Expansion Vessels Explained: How to Size, Check, and Replace Them

If a sealed heating system keeps losing pressure or the relief valve keeps venting, nine times out of ten the expansion vessel is the problem. It’s one of the most overlooked components in a sealed system — and one of the most important. This guide covers how expansion vessels work, how to size them correctly, when to replace them, and what to stock on the van.

What Does an Expansion Vessel Do?

Water expands when it heats up. In an open-vented system, that extra volume escapes into a feed-and-expansion tank in the loft. In a sealed system there’s no tank — the expansion vessel absorbs that volume instead.

Inside the vessel is a rubber diaphragm that separates two chambers: one side contains nitrogen gas pre-charged to a set pressure, the other side connects to the system water. As water expands, it compresses the gas side. When the system cools, the gas pushes the water back. Without a working vessel, expanding water has nowhere to go — pressure rises rapidly and the safety relief valve (PRV) dumps water to drain every time the boiler fires.

A waterlogged vessel — where the diaphragm has failed and the gas has escaped — is the most common cause of repeatedly dropping system pressure. The fix is straightforward: replace the vessel and re-pressurise the system.

Heating Vessels vs Potable Water Vessels

There are two distinct types and they are not interchangeable.

Heating expansion vessels (typically red) are designed for closed central heating circuits. They do not need to be WRAS approved because the water inside is not for drinking. The AquaSystem VR5 6Lt Sealed System Vessel and the AquaSystem VR24 24Lt Sealed System Vessel are typical examples — available from APM in Acton for same-day collection.

Potable expansion vessels are designed for unvented hot water systems (megaflow-type cylinders and pressurised cold water supplies). These must be WRAS approved because they contact drinking water. The AquaSystem AR18 18Lt Potable Expansion Vessel and the AquaSystem AR12 12Lt Potable Expansion Vessel carry full WRAS approval. Never fit a red heating vessel on a drinking water circuit — the rubber compound is not rated for potable contact.

How to Size an Expansion Vessel

Undersizing is the second most common mistake after fitting the wrong type. The correct vessel size depends on three things: total system water volume, the maximum operating temperature, and the system’s fill and relief valve pressures.

The full BS EN 12828 calculation is:

Ve = V × n / (1 – (P0 + 1) / (Pmax + 1))

Where:

• Ve = required expansion vessel volume (litres)
• V = total system volume (litres)
• n = expansion factor (0.034 at 70°C, 0.0576 at 90°C)
• P0 = vessel pre-charge pressure (bar, typically 0.5 bar for most domestic systems)
• Pmax = maximum working pressure before PRV lifts (typically 3 bar)

For a typical domestic sealed heating system with 100 litres of water volume, operating at 70°C, with a 0.5 bar pre-charge and 3 bar PRV:

Ve = 100 × 0.034 / (1 – 1.5/4) = 3.4 / 0.625 = 5.4 litres

So a 6L vessel covers it — the AquaSystem VR5 6Lt is the go-to for small domestic systems. A 150-litre system at the same parameters needs around 8 litres, making the AquaSystem VR18 18Lt a sensible choice with safety margin. Larger systems — multi-zone, underfloor, or commercial domestic — should use the AquaSystem VR24 24Lt.

When in doubt, go bigger. An oversized vessel causes no harm; an undersized one means the PRV keeps lifting and the homeowner keeps calling you back.

Pre-Charge Pressure: How to Check It

The pre-charge pressure in the gas chamber must match the static head of the system — typically 0.5 bar for most domestic installations (equivalent to approximately 5 metres of head). Manufacturers supply vessels pre-charged to 1.5 bar from the factory; you will need to adjust this before installation.

To check the pre-charge:

1. Isolate and drain the vessel from the system (or drain the whole system if the vessel has no isolation valve)
2. Attach a Schrader valve pressure gauge to the valve on the vessel — the same type used for car tyres
3. The reading is the nitrogen pre-charge. If no gas reads at all, the diaphragm has failed
4. Add nitrogen or adjust with a pump to reach the correct static head pressure

Never check the pre-charge with system pressure present — you’ll get a combined reading that means nothing. Always drain the water side first.

Signs a Vessel Needs Replacing

A failed expansion vessel is almost always the cause when:

• System pressure drops every time the boiler fires then recovers when cold
• The PRV discharges water regularly (brown staining below the relief valve outlet)
• Water comes from the Schrader valve when you press the pin — the diaphragm has ruptured
• The vessel feels uniformly heavy — a healthy vessel should feel lighter than expected because half of it is gas

If the diaphragm has failed, replacement is the only fix. You cannot repair a ruptured diaphragm. Some vessels have replaceable diaphragms — most domestic ones do not. Replace the whole unit.

Replacing an Expansion Vessel: Step by Step

Most domestic expansion vessels are teed off a short drop from the flow pipe, close to the boiler. The job is straightforward if the original installer fitted an isolation valve on the vessel connection (many didn’t).

1. Turn the boiler off and allow the system to cool
2. Close the isolation valve on the vessel (if fitted) or drain the system via the drain cock
3. Relieve any residual pressure via the filling loop or a bleed point
4. Unscrew the vessel from its connection — typically a 3/4" BSP male fitting
5. Pre-charge the new vessel to the correct static pressure before fitting
6. Screw in the new vessel using PTFE tape on the male thread. Do not over-tighten brass-to-brass — firm hand tight plus quarter-turn with a spanner
7. Re-open the isolation valve, repressurise to 1–1.5 bar cold fill pressure, and check for leaks
8. Fire the boiler and check pressure rise at operating temperature — it should climb no higher than 2.5 bar on a 3 bar PRV system

While you’re there, fit an isolation valve if one isn’t already in place. It makes future replacements a 10-minute job rather than a full drain-down.

Stock the Right Sizes

For domestic work, carry a 6L and an 18L heating vessel as standard. For unvented cylinder callouts, carry a WRAS-approved potable vessel. APM at 24 Western Avenue, Acton, W3 7TZ stocks the full AquaSystem expansion vessel range for same-day collection, or call ahead on 020 8702 8080 to confirm availability.

Browse the full Heating & Controls range at apmi.uk for magnetic filters, filling loops, pressure gauges, and everything else you need for sealed system work.