An unvented hot water cylinder is a sealed tank that stores hot water at mains pressure, fed directly from the cold mains rather than from a tank in the loft. Because it runs at mains pressure, it delivers strong flow to taps and showers across the house without a separate pump, and it does away with the cold-water storage cistern that vented systems rely on.
What an unvented cylinder actually is
"Unvented" means the cylinder is closed to the atmosphere — there is no open vent pipe and no cold tank sitting above it. The cold supply comes straight from the rising main, so the stored hot water is held at, or close to, incoming mains pressure.
The water can be heated in two ways: by a boiler circulating through a coil inside the cylinder (an indirect cylinder), or by an electric immersion heater — an electrically heated element that sits in the water like a giant kettle element. Many cylinders include an immersion heater as a backup even when a boiler does the main heating. Some run on electricity alone.
Because the system is pressurised and sealed, it must be fitted and serviced by someone holding the relevant competence (in the UK this is usually a G3 qualification). That requirement exists for safety reasons, which the section on safety parts explains.
How mains-pressure hot water works
An unvented hot water cylinder is a sealed tank that stores hot water at mains pressure, fed directly from the cold mains rather than from a tank in the loft.
In a vented system, hot water reaches the taps under gravity from a loft tank, so pressure depends on how high the tank sits above each outlet. Upstairs showers can feel weak as a result. An unvented cylinder removes that limitation: it is pushed by the same pressure that feeds your kitchen cold tap.
The practical upshot is balanced pressure at hot and cold taps, which suits mixer showers and lets several outlets run at once without one starving another. There is no loft tank to freeze, overflow, or collect debris.
One thing worth knowing: an unvented cylinder needs a good incoming mains supply to perform well. If the flow rate into the property is poor, the cylinder cannot conjure pressure it never receives. The supply is normally checked before installation for exactly this reason.
The safety parts: expansion vessel and tundish
Heating water makes it expand. In an open vented system that extra volume simply pushes back up the vent pipe into the loft tank. A sealed cylinder has nowhere obvious for it to go, so it relies on dedicated components to manage pressure safely.
- Expansion vessel: a small tank, part air and part water, separated by a flexible diaphragm. As the stored water heats and expands, it pushes into the vessel and compresses the air cushion, absorbing the rise in volume. Some cylinders use an internal air gap instead of a separate vessel, but the principle is the same.
- Temperature and pressure relief valve: a safety valve that opens if pressure or temperature climbs too high — for example if controls fail. It releases water to relieve the build-up.
- Tundish and discharge pipe: the tundish is a small open funnel fitted into the discharge pipe below the relief valve. Any water released passes through it, leaving a visible air gap so the discharge cannot be siphoned back. The discharge pipe then carries the (potentially very hot) water safely to a drain outside or to a suitable point.
If you ever see water dripping from the tundish, it usually signals a fault — commonly an expansion vessel that has lost its air charge, or a relief valve that needs attention. It is a sign to have the system looked at, not ignored.
Unvented compared with vented and combi systems
Each type suits different homes. The right choice depends on water demand, the incoming mains, and whether you have space for a cylinder.
- Unvented cylinder: strong, balanced mains pressure and good simultaneous use; stores a fixed volume of hot water, so a large draw-off can run out until the cylinder reheats. Needs a competent installer, periodic servicing, and a decent mains supply. No loft tank required.
- Vented cylinder: the traditional setup with a loft tank feeding the cylinder by gravity. Simpler and cheaper to maintain, and tolerant of poor mains, but pressure can be weak upstairs and it needs tank space in the loft.
- Combi boiler: heats water on demand with no cylinder at all, saving space and avoiding standing heat loss. Flow rate is limited by the boiler's output, so running two showers at once tends to disappoint. Best suited to smaller homes with one main bathroom.
In short: combis save space, vented systems are forgiving and low-maintenance, and unvented cylinders prioritise pressure and multi-outlet performance.
Sizing and running costs
Size is driven by how much hot water the household uses at once. A larger family with two bathrooms needs a bigger cylinder than a couple in a flat. Choose too small and you run cold during busy periods; too large and you pay to keep water hot that nobody uses.
As a rough guide, sizing is usually based on the number of bathrooms and occupants, with allowance for baths, which use far more than showers. An installer typically works through expected peak demand rather than guessing from floor area.
Running costs depend mainly on the heat source. A cylinder heated by a gas or oil boiler is generally cheaper per unit of heat than one heated solely by electric immersion. Standing heat loss matters too: a well-insulated cylinder holds its temperature for longer, so the boiler or immersion fires up less often.
Maintenance is an ongoing cost worth budgeting for. Unvented cylinders are normally serviced annually, during which the safety valves and the expansion vessel's air charge are checked. That service keeps the system safe and helps it run efficiently, and it is part of the trade-off for the pressure and convenience these cylinders provide.