PSSR 2000 · Pressure systems

Expansion vessel examination under PSSR

Independent examination of your expansion vessel under a Written Scheme of Examination, by a competent person under PSSR.

An expansion vessel keeps a sealed heating system safe by absorbing the swelling of the water as it heats, on a thin diaphragm with a gas cushion behind it. When that diaphragm fails the cushion is lost and the safety valve starts discharging hot water, which is why a competent examination matters.

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  • Independent and impartial
  • Competent engineer surveyors
  • Reports issued promptly
gas water
Written schemeExamined to a certified written scheme of examination
Diaphragm and chargeThe membrane and gas pre-charge that absorb expansion
Certified reportA written report of examination, within 28 days
Sealed systemPart of a pressurised heating or hot-water system
Pressure equipment we examine

Why your expansion vessel needs PSSR examination

An expansion vessel sits on a sealed, pressurised heating or hot-water system and takes up the expansion of the water as it warms, water swells by roughly four parts in a hundred from cold to hot, so that the system pressure stays steady and the safety valve is not forced to lift. Inside, a flexible diaphragm separates the system water from a gas cushion pre-charged with air or nitrogen.

Where the vessel is part of a pressure system in PSSR scope, Regulation 8 requires a written scheme of examination before it is used. The examination checks the vessel and its diaphragm, the gas pre-charge and its charging valve, and the system safety valve it works with, then reports on condition to the scheme. We examine it independently of whoever maintains the heating plant.

Shell and connection
Diaphragm or bladder
Gas pre-charge
Charging valve
System safety valve
Pressure gauge
Supports and mounting
Nameplate and SWP
How it works

How we examine your expansion vessel

A competent engineer surveyor examines the vessel and the way it works with the system: the shell and connection for corrosion, the diaphragm and the gas pre-charge behind it, the charging valve that sets and holds that pre-charge, and the system safety valve the vessel protects. The marked safe working pressure is checked so the vessel and the system match.

  • 1

    Get in touch

    Tell us the vessel, the heating or hot-water system it serves and whether a written scheme is in place.

  • 2

    On-site examination

    A competent engineer surveyor examines the vessel, its diaphragm and charge and the system safety valve, to the scheme.

  • 3

    Your report

    You receive a written report of examination, any defects and timescales set out, within the statutory window.

Why businesses choose SEIS

  • Independent and impartial: we examine the vessel, we do not supply or service the heating plant
  • Competent engineer surveyors used to sealed heating systems and PSSR
  • The vessel examined as part of the system, not in isolation
  • Certified reports issued promptly, the next examination date flagged
What we examine

Expansion vessel: what a thorough examination covers

Diaphragm or bladder

The membrane that separates the water from the gas, the part that fails most, because once it perforates the cushion is gone and the system has nowhere to put its expansion.

Gas pre-charge

The pre-charge pressure behind the diaphragm, checked against the system, since a vessel that has lost its charge cannot absorb expansion and forces the safety valve open.

Charging valve

The charging valve, often a schrader, checked so the pre-charge can be set and held, and because a damaged valve is also a leak path on a pressurised vessel.

System safety valve

The relief valve the vessel works with, checked so it lifts at the set pressure, the last line when expansion has nowhere else to go.

Shell, connection and supports

The shell, the system connection and the mounting, for corrosion, leaks and the strain of a vessel full of water that has lost its cushion.

Nameplate and safe working pressure

The marked safe working pressure, so the vessel matches the system it protects and the examination is built around the right figures.

Scheme and certification

How it works, and what you receive

Where an expansion vessel forms part of a pressure system in PSSR scope, Regulation 8 requires a written scheme of examination, drawn up or certified by a competent person, before the system is used. The vessel is then examined to the intervals the scheme sets, alongside the system it protects. After each examination you receive a written report on the condition of the vessel and its protective devices, with any actions and timescales, issued within twenty-eight days.

Step oneA Written Scheme of Examination, certified before use
To schemeExamined to the intervals the scheme sets
28 daysA written report, issued within the statutory window
IndependentWe examine it, we do not supply it

You receive a report on the condition of the system and its safety devices, with any actions and timescales set out clearly.

Full statutory cover

Part of our full PSSR inspection service

Expansion vessel is one of the many kinds of equipment we cover. We inspect the full range, across every sector, as an independent provider, one item or a whole site, anywhere in the UK.

See our full PSSR inspection service
Other services

Other statutory inspections we carry out

Many sites run more than one regime. We can examine all of it, under one independent provider.

LOLER inspectionThorough examination and certification of lifting equipment and accessories. PUWER inspectionInspection of work equipment against the five PUWER duties, with a written record. LEV testingLEV thorough examination and test of your extraction systems.
PSSR FAQs

Expansion vessel examination: common questions

Does an expansion vessel need a PSSR examination?
Where it is part of a pressure system in scope, yes. A larger sealed heating or hot-water system can put the vessel within PSSR, and then it must not be operated without a written scheme of examination. You can read the duty in the HSE guidance on the Pressure Systems Safety Regulations.
Do I need a written scheme of examination, or a certificate?
A written scheme. Regulation 8 requires a written scheme of examination, drawn up or certified by a competent person, before the system is used, and the examination produces a certified written report. Our guide to PSSR explains the scheme and what it must contain.
How do I know if my expansion vessel is in PSSR scope?
It depends on the system it serves. A small domestic vessel usually is not, but a commercial or industrial sealed heating system with hot water above its threshold can bring the vessel into scope. We can advise where the line falls and what the written scheme needs to cover.
What is the most common defect on an expansion vessel?
A failed diaphragm. Once the membrane perforates, the gas cushion is lost, the vessel fills with water and the system can no longer absorb expansion, so pressure rises and the safety valve discharges hot water. The diaphragm and the gas pre-charge are at the centre of the examination.
What does the examination cover?
The shell and system connection, the diaphragm and gas pre-charge, the charging valve, the system safety valve tested to confirm it lifts, the pressure gauge, the supports and mounting, and the nameplate and safe working pressure.
Who is competent to examine it?
A competent person with the training, skills, experience and knowledge for the system, independent of its operation. Our engineer surveyors examine sealed heating systems and their vessels regularly and report to the written scheme.
Can you check the gas pre-charge as part of the examination?
Yes. We check the pre-charge against the system and the charging valve that sets and holds it, because a vessel with the wrong charge, or none, cannot do its job and quietly throws the load onto the safety valve.
Do you examine expansion vessels across the UK?
Yes. Our engineer surveyors travel to plant rooms and commercial premises nationwide, a single vessel or a full heating system. Call 0330 043 8191 to arrange a visit around your maintenance.

Is your expansion vessel due an examination?

Talk to an engineer surveyor, get a quote and book your inspection anywhere in the UK.

0330 043 8191 Request a quote