7.2 Air Gaps and the Backflow Preventer Family

The air gap

IPC 608.14.1: the minimum required air gap shall be measured vertically from the lowest end of a potable water outlet to the flood level rim of the fixture or receptacle into which the outlet discharges. Air gaps comply with ASME A112.1.2; air gap fittings comply with ASME A112.1.3.

An air gap is the only method that cannot fail mechanically. There is no seat to foul, no check to stick, no diaphragm to tear. It is rated for both backsiphonage and backpressure at both low and high hazard. Everything else in Table 608.1 is a compromise made because a physical gap is not practical at that location.

Table 608.16.1 — minimum required air gaps

The close to a wall column exists because a nearby vertical surface lets water climb by capillary action and wall-following flow, defeating a gap that would otherwise work. Footnote a defines when the "away from a wall" column applies: walls or obstructions spaced from the nearest inside edge of the spout opening by more than three times the effective opening diameter for a single wall, or more than four times for two intersecting walls.

IPC 608.16.1 adds one limitation: openings and outlets equipped for hose connection shall be protected by means other than an air gap. A hose defeats a gap by definition — you can always lift the end.

Worked air gap

A 2-inch nonpotable fill line discharges into an open tank, with a wall 3 inches from the spout.

• Effective opening is greater than 1 inch, so the last row applies.
• Is it "close to a wall"? The wall is 3 inches away, and three times the effective opening (2 inches) is 6 inches. Since 3 inches is less than 6 inches, this is the close to a wall case.
• Required air gap: 3 times 2 inches = 6 inches, measured vertically from the lowest end of the outlet to the flood level rim.

The mechanical devices

Reduced pressure principle backflow prevention assembly (RPZ) — 608.14.2

Two independently acting check valves separated by a relief valve zone held at a pressure lower than the supply. If either check fouls, the relief valve dumps to atmosphere and the zone can never rise to supply pressure.

• Standards: ASSE 1013, AWWA C511, CSA B64.4, CSA B64.4.1. Detector version: ASSE 1047.
• Rated high or low hazard, backpressure or backsiphonage. Sizes 3/8 to 16 inches.
• Permitted under continuous pressure.
• The relief opening shall discharge by air gap and shall be prevented from being submerged.

IPC 608.15.2.1 Relief port piping: the termination of the piping from the relief port or air gap fitting must discharge to an approved indirect waste receptor or to the outdoors where it will not cause damage or create a nuisance, and the receptor and drainage piping must be sized to drain the maximum discharge flow rate from the relief port as published by the manufacturer. An RPZ dumping 40 gpm into a 2-inch floor drain rated for 20 gpm floods the room.

Double check backflow prevention assembly (DC) — 608.14.7

Two independently acting checks in series, with shutoffs and test cocks.

• Standards: ASSE 1015, CSA B64.5, CSA B64.5.1, AWWA C510. Detector fire protection version: ASSE 1048.
• Rated low hazard only, but handles both backpressure and backsiphonage. Sizes 3/8 to 16 inches.
• Capable of operating under continuous pressure.

Pressure vacuum breaker assembly (PVB) — 608.14.5

A spring-loaded air inlet plus a check valve, with shutoffs and test cocks.

• Standards: ASSE 1020 or CSA B64.1.2. Spill-resistant version (SVB): ASSE 1056 or CSA B64.1.3.
• Rated high or low hazard, backsiphonage only. Sizes 1/2 to 2 inches.
• Installed with the critical level not less than 12 inches above all downstream piping and outlets.
• Shall not be installed where spillage could cause damage to the structure.

Atmospheric vacuum breaker (AVB) — 608.14.6, 608.16.4

A simple float or poppet that opens to atmosphere when supply pressure drops.

• Pipe-applied: ASSE 1001 or CSA B64.1.1. Hose connection types: ASME A112.21.3, ASSE 1011, 1019, 1035, 1052, CSA B64.2 family.
• Rated high or low hazard, backsiphonage only.
• Critical level set not less than 6 inches above the flood level rim of the fixture or device (608.16.4). Pipe-applied vacuum breakers are installed not less than 6 inches above the flood level rim of the fixture, receptor or device served.
• No valve may be installed downstream of an AVB, and it may not be under continuous pressure — the poppet will stick. IPC 608.17.5 states the downstream valve prohibition explicitly for irrigation.
• Shall not be installed under exhaust hoods or similar locations containing toxic fumes or vapors (608.16.4).

Deck-mounted and integral vacuum breakers — 608.16.4.1

Approved deck-mounted or equipment-mounted vacuum breakers, and faucets with integral atmospheric vacuum breakers or spill-resistant assemblies, are installed per the manufacturer with the critical level not less than 1 inch above the flood level rim.

Other devices

The barometric loop — 608.14.4

Barometric loops shall precede the point of connection and shall extend vertically to a height of 35 feet. A barometric loop shall only be utilized as an atmospheric-type or pressure-type vacuum breaker.

Thirty-five feet is not arbitrary: it is just above the theoretical maximum height (33.9 feet) that atmospheric pressure can lift a column of water. Above that height, siphonage is physically impossible. It is backsiphonage protection only.

Location and protection of devices

• 608.15: access to backflow preventers as specified by the manufacturer's instructions.
• 608.15.1: outdoor enclosures for backflow prevention devices comply with ASSE 1060.
• 608.15.2: backflow preventers shall not be located in areas subject to freezing, except where they can be removed by means of unions or are protected from freezing by heat, insulation or both. In Colorado this is a working constraint, not a theoretical one.