Ceramic tap means a tap which use a rotating ceramic disc to control the flow of water, requiring only a quarter-turn rotation.

Check valve means a mechanical device which allows water to flow in one direction only.

Cold water storage tank means a tank for holding water at atmospheric pressure, usually used for providing a feed to domestic cold water and/or to a vented domestic hotwater system.

Combined temperature and pressure relief valve means a mechanically operated valve that opens to discharge water when a set temperature or pressure is exceeded.

Direct heating means a method of heating in which the heat souce is integral with the hot water vessel. An example is a hot water storage vessel fitted with an immersion heater, or a gas burner with a flue arrangement that psses through the vessel so that the flue transfers heat to the water.

Expansion vessel means a vessel used to temporarily accommodate the expansion of water from an unvented hot water storage vessel as it is heated.

Fused spur means an electrical spur connection to an electrical ring main which is protected by a dedicated fuse and also, optionally, a switch.

Immersion heater means a electrical heating element with thermostat used to heat water in a storage tank.

Indirect heating means a method of heating stored water through a heat exchanger coil. The source of heat is typically a gas or oil boiler that is not part of the hot water heater.

Instantaneous water heater means an device that heats water on demand as water flows through it. They are normally unvented (i.e. operate at mains pressure).

Manifold means a pipe fitting that permits has a number of branch connections to be made to the main.

Multipoint water heater means a water heater which can supply water to more than one outlet.

Pressure reducing valve means a valve that reduces high inlet water pressure to a fixed lower value. They are typically used in geographical areas of high water pressure to reduce pressure to a level that prevents safety devices such as a pressure relief valve from activating in normal operation. The design of these devices normally incorporates a spring loaded valve seal that isolates the inlet flow stream if the outlet stream pressure exceeds the set pressure value.

Pressure relief valve means a safety device to prevent excessive pressure. The pressure relief valve is designed or set to open ('lift') at a predetermined set pressure to prevent vessels from being subjected to pressures that exceed their design limits.

Ring main means an electrical circuit used to link electrical sockets and loads which starts at the consumer unit and returns to the consumer unit. This arrangement provides two independent conductors for live, neutral and protective earth within a building for each connected load or socket.

Saddle valve means a valve mounted on a pipe run by a clamping device, or fitting that taps into the side of a pipe, used to make quick connection to an existing line to provide a water supply for a low-demand device.

Sacrificial anode or sacrificial rod means a metallic anode that is designed to be more easily oxidized than the protected metal. It is an anti-corrosion device which prevents oxidation (corrosion) of a steel tank. As the sacrificial anode is intended to preferentially corrode, it is designed to be replaced from time to time. Typically used in glass (i.e. vitreous enamel) lined steel water tanks to provide effective corrosion protection without incurring the higher costs associated with stainless steels.

Stainless steel means is a term to describe a type or variety of alloy steels that are resistant to corrosion or staining. They are characterised by a shiny appearance and are produced by forming steel alloys containing at least 11% chromium and smaller amounts of other additives (carbon, magnesium, nickel etc.)

Stainless steel (Duplex) means a type of high performance stainless steel that offers greater strength and resistance to stress-corrosion cracking than most other types of stainless steels. It was developed for chemical proccessing applications such as desalination plants, heat exchangers, and marine applications requiring a high degree of corrosion resistance and it also used in premium unvented water cylinders as it combines high strength and excellent corrosion properties. Duplex stainless steel is more expensive than standard stainless steels.

Thermostat means a device for regulating the temperature of a system so that the system's temperature is maintained near a desired set point temperature.

Thermostatic mixing valve means a hot and cold water mixing device which automatically adjusts for variations in the temperature and/or pressure of the incoming supplies to maintain a selected temperature at the outlet.

Tundish means a device, installed in the discharge pipe from a valve, that provides an air break allowing discharge to be conducted safely to place of termination. The tundish also provides a visible indication of a discharge and functions as a backflow prevention device.

Usable storage means the proportion of hot water that can be drawn from a tank before the temperature drops to a point that it is no longer considered hot.

Unvented storage water heater means a storage water heater that is designed to be connected directly to the mains water supply resulting in a stored hot water pressure of typically 3 bar. This dispenses with the need for water storage tanks in the roof space and results in high pressure hot and cold water to all outlets, including showers, without the need for a pump. Cold water accessories, such as an expansion vessel, allow for expansion of hot water and prevent excessive pressures. Tank construction is normally steel. Today most new homes have mains pressure systems installed as standard.

Vented tap means a tap specifically for use with a vented water heater, and which operates by controlling the flow of cold water into the water heater which in turn displaces hot water out the tap. During the warm-up cycle it is normal for expanded water to drip from the spout or tap.

Vented storage water heater means an open vented cylinder used to store hot water at low pressure. Water is supplied to these cylinders from a cold water storage cistern/tank (usually located in the loft/roof space) and not directly from the water mains like the mains pressured unvented cylinders. The flow rate of water from an open vented cylinder is dependent on the pressure (static head) between the cylinder and the cold water storage cistern/tank, with the pressure increasing by 0.1 bar for every 1m of height difference between the cylinder and the cistern. This pressure is relatively low compared with an unvented water heater. Today most new homes have the alternative unvented mains pressure system installed as standard.

Vented water heater (point of use) means a water heater that stores water at low pressure which is used with an open outlet spout or special vented tap, and typically installed above or below a sink. The mode of operation relies on the opening of a tap or valve on the inlet side to displace the hot water in the heater with incoming cold water, with the open outlet preventing the possibility of water pressure building up inside the storage tank. During the warm-up cycle it is normal for expanded water to drip from the spout or tap. Larger vented water heaters drip more than smaller units as the volume of expanded water is greater. A point of use vented water heater can only be used with a single open outlet tap or spout.

Vitreous enamel means a thin layer of glass fused by heat on to the surface of a metal by melting tiny coloured glass particles to form a durable protective finish. It is used to line steel (non-stainless) water heater storage tanks to prevent corrosion, usually in conjuction with a sacrificial anode.