An unvented cylinder is a sealed hot water storage vessel fed directly from the cold mains, so it delivers hot water at mains pressure — commonly 3–3.5 bar after the pressure-reducing valve — to every tap in the house. Because there is no open vent for heated water to expand into, it relies on an engineered safety chain: expansion provision, relief valves and a visible tundish discharge.
How does an unvented cylinder work?
An unvented cylinder is connected straight to the cold water mains through an inlet control set, so the stored hot water sits at — and is delivered at — mains pressure. There is no loft header tank and no open vent pipe. A pressure-reducing valve first drops the incoming mains to the cylinder's design working pressure, commonly 3–3.5 bar, which balances hot and cold supplies and gives strong, even flow at upstairs showers without booster pumps.
Because the vessel is sealed, thermal expansion has to be managed deliberately. Water heated from 10 °C to 65 °C expands by roughly 2% — about 4 litres in a 200-litre cylinder — and that volume must go somewhere. It is absorbed either by an external expansion vessel or by an internal air bubble maintained at the top of the cylinder, depending on the design.
What safety devices does an unvented cylinder need?
An unvented cylinder needs a layered safety chain, because a sealed vessel of hot water must be protected against both over-pressure and over-temperature with no open vent to fall back on. Each layer only acts if the one before it fails:
| Layer | Device | What it responds to |
|---|---|---|
| Working pressure | Pressure-reducing valve | Mains pressure above the design value |
| Normal expansion | Expansion vessel or internal air gap | Volume growth of heated water |
| Over-pressure | Expansion relief valve | Failed or waterlogged expansion provision |
| Over-temperature | Thermal cut-out on each heat source | Control thermostat failure |
| Last line | Temperature & pressure relief valve | Excessive temperature or pressure |
| Visibility | Tundish | Makes any discharge safe and visible |
European products are built and tested to EN 12897, and most markets regulate installation — in the UK, for example, unvented cylinders fall under Building Regulations Approved Document G3 and must be installed by a competent person. Relief-valve settings differ by market and approval scheme, so always follow the values stamped on the fitted devices rather than a generic figure.
What are the advantages of an unvented cylinder over a vented one?
The headline advantage is pressure. A gravity-fed vented cylinder develops only about 0.1 bar per metre of header-tank height — typically 0.2–0.3 bar at an upstairs shower — while an unvented cylinder delivers roughly ten times that from every outlet. Other gains follow: no loft tank to freeze or contaminate, hot and cold supplies at balanced pressure for thermostatic showers, and flexible siting anywhere in the building.
The trade-offs are real but manageable: performance depends on the quality of the incoming mains supply, discharge pipework must be routed to a safe visible point, and the safety chain needs an annual service. The most common faults are covered in common problems with unvented cylinders.
What maintenance does an unvented cylinder need?
An annual service keeps the safety chain trustworthy: check and recharge the expansion vessel pre-charge, briefly lift each relief valve to confirm it reseats cleanly, clean the inlet strainer, and confirm the tundish is dry in normal operation. On enamel-lined cylinders the sacrificial magnesium anode should be inspected on the schedule the manufacturer specifies — typically every 1–2 years — because it, not the relief valves, is what protects the tank shell itself from corrosion. A well-maintained unvented cylinder routinely reaches a 15–25 year service life.
Frequently asked questions
Do I need good mains pressure for an unvented cylinder?
Yes. The cylinder can only deliver what the mains supplies, so a weak or low-flow main undermines the system's main advantage. As a rule of thumb installers look for at least around 1.5–2 bar dynamic pressure and 20 litres per minute of flow before recommending unvented storage.
Why is water coming out of my unvented cylinder's tundish?
A visible discharge means a relief valve has opened. The most common cause is a lost expansion vessel pre-charge, which forces expansion water out through the expansion relief valve during heat-up. Steady or hot discharge should be investigated promptly by a competent installer.
Can an unvented cylinder be used with a heat pump or solar thermal?
Yes — unvented cylinders are routinely built with large-surface coils for heat pumps and with twin coils for solar plus boiler operation. The safety chain is unchanged; only the coil design and control strategy adapt to the heat source.
How often should an unvented cylinder be serviced?
Annually. The service checks the expansion vessel charge, relief valve operation, inlet strainer and discharge pipework. Skipping it is the leading cause of nuisance tundish discharge and premature valve failure.