The traditional domestic hot water vented cylinder is gradually being replaced where possible with unvented units. Isaac Occhipinti, Head of External Affairs at the Hot Water Association, explains why.

A cylinder has to be the correct specification for its use and fuel type. Manufacturers will typically offer of full range of direct electric, indirect (for boilers), and renewable heat source cylinders (for heat pump and solar thermal). Most of these will be offered in capacities typically between 100-300l.

The most immediately obvious difference between cylinder types is the method of accommodating the expansion water. The industry standard has generally been a cylinder with a separate, wall-mounted expansion vessel. The system works – it is reliable, but it takes up space in a cylinder cupboard that could otherwise be used for storage.

By contrast the ‘bubble top’, or ‘air gap’ system dispenses with the need of a separate expansion vessel. The system works by having a bubble of air trapped in the top of the cylinder, and the hot water is drawn from the unit from a side or top mounted internal pipe that reaches into the water.

The air gap is often kept separate from the water by a plastic floating baffle that sits on the water, which moves vertically with the expansion of the water, according to heat. The air gap should be regenerated annually when the unit is serviced by an approved installer, thus preventing a loss of gap that would otherwise result in over pressurisation of the cylinder, or an operational discharge from the expansion relief valve.

Cylinders are also available with an ‘all on top’ design, with heating coil, hot and cold water services, safety valves, and expansion vessels all concealed and contained under a removable lid. The purpose of this design is to minimise the impact of external pipework and connections, and optimise space.

The more advanced a product is in terms of design and efficiency, the more expensive it is likely to be. Some cylinder ranges are made to be as economic as possible to buy, others will be more sophisticated in design to improve the efficiency and ease of fitting.

It’s not just efficiency and heat retention that is important to a householder, space saving is also important. A small increase in diameter can significantly reduce the height of a cylinder, allowing more storage or appliances within the space.

At the opposite end, slimline, and even horizontal products, are available to fit into specific areas. Again, more expensive, but they are problem solvers.

Heat up time from cold, and heat retention, are generally the benchmark of performance. However, if large volumes of hot water are required, such as in commercial applications, you should look for cylinders with even higher specification coils to provide a faster recovery rate.

The ‘horses for courses’ argument is also relevant for coil type. In hard water areas, heat exchangers that could harbour and encourage limescale deposits should be avoided. Manufacturers have invested in heat exchange coil design, along with tank-in-tank technology, to optimise economic heat recovery, and reduce scale buildup.

Unvented cylinders are not all the same. Quality, efficiency, design ingenuity, and price all vary. The end user will not always be interested in the same features as an installer, and giving alternatives at differing prices can lead to both a happier customer, and a more profitable project for the installer.

The introduction of ErP has had a significant effect on the cylinder industry, and the products available. The updated regulations from September 2017 have effectively outlawed products below a C rating.

This has had a dramatic effect on the performance and efficiency of products that are available. This leads to cost savings for the consumer on a gas-fired central heating system, with greater savings on electrically-heated cylinders.

But most modern cylinders are insulated with polyurethane foam however, and efficiency levels, shown by the ErP label and supporting fiche, do vary according to thickness, quality, and type of insulation, with higher rated products also available with vacuum panel insulation.