“backflow” means flow upstream, that is in a direction contrary to the intended normal direction of flow, within or from a water fitting;

“cistern” means a fixed container for holding water at atmospheric pressure;

“combined feed and expansion cistern” means a cistern for supplying cold water to a hot water system without a separate expansion cistern;

“combined temperature and pressure relief valve” means a valve capable of performing the function of both a temperature relief valve and a pressure relief valve;

“contamination” includes any reduction in chemical or biological quality of water due to a change in temperature or the introduction of polluting substances;

“distributing pipe” means any pipe (other than a warning, overflow or flushing pipe) conveying water from a storage cistern, or from hot water apparatus supplied from a cistern and under pressure from that cistern;

“expansion cistern” or “expansion vessel” means a cistern or vessel connected to a water heating system which accommodates the increase in volume of water in the system when the water is heated from cold;

“expansion valve” means a pressure-activated valve designed to release expansion water from an unvented water heating system;

“flushing cistern” means a cistern provided with valve or device for controlling the discharge of the stored water into a water closet pan or urinal;

“overflow pipe” means a pipe from a cistern in which water flows only when the water level in the cistern exceeds a predetermined level;

“pressure relief valve” means a pressure-activated valve which opens automatically at a specified pressure to discharge fluid;

“primary circuit” means an assembly of water fittings in which water circulates between a boiler or other source of heat and a primary heat exchange inside a hot water storage vessel, and includes any space heating system;

“secondary circuit” means an assembly of water fittings in which water circulates in supply pipes or distributing pipes of a hot water storage system;

“secondary system” means an assembly of water fittings comprising the cold feed pipe, any hot water storage vessel, water heater and pipework from which hot water is conveyed to all points of draw-off;

“servicing valve” means a valve for shutting off for the purpose of maintenance or service the flow of water in a pipe connected to a water fitting;

“stopvalve” means a valve, other than a servicing valve, used for shutting off the flow of water in a pipe;

“storage cistern” means a cistern for storing water for subsequent use, not being a flushing cistern;

“temperature relief valve” means a valve which opens automatically at a specified temperature to discharge fluid;

“terminal fitting” means a water outlet device; and

“vent pipe” means a pipe open to the atmosphere which exposes the system to atmospheric pressure at its boundary.