Expansion Vessels

Prevent pressure issues and protect your system with our range of expansion vessels. Fitted to central heating and hot water systems, these units absorb excess pressure caused by water expansion as your boiler heats up. Whether you need a replacement boiler expansion vessel or you’re fitting out a new system, we stock sizes to suit most domestic and light commercial setups. Each expansion vessel is built for reliability, easy to install, and helps extend the life of your system. A must-have for closed-loop heating jobs done right.

Q: What is an expansion vessel and how does it work?

A: An expansion vessel is a sealed tank used in sealed central heating systems to absorb excess pressure as water heats and expands. Inside, a diaphragm separates air from water. As heated water increases in volume, the air side compresses, helping to regulate pressure and prevent damage to pipes, valves, or the boiler.

Q: What types of expansion vessels do you stock?

A: We stock a full range of boiler expansion vessels, including:

• Central heating expansion vessels for sealed systems.

• Potable water expansion vessels for hot water systems.

• Combi boiler expansion vessels, sized for residential use.

• Mini shock arrestors for reducing water hammer.

• Zilmet expansion vessels in a wide range of litre capacities.

You’ll also find boiler mounting brackets, service valves, and vessel kits for safe and easy installation.

Q: What size expansion vessel do I need?

A: Expansion vessels range from 8L to 50L and should be chosen based on your system’s water volume and boiler size. As a rough guide:

• 8–12 litre vessels suit small to medium combi boiler systems.

• 18–24 litre vessels suit larger systems or small commercial setups.

• 35–50 litre vessels are ideal for higher-demand or multi-zone systems.

If you're unsure on sizing, your installer or heating engineer can advise based on system calculations.

Q: How do I know if my expansion vessel needs recharging?

A: Common signs your vessel may need recharging include:

• Frequent pressure loss or pressure too high on your boiler gauge.

• Water discharging from the pressure relief valve.

• The expansion vessel feels full of water with no air at the top.

To recharge, isolate the system, check the vessel’s pressure using a tyre gauge on the Schrader valve, and top up with a hand pump if needed. Always follow manufacturer instructions or consult a heating engineer if you’re unsure.

Q: What pressure should an expansion vessel be set to?

A: Most domestic expansion vessels are pre-charged to 0.75–1.5 bar, but this should match your system’s static pressure. Always check the boiler or heating system manual for the correct setting, and use a pressure gauge when adjusting.

Q: Can you replace just the expansion vessel on a boiler?

A: Yes – if your current vessel is faulty, you can often replace just that part. We stock individual boiler expansion vessels, as well as accessories for wall mounting or remote connection if internal space is limited.

Q: Do I need a different vessel for potable water systems?

A: Yes – potable water expansion vessels are designed for clean, drinkable water and are WRAS-approved. These are typically used in unvented hot water cylinders or systems where hot water is stored under pressure.

Q: How much does it cost to replace a boiler expansion vessel?

A: Replacement expansion vessels start from around £30–£50, depending on size and type. Additional costs may apply for labour. We recommend checking compatibility with your boiler model or asking at your local JT Atkinson branch for guidance.

Q: Do you deliver expansion vessels across the UK?

A: Yes – we offer UK-wide delivery on all expansion vessels and heating accessories. You can also check stock and click and collect from your local branch.