Water Regulations · 9 min read
Updated 28 May 2026
Expansion Vessels and Sealed System Pressure
Every sealed plumbing system — sealed central heating, unvented hot water — needs an expansion vessel to absorb the volume increase as water heats. The vessel uses a rubber diaphragm with pre-charged air on one side; as water expands, it pushes the diaphragm and compresses the air. Set the pre-charge right and it works quietly for years; get it wrong, or let the air leak out, and you'll see climbing pressure, a weeping pressure relief valve, or a steady drip from the tundish.
Expansion vessels are the unsexy part of any sealed system but they're where most nuisance failures start. This guide covers what they do, the right pre-charge values, the failure symptoms, and the operating pressures you should see on a healthy system. For the wider context on unvented cylinder safety, read G3 unvented hot water cylinders.
Why expansion has to be absorbed
Water expands as it heats. The numbers look small but the effect is significant in a closed vessel:
| Temperature change | Volume expansion | Example — 200 L cylinder |
|---|---|---|
| 10°C → 60°C | ~1.7% | +3.4 L |
| 10°C → 80°C | ~2.9% | +5.8 L |
| 10°C → 95°C (overheat) | ~4.0% | +8.0 L |
No vessel = no system
How the vessel works
Inside the steel shell is a rubber diaphragm dividing the vessel into two chambers:
- Air side — pre-charged through a Schrader valve (like a car tyre valve) to a known pressure.
- Water side — connected to the system. As water expands, it pushes the diaphragm and compresses the air on the other side.
For the vessel to actually accept water, the system pressure must exceed the air-side pre-charge. If the pre-charge is set too high, water can't get in and the vessel contributes nothing. If it's set too low, the vessel fills with water at cold fill and has nothing left to absorb on heating. Pre-charge has to match the cold fill pressure of the system.
Setting the pre-charge
| System type | Typical pre-charge | Cold fill / operating pressure |
|---|---|---|
| Sealed central heating (small/medium domestic) | 1.0 bar | Cold fill 1.0-1.2 bar; hot 1.5-2.0 bar; PRV ~3 bar |
| Sealed central heating (large / multi-storey) | 1.5 bar | Cold fill 1.5 bar; hot 2.0-2.5 bar |
| Unvented hot water cylinder | 3.0-3.5 bar | Cold mains pressure at inlet group (typically 3.0-3.5 bar after reducing valve) |
| Commercial pressurisation set | Variable | Set per design — match cold static pressure at the vessel |
The pre-charge is always set with the vessel isolated and depressurised on the water side. Trying to "top up" a vessel through its valve while it's still full of water reads garbage and achieves nothing. On an unvented cylinder service, isolating, draining to the vessel, checking and resetting pre-charge is one of the standard annual jobs.
How vessels fail
Two main failure modes:
- Lost air charge — slow leak through the Schrader valve or diaphragm. The vessel fills with water at cold and has no capacity to absorb expansion. Most common.
- Ruptured diaphragm — air and water mix. Pressing the Schrader valve releases water rather than air; the vessel is dead and needs replacing.
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Symptoms of a failed vessel
- 1
Pressure gauge that swings dramatically
Cold the system reads 1.0-1.2 bar; the moment it heats, it climbs to 2.5, 3.0 bar and the PRV opens. That's the textbook failed-vessel pattern on sealed central heating.
- 2
Visible discharge from the boiler PRV pipe outside
A small puddle or a drip under the white plastic discharge pipe on the outside wall whenever the boiler runs. Often blamed on the PRV itself; usually the PRV is fine and the vessel behind it has failed.
- 3
Steady drip from the tundish on an unvented cylinder
Every time the cylinder reheats, you see water trickling through the tundish. Failed expansion vessel (or, on bubble-top designs, a lost bubble) is the first thing to check.
- 4
Repeated need to top up
If you're refilling a sealed heating system every week and there's no visible leak, the PRV has been quietly venting through a failed vessel.
- 5
Press the Schrader valve and water comes out
Definitive test for a ruptured diaphragm. Replace the vessel — there's no fix.
Checking a vessel — the quick method
- 1
Note the system pressure cold
On a sealed heating system, that's the pressure gauge with the boiler off and the system cool. Should be around 1.0-1.2 bar.
- 2
Isolate and depressurise the water side
On heating: isolate the boiler from the system, drain a small amount via the drain-off. On unvented HW: turn off the cold mains feed, open a hot tap, drain enough to drop pressure to zero at the vessel.
- 3
Check pre-charge via the Schrader valve
Use a dry-pressure (tyre) gauge. Should match the system cold fill — 1.0 bar heating, 3.0-3.5 bar unvented HW.
- 4
Re-charge if low; replace if water comes out
Top up with a foot pump or compressed air to the target. If water comes out instead of air, the diaphragm is ruptured — replace the vessel.
- 5
Recommission
Close drains, refill the system to the right cold pressure, vent radiators, run and check the pressure swing on heating.
Ninja Tip
Sizing — quick sanity check
On a sealed central heating system the rule of thumb is that vessel volume should be around 10% of total system water volume for a system filled at 1.0 bar and operating to 80°C. On an unvented cylinder, the manufacturer specifies the vessel — typically about 10-12% of cylinder volume (a 200 L cylinder needs around a 24 L vessel). Undersized vessels behave like failed ones: the system can't absorb expansion and the safety valves end up doing the work.
Frequently Asked Questions
What does an expansion vessel do?
It absorbs the volume increase when water is heated in a sealed system. Water expands about 1.7% from 10°C to 60°C; in a sealed cylinder or heating system that extra volume has nowhere to go and pressure would rise sharply. The vessel has a rubber diaphragm with pre-charged air on one side; as the system heats, water pushes the diaphragm and compresses the air, holding pressure within safe limits.
What pre-charge pressure should an expansion vessel be set to?
On a sealed central heating system, typically 1.0 bar — matching the cold fill pressure. On an unvented hot water cylinder, typically 3.0-3.5 bar — matching the cold mains pressure at the inlet group. The pre-charge is always set with the vessel isolated and depressurised on the water side. Wrong pre-charge is the most common cause of nuisance discharge.
How do I know my expansion vessel has failed?
Three classic signs. On heating: cold pressure sits OK at around 1 bar but climbs to 2.5-3 bar when hot and you see water from the boiler PRV pipe outside. On unvented hot water: a slow steady drip from the tundish when the cylinder is heating. On either: a pressure gauge that's far more sensitive than it should be — small changes in temperature cause big pressure swings.
What pressure should a sealed heating system run at?
Cold fill around 1.0-1.2 bar. Hot operating pressure should rise to about 1.5-2.0 bar. The pressure relief valve typically opens at 3 bar. If your gauge sits at 0.5 bar cold you have a leak or a topping-up issue; if it climbs above 2.5 bar when hot the expansion vessel is failing or undersized.
Sources & further reading
Guidance only. This article is general information for UK readers, not a substitute for a site-specific assessment by a competent person. Regulations and best practice change — always check the current official guidance and, for compliance work (Legionella risk, unvented cylinders, water regulations), use a suitably qualified professional. Pipe Assassin is an electric-boiler and water-hygiene specialist and is not Gas Safe registered; we do not carry out gas work.
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