Knowing subtle differences between domestic hot water cylinders is crucial to meet individual homeowner requirements both in terms of space and efficiency, explains Isaac Occhipinti, head of external affairs at the Hot Water Association (HWA).
The domestic hot water cylinder industry offers choice to both the homeowner and installers, and while the overwhelming majority of cylinders look the same, and in most cases they are fitted in the same way, there are many differences that the installer and end user should be aware of when selecting a product. It is a challenge for cylinder manufacturers to show the USPs of their product. The traditional vented cylinder is gradually being replaced where possible with unvented units, and the following discusses the merits and choices of this growing section of the market.
Clearly 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 and 300 litres.
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 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 and 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 and 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 usable 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 the basic yet compliant cylinder 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 even a washing machine to be accommodated.
At the opposite end, slim line 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, perhaps in commercial applications, customers 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 build up.
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. Most modern cylinders are insulated with Polyurethane foam however 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.
The Hot Water Association Charter identifies manufacturers whose products have passed a high level of audit and scrutiny.