Compatible expansion vessels for boilers and closed heating systems. Choose by brand compatibility, volume (litres), connection type, and pre-charge pressure. Trade prices after login.
Choose by
- Compatible brand: Vaillant · Bulex · Saunier Duval · Universal
- Volume (litres): 5 L · 8 L · 10 L · 12 L · 18 L · 24 L
- Connection: internal thread 3/4″ · flat seal · flange · other
- Pre-charge pressure: 0.5 bar · 0.75 bar · 1.0 bar · adjustable
What is an expansion vessel and why does it fail?
The expansion vessel absorbs the volume change of the heating water when it heats up and cools down. A membrane separates a water side from a pressurised gas side. When the membrane fails or the pre-charge pressure drops, system pressure swings, the safety valve leaks, and the boiler locks out on pressure.
How to choose the right one
- Match the volume to the system, not just the boiler. A larger installed volume needs a larger vessel.
- Match the connection to what’s on the boiler or system: internal thread with a flat seal, flange, or a fitting kit.
- Set the pre-charge pressure at install to the value your boiler manufacturer specifies (typically 0.5-1.0 bar). Adjustable vessels are useful when the target value is uncommon.
- Check compatibility against the boiler model, not just the brand. Our product pages list VERIFIED and HIGH-CONFIDENCE model matches.
Product grid
Compatibility context
Where a product page lists compatibility with selected models of Vaillant, Bulex or Saunier Duval, every match is either VERIFIED (confirmed via an official technical source or our own testing) or HIGH CONFIDENCE (multiple reliable sources agree).
Frequently asked questions
Which volume should I fit?
Roughly ten percent of the total system water volume, minimum, or the value the boiler manufacturer specifies. Undersized vessels are the most common cause of “vessel replaced, pressure still swings”.
Can I replace a Vaillant expansion vessel with a compatible one?
For selected models, yes. Our product pages state exactly which models are covered, at what confidence level, and which OEM references are replaced.
How do I set the pre-charge pressure?
With no water pressure in the vessel. Deflate to zero, then charge to the value the boiler manufacturer specifies for that model.