Mains pressurised hot water systems are increasingly being used to replace traditional low pressure open vented systems in the UK. Electrical installers are becoming more familiar with these systems, leading to them being used in an increasing number of domestic upgrades, particularly in larger homes and properties that are being extended.

The market for small unvented and vented electric water heaters is particularly buoyant, having experienced steady growth over the last five years, thanks to positive influences such as raised awareness of energy efficiency. Instantaneous water heaters have also experienced a period of moderate growth, particularly in the commercial sector.

Small unvented hot water systems are increasingly popular and are fed directly from the cold water mains supply. They offer a number of benefits including high flow rates from all hot taps and balanced water supplies. Such products are useful for applications where a suitable hot water store is a considerable distance away or where there is no hot water store at all. They can be installed both over and under a sink (with pipe connections located accordingly) and are available in a number of sizes and capacities.

But when it comes to actually fitting electric water heaters, what should electrical installers be aware of?

This article uses Ariston’s Andris Lux Eco unvented electric water heaters as an example throughout.

Units above 15 litres in capacity need to be fitted by G3 accredited installers. As a result, any electrician wanting to work with such products requires suitable training and certification. As Part G3 of the Building Regulations defines unvented hot water systems as a “controlled service”, it is a legal requirement for all relevant installations to be reported to the local Building Control department.

Unvented units of 15 litres and under are not subject to the same regulations, but should only be fitted by a suitably trained person.

Products should be supplied with energy labels, in accordance with the Ecodesign and Energy Labelling Directives (commonly referred to as ErP). This legislation applies to water heaters with outputs up to 70kW. Manufacturers of water heaters also need to supply the relevant performance and efficiency parameters in the form of the ‘technical fiche’.

Unvented electric waters are available nationwide from builders’ and plumbers’ merchants, as well as some electrical wholesalers.

This is not really the case. Once the unit has been fixed to the wall (via hanging brackets) and dielectric junctions fitted to the inlet and outlet water connections, the cold and hot water pipework can then be connected.

A 6bar pressure relief valve (PRV) is needed for the cold water supply, while pipework should be secured with a mixture of compression and solder fittings. The hot water outlet should be connected using 15mm copper pipe, going to an approved flexible tap connector.

Discharge pipework also needs to be fitted, the requirements of which vary depending on the capacity of the electric water heater. One method is to run the (sleeved) discharge pipe straight through the wall to a drain outside, conforming to G3 and British Standards. Another option (for units above 15 litres) is to run the discharge pipe through the wall via a tundish – this is done when it isn’t possible to terminate at a safe location, or if pipe lengths are excessive.

G3 stipulates: The tundish must be vertical and fitted within 600mm of the temperature and pressure relief valve. The tundish must also be in a position visible to the occupants, and positioned away from any electrical devices. The discharge pipe should terminate in a safe place where there is no risk to persons in the vicinity of the discharge and to be of metal.

For a more comprehensive guide to installation, we also have a useful YouTube video:

To connect the electrics on an Ariston unit, first unscrew and remove the front fascia panel. Then, connect the 13 amp electrical supply to the water heater. The cable is threaded through the back of the unit, through a tube embedded in the foam insulation, and connected as per the wiring instructions in the product manual. Heat resistant cable, round 3 core 1.5mm (to BS 6141 table 8) should be used to connect to the electrical supply.

The electrical connection is via a 13 amp spur – in most cases, a standard switched fuse spur is sufficient. Once the wiring has been connected, the fascia panel can be reaffixed and the unit can be commissioned for use.

Before switching on an electric water heater, it needs to be commissioned. The cold water mains supply should be turned on to fill the unit, allowing any air to flush through the pipework. All pipes should be checked for any leaks and connections double checked. If everything is satisfactory, the electrical supply can then be turned on at the fuse spur.

Once the heater is on, the thermostat should be set to 60°C, to help reduce the build-up of scale in hard water areas. (If the heater has an energy saving or economy setting, be sure to activate it.) Once the water has heated up, open the hot tap and check warm water is available to complete the installation process.

Electric water heaters must be earthed in line with the current I.E.E. wiring regulations.

Ariston’s Andris Lux Eco is part of the UK’s number one range of small unvented electric water heaters, encompassing leading Italian design, energy efficiency, durability, usability and safety. It boasts features unique to the market such as anti-legionella and anti-overheating functions, which have resulted in it being installed in a variety of specification-led locations, including commercial properties, building sites, kitchens, bathrooms and offices.

For more information on the products available from Ariston, click here.

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