What is an expansion vessel in a central heating system?

An expansion vessel is a sealed steel cylinder, typically red, containing a rubber diaphragm. Nitrogen gas is pre-charged on one side of the diaphragm; heating system water is on the other. When the central heating heats up, the water expands - the expansion vessel absorbs this volume increase by compressing the gas through the diaphragm. Without an expansion vessel in a sealed system, water expansion would rapidly raise the pressure until the pressure relief valve lifts and discharges water.

What pressure should an expansion vessel be?

The nitrogen gas pre-charge pressure in the expansion vessel should match the static fill pressure of the heating system - typically 1.0 bar for a single-storey system or 1.5 bar for a two-storey system. The system cold fill pressure (shown on the boiler pressure gauge when cold) should be 0.2-0.5 bar above the vessel pre-charge. Most combi boilers operate at a cold fill pressure of 1.0-1.5 bar, rising to 1.5-2.5 bar when hot.

How do I know if my expansion vessel has failed?

Signs of a failed expansion vessel include: the pressure relief valve frequently discharging water (often noticed as a pipe dripping outside); the boiler pressure rising to over 3 bar when hot then dropping back when cool; and the system needing regular topping up via the filling loop. When the rubber diaphragm perforates, water fills both sides of the vessel and it can no longer absorb expansion. The Schrader valve on the top of the vessel (like a car tyre valve) can be pressed to check - if water comes out, the diaphragm has failed.

Can I replace an expansion vessel myself?

Replacing an expansion vessel on a sealed central heating system does not require a Gas Safe registration as it does not involve gas pipework, but it does involve draining and re-pressurising the heating system. For a combi boiler where the expansion vessel is inside the boiler casing, access requires opening the boiler - this work is better left to a Gas Safe engineer who can carry out a full boiler service at the same time and ensure all safety devices are functioning correctly after the repair.

Plumbing & Heating noun

Expansion Vessel

/ ɪkˈspanʃ(ə)n ˈvɛs(ə)l /

Also known as: expansion tank, diaphragm vessel, pressurisation vessel

Definition

An expansion vessel is a sealed steel cylinder - typically red, ranging from 2 to 18 litres in domestic systems - containing a rubber diaphragm. Nitrogen gas pre-charged to the system static pressure occupies one side; heating system water occupies the other. As the system heats up and the water expands, the extra volume compresses the gas through the diaphragm, absorbing the expansion without raising system pressure to the relief valve lift point.

In practice

In combi boilers, the expansion vessel is typically housed inside the boiler casing. In system boilers and unvented hot water cylinders, a separate external expansion vessel is also required for the domestic hot water circuit. The vessel is connected to the return pipe of the heating circuit, as this is the lowest temperature and pressure point in the system.

The most common failure mode is the rubber diaphragm perforating with age, allowing water to fill both sides of the vessel. When this happens the vessel can no longer absorb expansion - pressure rises rapidly when the boiler fires, the pressure relief valve lifts and discharges water, and the system then drops in pressure as it cools. The Schrader (car tyre style) valve on the dry side of the vessel can be pressed to check - if water emerges, the diaphragm has failed and the vessel needs replacement.

Building Regulations

Relevant PartPart L - Conservation of fuel and power

StandardBS EN 13831 - sealed expansion vessels

Cold fill pressureTypically 1.0-1.5 bar (confirm with boiler manual)

Relief valve settingTypically 3 bar

Expansion vessels in sealed central heating systems must comply with BS EN 13831. The vessel size must be correctly calculated for the system water volume and the temperature range from cold fill to maximum operating temperature. An undersized vessel will cause the pressure relief valve to lift regularly. Part L requires that sealed heating systems are properly commissioned, which includes setting the expansion vessel pre-charge pressure correctly.

Full Building Regulations guidance

Expansion Vessel

Just so you know...