Glossary

Glossary

Plain-language definitions of the terms used across this Knowledge Hub — from air gaps and BA devices to WRAS, AIM and HTM 04-01.

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Closed system

Heating or chilled-water circuit with no permanent connection to atmosphere. Filling points are typically Category 4.

Check valve

Single or double mechanical device that allows flow in one direction only. Double check (ED) is Category 3 maximum.

Category 1–5

Five fluid risk classes in BS EN 1717 / Schedule 2. Category 1 is wholesome water; Category 5 is a serious health hazard (pathogenic organisms).

Break tank

Storage cistern that physically separates a wholesome supply from a downstream system via an air gap. Required for Category 5.

Back-siphonage

Reversal of flow caused by a reduction of pressure in the supply, drawing downstream fluid back into the wholesome system.

Back-pressure

Pressure in the downstream system exceeding the supply pressure, forcing fluid back toward the wholesome main.

Backflow

Unwanted reversal of flow in a water fitting, caused by back-pressure or back-siphonage, that can contaminate the wholesome supply.

BA device

EN 1717 designation for a reduced pressure zone (RPZ) valve. Provides Fluid Category 4 protection against back-pressure and back-siphonage.

Assembly

A complete backflow prevention unit as supplied, including isolation valves, test cocks and strainer where specified.

AUK3

UK-specific air gap arrangement combining a Type A air gap with a weir overflow. Used on break tanks serving Category 5.

AIM

WRAS Water Suppliers’ Approved Installation Method. Currently January 2025 Issue 3 for RPZ valves. Mandatory for Category 4 mechanical protection.

Air gap

Unobstructed vertical distance between the lowest point of a water inlet and the spillover level of a receiving vessel. Type AA, AB, AD, AF, AG.

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