Expansion vessel

1 How it works

Water grows as it heats. In a sealed system there is nowhere for that to go unless something gives way, and the vessel is the part designed to give. Vaillant describes the usual design as a chamber split by a thin rubber diaphragm, with system water on one side and nitrogen on the other, the gas compressing as the water expands².

Vaillant's guidance on pressure loss sums it up: unlike a loft header tank, the vessel absorbs the pressure rise as the boiler heats the system³. A gauge moving a little with heat is what this looks like day to day; see boiler pressure.

2 When a vessel is failing

Vaillant's website lists a thinning, perforated diaphragm as the more common failure, along with air leaking from the valve, and says hard water can damage the diaphragm over time². It names a faulty vessel among the possible causes when a relief valve is letting water out⁴.

The Vaillant 2023 ecoTEC plus manual gives the clearest user-facing clue: if the vessel no longer compensates properly, the pressure relief valve may open already at 2.5 bar⁵. A faulty vessel is one possible cause of symptoms such as these, not the only one, and the diagnosis is for an engineer:

• Pressure swinging widely between cold and hot, or reaching the top of the range when hot.
• Water from the relief pipe outside, see pressure too high.
• Pressure falling after that, then needing repeated top-ups, see pressure keeps dropping.

3 Who may work on it

Vaillant says only a Gas Safe registered engineer may fit a new vessel on a gas appliance, because the appliance has to be partly dismantled². Regulation 3 of the Gas Safety (Installation and Use) Regulations 1998 sets the competence rule for England, Wales and Scotland; who can work on a boiler covers other parts of the UK⁶. Do not take the casing off to look for it.

The 5-10 year figure is from a maker's web page and is not a service interval. Vaillant itself says some last longer².