The laws of physics cannot be broken, warns HWA member Isaac Occhipinti. Here, he explains why it’s important to work with the mechanics of vented and unvented hot-water systems.

When water is heated it expands; it’s the law of physics. And if a hot water system does not have provision to allow for the increase in volume, the pressure within a system will increase. These increases in pressure can have a devastating effect.

Hot water storage can be split into two categories: open vented systems and unvented systems.

Open vented

These systems incorporate a vent pipe, which rises from the top of the cylinder to a point that is open to atmosphere above the cold water storage cistern. As the water in the cylinder is heated, the increase in volume is displaced equally up the feed and vent pipe into the cold-water storage cistern. In this system, the pressure within the cylinder is governed by the height of the cold water storage cistern. Building Regulations reference BS1566-1:2002, the British Standard for copper cylinder design, details minimum material thicknesses that should be used for different static heads in cylinders.


These systems are closed and designed to operate at a higher pressure. The inlet is closed with a non-return check valve to prevent the heated, expanded water from returning to the mains supply. It also includes a pressure-reducing valve, before the non-return valve on the inlet to the cylinder, to limit the pressure on mains water entering the cylinder.

Unvented cylinders are not fitted with a vent pipe; instead they are provided with an expansion vessel, or designed with an internal air pocket. Both of these methods use a pocket of trapped gas, which compresses as the water expands. As a requirement of Building Regulations, unvented cylinders must have two independent safety devices, to minimise the danger of excessive pressure attached to the cylinder.

In addition, the regulations require the hot water storage to have precautions to prevent the water from exceeding 100°c. This is normally controlled by the use of non-self-resetting energy cut-outs on electric heat sources such as immersion heaters and through the use of temperature relief valves. The temperature relief is most often combined with a pressure relief valve to form a temperature and pressure relief valve.

As part of the installation, the installer must ensure that a suitable method of discharge is provided from the safety valves in the unvented system, or as overflow from the cold water storage cistern in vented system. Both of these discharges must be checked on installation to ensure they can convey the discharge to where it is visible, but will not cause danger to people in, or about the building.

Familiarise, but don’t meddle

With all of these requirements, it is important that the installer is familiar with the system they are using, the appropriate Building Regulations and the limitations of the product being fitted.

When installing a vented system, the installer must ensure the cylinder is the correct grade for the static head of water it will be subjected to, and that the cylinder being used meets the requirements of the standard referenced in the Building Regulations.

If replacing an existing cylinder, or working on an existing system, check the vent pipe meets the requirements of the regulations. Old immersion heaters may not be compliant with current regulations, as they do not have a cut-out. On both systems, ensure discharge pipes perform correctly.

There have been cases where conversion of a vented cylinder to an unvented cylinder has been attempted. The HWA wants to highlight why this is a dangerous idea:

•  Due to the differences in design and pressure, and the strength of materials between the two systems, the common failure mode appears quickly, when the pressure of the system pushes out the concave base of the vented copper cylinder causing it to move, fall and leak

•  If excessive pressure was applied to a grade 3, standard vented cylinder, it would begin to distort the bottom of the cylinder. This is the result of the incorrect grade of cylinder being installed in a vented installation

•  If a grade 3, standard vented cylinder was modified, with no means of expansion provided and mains pressure was applied to the cylinder, it would severely distort at the bottom of the cylinder before eventually rupturing at approximately 7-bar.

So, to ensure an installation is safe and compliant with regulations, make sure the product has approval from third-party test centre, confirming it meets the requirements of the Building Regulations. Always follow the installation instructions provided with the product.

As always, HWA members will be happy to offer advice and support, visit: