Expansion vessel
In plumbing
A steel vessel divided inside by a flexible diaphragm, with system water on one side and a cushion of air or nitrogen on the other. As heated water expands, the cushion compresses and takes up the extra volume. Once the cushion is lost, the pressure jumps when the system heats and the relief valve may open.
Using Expansion vessel in a plumbing system
Updated
A distinction worth remembering
Most heating faults make sense once you picture the loop: out hot along the flow, through each radiator, back cooler along the return. Then ask how the system is kept full, from a loft tank or through a sealed vessel and gauge.
Questions to take further
- How can I tell whether my heating is sealed or open-vented?
- A sealed system usually has a pressure gauge on or near the boiler and no small tank in the loft feeding the heating. An open-vented one has that loft tank and no pressure gauge to watch.
Sources
- Skills England: plumbing and domestic heating technician — The systems, materials, planning and customer responsibilities in the England occupational standard.
- HSE: domestic gas safety FAQs — Gas Safe registration and checking the categories of work on an engineer's ID card.
Where Expansion vessel fits in the learning path
The Central Heating section covers these topics:
- Wet Central Heating Systems
- Radiators and Heating Controls
- Heating Pressure and Safe Fault Checks
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- Wet Central Heating SystemsCentral Heating
- Heating Pressure and Safe Fault ChecksCentral Heating
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A heating system with no loft cistern, filled from the mains through a filling loop and kept under pressure. An expansion vessel absorbs the water's expansion as it heats, a gauge shows the pressure, and a relief valve opens if that pressure climbs too high.
Sealed heating system
Read the Sealed heating system entryCompare related terms
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Sealed heating system
A heating system with no loft cistern, filled from the mains through a filling loop and kept under pressure. An expansion vessel absorbs the water's expansion as it heats, a gauge shows the pressure, and a relief valve opens if that pressure climbs too high.
Read the Sealed heating system entryPressure relief valve
A safety valve on a sealed heating system that opens, commonly at about 3 bar, releasing water so the pressure cannot climb to a dangerous level. Its discharge pipe usually ends outside. Water dripping from it is a sign that the pressure has been too high, or that the valve no longer seals.
Read the Pressure relief valve entryBefore you start: some plumbing work is regulated or notifiable
Some water fittings work needs notice to, and consent from, the water supplier before it starts. Check the rules where you live.
Gas work is for competent workers covered by a Gas Safe registered business in Great Britain. Northern Ireland has separate gas regulations. Do not work on gas pipes or appliances yourself.
In England, installing an unvented hot-water system over 15 litres is notifiable building work: use a competent installer registered with a competent-person scheme, or tell building control before work starts. Leave this work to a suitably skilled person.
Wales, Scotland and Northern Ireland have their own water and building rules; check which apply before work starts. Electrical connections belong with an electrician.
This guide does not qualify you or make you competent, approved or registered for plumbing, gas or unvented work.
A definition is a start. Using it in a real question is where it sticks.