An unvented hot water cylinder is a sealed hot water tank that takes water directly from the mains and stores it at mains pressure, so taps and showers run strongly without needing a tank in the loft. It differs from older systems by having no cold-water storage tank feeding it, which is why the hot water it delivers is pressurised rather than gravity-fed.
What an unvented hot water cylinder actually is
An unvented cylinder is an insulated steel tank, usually lined with stainless steel, that holds heated water under pressure. Because it is connected straight to the cold mains supply, the water inside is at roughly the same pressure as the cold tap in the kitchen. When you open a hot tap, mains pressure pushes the stored hot water out.
This is the main contrast with a traditional "vented" or open system, where a tank in the loft feeds the cylinder by gravity. In a vented setup, hot water pressure depends on how high the loft tank sits above the tap — which is why upstairs showers can feel weak. An unvented system removes that dependency on height.
The water can be heated in two common ways: by a boiler circulating hot water through a coil inside the cylinder, or by an electric immersion heater. An immersion heater is an electric element fitted into the tank, much like a giant kettle element, controlled by a thermostat. Many cylinders include an immersion heater as a backup even when a boiler does the main heating.
Why it delivers stronger mains-pressure hot water
It differs from older systems by having no cold-water storage tank feeding it, which is why the hot water it delivers is pressurised rather than gravity-fed.
The strength comes from the mains itself. Because the cylinder is sealed and fed by the incoming mains, the hot water leaves at close to mains pressure rather than being limited by gravity from a loft tank. That tends to mean better flow at high-level outlets and the ability to run more than one outlet at once.
A practical benefit is that an unvented system can supply a balanced pressure of hot and cold water to a shower, so mixer and thermostatic showers behave predictably. It also frees up loft space, since there is no cold-water storage cistern and no separate feed-and-expansion tank.
The trade-off is that performance depends on the incoming mains. If the property has low mains pressure or a poor flow rate, an unvented cylinder cannot create pressure that the mains does not provide. For that reason the mains supply is usually checked before this type of system is chosen.
The safety components and the G3 rules
Heating water in a sealed vessel raises both temperature and pressure, so unvented cylinders carry safety parts that a vented system does not need. These are not optional extras — they are required for the system to operate safely.
- Expansion vessel: a small tank, often containing an air-filled rubber diaphragm, that absorbs the extra volume created as water expands when heated. Without somewhere for that expansion to go, pressure inside the cylinder would rise sharply.
- Pressure and temperature relief valves: these open automatically to release water if pressure or temperature climbs beyond safe limits, preventing the cylinder from becoming dangerous.
- Tundish: a small open funnel fitted into the discharge pipework from the relief valves. Its visible air gap shows when water is being released and prevents waste water from siphoning back into the system. A tundish that frequently has water passing through it is a warning sign that something needs checking.
Installation and maintenance fall under Part G3 of the Building Regulations in England and Wales, with equivalent rules elsewhere in the UK. G3 covers how unvented systems must be installed, the discharge pipework that carries released hot water safely away, and the requirement that the work is done and certified by a suitably qualified person. Fitting or substantially altering an unvented cylinder is notifiable work, meaning building control must be informed, usually through a registered installer. An installer working on these systems should hold a specific G3 qualification.
Signs a cylinder needs attention
Several symptoms suggest the system is no longer working as it should. Water discharging from the tundish or the outside discharge pipe, beyond an occasional small amount, often points to a failed expansion vessel or a relief valve that is not seating correctly.
Other warning signs include hot water that runs out faster than usual, fluctuating pressure at the taps, knocking or banging noises as the cylinder heats, or visible corrosion and damp around the unit. A cylinder that has lost its expansion capacity may also cause pressure to spike, which can damage other parts of the system over time.
Unvented cylinders are generally recommended for an annual service, during which a competent person checks the expansion vessel charge, tests the relief valves, and inspects the discharge arrangements. Keeping to that schedule helps spot problems before they become a leak or a breakdown.
Sizing and cost considerations
Choosing the right size matters because the cylinder stores a fixed amount of hot water. Too small, and a household runs out during peak demand; too large, and energy is wasted keeping unused water hot. Sizing is usually based on the number of bathrooms, the number of occupants, and how many outlets might be used at once.
As a rough guide, a single-bathroom home might suit a smaller cylinder, while a property with several bathrooms and a high-flow shower needs considerably more capacity. The available mains flow rate also influences the choice, since a large cylinder is of little use if the mains cannot refill it quickly enough.
Costs vary with capacity, the quality of the tank, whether an existing system is being replaced or a new installation is being created, and the amount of pipework and electrical work involved. Ongoing costs include the annual service and occasional replacement of wearing parts such as the expansion vessel diaphragm or relief valves. Anyone comparing options should ask an installer for a written breakdown covering the unit, the safety components, the discharge pipework, and the building control notification.