Water Supply Industry Interpretations and Advice

  • Area of Interest : Enforcement
  • Topic : Backflow prevention
  • Title : Type AG air gaps
  • Clause Reference : Regulation 4 & Schedule 2 paragraph 15
  • Reference Number : E04
  • Decision Date : March 2014

In accordance with the Regulators’ Specification for backflow prevention arrangements and devices (please refer to interpretation E03) to be considered as ‘adequate’ a Type AG air gap must have:

'a visible, unobstructed and complete physical air break between the lowest level of water discharge and the level of potentially contaminated fluid downstream (critical water level) within a cistern, vessel, fitting or appliance, that:-

  • is not less than 20mm or twice the internal diameter of the inlet pipe whichever is the greater; and
  • from which water discharges at not more than 15° from the vertical centreline of the water stream.'

In addition to satisfying this requirement a Type AG air gap must also comply with the requirements of Regulation/Byelaw 4(2). That is to say a Type AG air gap must conform to an appropriate performance specification or standard.

Water suppliers recognise the following performance specifications as meeting both these requirements:-

For all pipe/valve connection sizes:

  • Conformity with Regulators’ Specification Test Code Sheet (TCS) 2212.3 – verification by measurement only

Alternatively for single pipe/valve sizes greater than DN 22 (G¾) only:

  • Conformity with BS EN 14623:2005 – verification by vacuum test or measurement