Glossary · Central heating, radiators and hot water
Vented vs unvented cylinder: what is the difference?
A vented cylinder is fed from a cold water storage cistern, usually in the loft, and has a vent pipe open to the air. An unvented cylinder is fed straight from the mains with no vent pipe, and relies on safety devices instead. The difference decides water pressure, loft space needs and who may install it.
Key points
- A vented cylinder is fed from a storage cistern and has a vent pipe. An unvented cylinder is fed from the mains and has none.
- Vented systems run at lower pressure, set by the height of the tank. Unvented systems deliver at mains pressure.
- An unvented cylinder needs an expansion route, safety devices and a visible, safe discharge pipe.
- Unvented installation is notifiable building work and needs a skilled, registered installer.
- A replacement cylinder of either type should have an electronic temperature control such as a cylinder thermostat.
How the two types work
Both types store heated water. They differ in how the water gets in and how it expands when it warms.
Approved Document G defines a vented (open) hot water storage system as a vessel fed with cold water from a dedicated storage cistern. The water expands back up the cold feed pipe, and a vent pipe connects the top of the vessel to a point open to the air above the cistern.[1]Ministry of Housing, Communities and Local Government, Sanitation, hot water safety and water efficiency: Approved Document G (2024 amendments) BASI describes it as a system fed from a header tank in the loft that runs at low pressure, and says it is what most older Yorkshire homes have.[2]BASI Heating, Hot water cylinders
An unvented (closed) system is fed from a supply pipe with no vent pipe. Expansion is taken up inside the cylinder or in an external expansion vessel, and safety devices stop the water going above 100°C.[1]Ministry of Housing, Communities and Local Government, Sanitation, hot water safety and water efficiency: Approved Document G (2024 amendments) Read more on the unvented cylinder page.
Side by side
| Vented | Unvented | |
|---|---|---|
| Cold water source | Storage cistern, often in the loft | Mains supply pipe |
| Vent pipe | Yes, open to the air | None |
| Pressure at the taps | Lower, as BASI describes it | Mains pressure |
| Safety extras | Open vent pipe | Two independent safety devices plus a safe, visible discharge route |
| Installer | No unvented-specific rules | Registered or skilled installer, work notifiable |
BASI says vented cylinders are simpler and cheaper but the pressure is reduced, while unvented cylinders run at mains pressure straight to the taps. It adds that most replacement jobs it quotes are unvented unless the loft layout or property type suits a vented system.[2]BASI Heating, Hot water cylinders
What either type needs
A replacement hot water storage vessel should have an electronic temperature control, such as a cylinder thermostat, under the energy efficiency guidance in Approved Document L for England.[3]Ministry of Housing, Communities and Local Government, Approved Document L, Conservation of fuel and power, Volume 1: Dwellings, 2021 edition incorporating 2023 amendments For an unvented cylinder, Approved Document G adds the extra safety devices and the discharge pipework rules, and tells owners to choose an installer with proven skills, such as a competent person scheme registration.[1]Ministry of Housing, Communities and Local Government, Sanitation, hot water safety and water efficiency: Approved Document G (2024 amendments)
Example
Imran and Sana live in a Leeds terrace. Their loft is small and their shower is weak. A survey finds good mains pressure, so they are offered an unvented cylinder. A neighbour with a large loft and an existing tank decides to keep a vented cylinder, because the system works and costs less to change. Both choices are reasonable.
What people get wrong
People often say unvented is always better because showers are stronger. The stronger flow depends on the incoming mains supply. If the mains is weak, an unvented cylinder cannot fix it.
The other mistake is thinking a vented cylinder is out of date or unsafe. It is a long-standing design that the regulations describe in the same document as unvented systems. Its limit is pressure, not safety.
The bottom line
The choice comes down to pressure, space and cost. Vented is simpler and relies on a tank; unvented uses the mains and needs more safety equipment and a skilled installer. A survey that checks your mains supply and your loft will point to the right answer for your house.
Common questions
Which is better, vented or unvented?
Neither wins on every house. Unvented gives mains pressure and frees the loft. Vented is simpler and cheaper, as BASI describes it. BASI says it talks customers through both at survey.
Do I need a loft for a vented cylinder?
A vented cylinder needs a cold water storage cistern, and Approved Document G describes the vent pipe running above it. In most homes that cistern sits in the loft.
Is an unvented cylinder more dangerous?
It holds hot water under pressure, so the regulations require protection. That means at least two independent safety devices and a safe discharge route. Correctly installed by a registered installer, these devices do their job.
Can I swap vented for unvented myself?
No. Fitting an unvented cylinder is notifiable building work, and Approved Document G tells owners to choose installers registered with a competent person scheme or holding a current skills card.
Sources
We cite the original source for every figure: government, regulators, standards bodies and manufacturers' own documents. Each one shows the date we last checked it.
- Ministry of Housing, Communities and Local Government. Sanitation, hot water safety and water efficiency: Approved Document G (2024 amendments). Checked 11 November 2026. ↩
- BASI Heating. Hot water cylinders. Checked 11 November 2026. ↩
- Ministry of Housing, Communities and Local Government. Approved Document L, Conservation of fuel and power, Volume 1: Dwellings, 2021 edition incorporating 2023 amendments. Checked 11 November 2026. ↩