How open-vented and unvented stored hot water systems differ and what drives the choice at Level 3.
Two ways to store hot water
At Level 3 you should be able to select between an open-vented and an unvented stored hot water system and justify the choice.
Open-vented. A cold-water storage cistern supplies the cylinder and an open vent provides the safety path to atmosphere. In the familiar gravity-fed arrangement, head comes from the height of the cistern’s water surface above the outlet. Only the surface is at atmospheric pressure; below it, pressure rises with depth. The cistern may be in a loft or combined with the cylinder; trace the actual layout.
Unvented. There is no open vent. A common domestic installation takes its supply from the cold main through an inlet control group, without a roof cistern. Other suitable designs can use a dedicated cistern supply. Useful outlet performance depends on the available flow, regulated pressure and pipework, so an unvented cylinder does not guarantee strong or balanced delivery.
What drives the choice
- Available supply: the common mains-fed arrangement needs adequate flow and working pressure under simultaneous demand.
- Space and layout: a gravity-fed vented arrangement needs suitable cistern height above the outlets. A pumped arrangement has different performance requirements; trace the actual supply.
- Performance wanted: compare the required simultaneous flows and provide compatible hot and cold supplies at mixers.
- Competence and notification: in England, installation of storage over 15 litres normally needs notification through building control or a competent-person scheme. The exemption at 15 litres or less also requires any associated electrical work to be non-notifiable. Safety requirements still apply.
A typical domestic unvented cylinder over 15 litres combines expansion provision, operating temperature control, an independent energy cut-out and pressure/temperature relief, with discharge through a tundish. Smaller units also need safety protection, but the permitted arrangements depend on their design. Always confirm sizing and safety requirements against current Approved Document G and the manufacturer instructions.
Unvented details that come up
- Unvented storage needs designed expansion provision, such as an external vessel or an internal air volume; normal heat-up is not dealt with by routine relief discharge.
- The notification boundary above concerns the installation work. The safety concern is overheated water confined under pressure: a failure can release it violently, whatever the designed source of supply.
- The Water Fittings Regulations require safety arrangements appropriate to the system to prevent secondary hot water exceeding 100°C. An open vent belongs to a vented arrangement; an unvented vessel uses its specified controls and relief devices. On an unvented vessel over 15 litres, the temperature and pressure relief valve sits directly on the vessel, can discharge enough to keep the stored water below 100°C, and must not open on pressure alone until the pressure is at least 0.5 bar above the higher of the vessel's maximum working pressure and the expansion valve's operating pressure.
- Keep the relief termination safe, make discharge visible at the tundish and keep controls accessible for the specified service checks.
- A cold branch taken from the specified balanced outlet of the inlet group can provide compatible supplies to mixers. Check the layout, flow demand and valve limits; connection to one group does not eliminate all pressure losses.