Unvented Cylinder Expansion Vessel in London: What It Does and When It Fails

Unvented Cylinders and System Pressure

An unvented hot water cylinder stores mains-pressure hot water. Unlike a vented copper cylinder in an airing cupboard that is fed by gravity from a cold water tank in the loft, an unvented cylinder is connected directly to the mains cold water supply and operates at mains pressure — typically 2 to 3 bar in London. This means hot water is delivered at mains pressure throughout the property, giving powerful showers without a pump and consistent hot water flow at all outlets simultaneously.

Because the unvented cylinder operates at mains pressure and the water inside it is heated (expanding as it heats), the cylinder requires two specific components that a vented cylinder does not: an expansion vessel to absorb the thermal expansion of the domestic hot water, and a combination temperature and pressure relief valve (T&P valve or TPR valve) to discharge water safely if the temperature or pressure exceeds safe limits.

The DHW Expansion Vessel

As cold water enters the cylinder and is heated, it expands. In a vented system, this expansion is accommodated by water rising in the cold feed pipe back to the tank in the loft — there is effectively an open expansion path. In an unvented cylinder, the mains supply pipe is sealed and the expansion cannot return to the mains. The DHW expansion vessel — a small red vessel typically mounted on the cold water inlet pipework adjacent to the cylinder — absorbs this expansion in the same way a heating system expansion vessel absorbs the expansion of heating water: via a rubber diaphragm and nitrogen pre-charge.

The DHW expansion vessel for an unvented cylinder is separate from the central heating expansion vessel inside the boiler. They are different components serving different water circuits, and a fault in one does not affect the other. Both can fail independently.

How the DHW Expansion Vessel Fails

The rubber diaphragm in the DHW expansion vessel perforates with age — the same failure mechanism as the heating system vessel. When the diaphragm perforates, the vessel fills with water and the nitrogen cushion is lost. Thermal expansion of the domestic hot water has no absorption path, so the pressure in the cylinder rises when the water is heated. When the pressure exceeds the T&P valve set point (typically 6 or 8 bar for domestic cylinders), the T&P valve opens and discharges hot water through the discharge pipe.

The symptom is a T&P valve discharge pipe that drips or flows when the cylinder is heating. The discharge pipe from an unvented cylinder T&P valve is typically a 22 mm or 28 mm copper or plastic pipe that discharges via a tundish — a visible open connection — into a drain or outside the property. If this pipe is dripping when the cylinder is actively heating, the DHW expansion vessel is the first component to investigate.

Diagnosing the DHW Expansion Vessel

With the cylinder cold and the mains cold water supply isolated at the service valve on the cylinder inlet, the expansion vessel Schrader valve is tested. Depressurise the vessel side fully first, then press the Schrader pin. If water comes out, the diaphragm has failed and the vessel must be replaced. If air comes out, the nitrogen pre-charge has bled off and the vessel can be re-charged — first verify the pre-charge pressure against the cylinder manufacturer specification, typically 3.5 bar for a 3-metre head installation.

Replacement and Sizing

DHW expansion vessels for unvented cylinders must be sized to the cylinder volume — typically 1 litre of vessel capacity per 10 litres of cylinder volume, adjusted for the mains pressure. An undersized vessel will cause T&P valve discharge even when new. Most domestic London unvented cylinders of 150 to 250 litres require an expansion vessel of 12 to 25 litres. Prestige Engineers replace DHW expansion vessels and carry out T&P valve discharge investigations on unvented cylinders across all London boroughs.