of the check valve, as set out in the cylinder instructions. Positioning it in the wrong place means it cannot absorb the expanded water effectively.
Why the vessel pre-charge matters as much as its size
A correctly sized expansion vessel will still underperform if its air charge is wrong. Pre-charge is checked with no water pressure acting on the vessel side. If you test it while the system remains pressurised, the reading is not reliable.
If the pre-charge is too low, the vessel can take in water too early and lose useful expansion capacity as the cylinder heats. If it is too high, water may not enter the vessel until system pressure rises excessively. Both situations can lead to discharge from the relief valve.
A failed diaphragm creates a similar symptom. If water comes from the air valve when briefly checked by a competent installer, the vessel has failed and needs replacement. Repeatedly re-pressurising a defective vessel will not solve the underlying problem.
Signs your current expansion vessel may be too small or faulty
The most common sign is water dripping or flowing from the expansion relief discharge pipe while the cylinder heats, even though it stops once the water cools. You may also see pressure rising sharply during a heat-up cycle, hear intermittent discharge outdoors, or notice a pressure gauge moving close to the relief setting.
Do not cap, block or alter a discharge pipe to hide the issue. Relief-valve discharge is a safety warning that needs investigation. It may indicate an undersized vessel, lost pre-charge, a failed diaphragm, excessive incoming pressure or a problem with the control valves.
Expansion vessels for sealed heating systems are different
The phrase expansion vessel is also used for sealed central-heating systems. The principle is similar, but the sizing method is not identical. A heating vessel is based on the total volume of system water - including radiators, pipework, boiler or heat source, buffer tank and any underfloor-heating circuits - as well as the temperature range and safety-valve pressure.
A domestic hot-water vessel serves the cylinder and potable-water side of the system. A heating vessel serves the closed heating circuit. Where a home has both an unvented cylinder and a sealed heating system, it may need two separate vessels. Each must be correctly rated, installed and charged for its own circuit.
This distinction matters when adding a buffer tank or changing to an air-source heat pump. Extra water volume on the heating side can alter the required heating expansion capacity, but it does not automatically change the vessel needed for the hot-water cylinder.
When to ask an installer before ordering
Replacing an existing vessel with the same approved size and specification is usually straightforward for a qualified professional. A change in cylinder capacity, conversion from vented to unvented hot water, new pressure controls or a heat-pump installation deserves a full system check.
Unvented hot-water work must comply with the relevant Building Regulations and manufacturer instructions. In England and Wales, work on an unvented hot-water storage system is generally carried out by a competent person with the appropriate qualification, with notification requirements handled where applicable. Local rules may differ elsewhere in the UK.
Have the cylinder model, vessel size, valve settings and photos of the existing installation ready before seeking advice. That gives an installer or supplier enough information to identify whether you need a direct replacement or a properly recalculated specification. Modern Heat can help customers identify suitable TESY accessories from the published product and cylinder information, but the final installation decision should always follow the system manufacturer’s instructions.
A vessel is inexpensive compared with the disruption caused by repeated relief-valve discharge or an incorrectly commissioned unvented cylinder. Choose from confirmed system data, allow for the real pressure range and have the installation checked properly - especially where hot water is relied on every day.