provide the same function as an air gap; this is not true, because the continuous connection in such a case will still allow backflow through siphoning. According to the International Residential Code 2003, an air gap length must meet the requirements of being two times the effective inner diameter of the pipe (2×D) in order to be sufficient. A standard widely use in the United States is: A112.1.2 Air Gaps in Plumbing Systems (For Plumbing Fixtures and Water-Connected Receptors) In the United Kingdom, legislation is by statutory instrument and varies by country, but includes Water Supply (Water Quality) Regulations 2016 and Water Supply (Water Quality) Regulations (Wales). The categorization of air gaps is standardized by European standards, which cover the basic design and dimensions for appropriate to different uses. EN 13076 -- Devices to prevent pollution by backflow of potable water - Unrestricted air gap - Family A - Type A EN 13077 -- Devices to prevent pollution by backflow of potable water - Air gap with non-circular overflow (unrestricted) - Family A - Type B EN 13078 -- Devices to prevent pollution by backflow of potable water - Air gap with submerged feed incorporating air inlet plus overflow - Family A, type C and others for each family and type of air gap See also Hydrostatic loop Pressure vacuum breaker Double check valve Chemigation valve Reduced pressure zone device Atmospheric vacuum breaker Upstream contamination References Plumbing Backflow