Water-Supply-air-gaps class "A" & "B" as per the UK Bye laws

Water Supply byelaws within the UK

Bywlaw 22.

1. Every clothes washing machine, dishwasher or tumble dryer connected permanently or temporarily to the water supply service in any premises; shall incorporate either a Type "B" air gap or pipe inerrupterwhich, if removed , renders the machine inoperable.

2. Subject to para 3, every machine of a kind mentioned in para 1 which is connected permanently or temporarily to the water service elsewhere other than a domestic dwelling shall draw water by gravity only from a storage cistern.

3. Par 2 shall not apply where any machine mentioned in that paragraph incorporates a Type "A" air gap.

Byelaw 23

The inlet pipe of every Ion-exchange common salt regenerated type water softener used in connection with a clothes Washing machine or Dishwasher shall be fitted with a check valve and vacuum breaker or some other no less effective back flow prevention device; except where the supply of water to such a softener passes first through the backflow prevention device incorporated with that washing machine or dishwasher in accordance with byelaw 22 (par 1)

The diagram below shows the different class of air gap required by the byelaws.