5.3 Air Gaps, Critical Level, & Specific Backflow Applications (UPC Table 603.3.1, 603.5)
Key Takeaways
UPC Table 603.3.1 requires minimum air gaps of 1, 1-1/2, and 2 inches for effective openings up to 1/2, 3/4, and 1 inch, and twice the opening for openings over 1 inch.
Where a sidewall affects the outlet, the minimums rise to 1-1/2, 2-1/4, and 3 inches, or three times the opening (UPC Table 603.3.1).
A single sidewall affects an air gap when it is within three times the effective opening of the spout's inside edge; two intersecting walls when within four times (Table 603.3.1 note 1).
Where a backflow device has no critical level marking, the bottom of the device is the critical level (UPC Chapter 2 definition).
Hose bibbs need a nonremovable hose-bibb vacuum breaker or backflow preventer, or an AVB at least 6 inches above the highest point of use (UPC 603.5.7).
Air Gaps (UPC 603.3.1 and Table 603.3.1)
The UPC defines an air gap (water distribution) as "the unobstructed vertical distance through the free atmosphere between the lowest opening from a pipe or faucet conveying potable water to the flood-level rim of a tank, vat, or fixture." Because nothing connects the outlet to the receptor, an air gap protects against every hazard level and both backsiphonage and backpressure.
| Effective opening | Not affected by sidewalls | Affected by sidewalls |
|---|---|---|
| Not greater than 1/2 inch | 1 inch | 1-1/2 inches |
| Not greater than 3/4 inch | 1-1/2 inches | 2-1/4 inches |
| Not greater than 1 inch | 2 inches | 3 inches |
| Greater than 1 inch | 2 × the effective opening | 3 × the effective opening |
Notes to the table:
- When walls do not count: Sidewalls, ribs, or similar obstructions do not affect the gap if they are farther from the inside edge of the spout than 3 times the effective opening (one wall) or 4 times the effective opening (two intersecting walls).
- When walls do count: A wall, rib, or obstruction rising from the water surface to or above the spout's horizontal plane, and closer than note 1 allows, counts. The effect of three or more walls has not been determined; in that case, measure the gap from the top of the wall.
- Effective opening: The minimum cross-sectional area at the control valve seat, or of the supply pipe or tubing feeding the outlet. With two or more supply lines, use the smaller of the sum of their areas or the area of the single outlet.
- Gaps under 1 inch: Permitted only as a permanent part of a listed assembly tested under actual backflow conditions from 0 to 25 inches of mercury vacuum.
Worked examples
- 3/4-inch fill spout 6 inches from the tank wall: 3 × 0.75 = 2.25 inches; 6 inches is more than that, so the wall does not affect it. Minimum gap: 1-1/2 inches.
- 3/4-inch spout 1 inch from a single wall: the wall counts. Minimum gap: 2-1/4 inches.
- 1-1/2-inch fill line in a corner, 5 inches from both intersecting walls: 4 × 1.5 = 6 inches, and 5 inches is within that, so the walls count. Minimum gap: 3 × 1.5 = 4-1/2 inches.
- 2-inch line well clear of walls: 2 × 2 = 4 inches.
Drainage air gaps differ: Potable water discharged to a drainage system needs a gap of 2 pipe diameters, not less than 1 inch (603.4.4). Indirect waste air gaps and air breaks are covered in Section 7.3.
Critical Level
The critical level (C-L or C/L) marking on a vacuum breaker or backflow device is set by the testing laboratory. It determines the minimum elevation above the flood-level rim at which the device may be installed. If a device has no critical level marking, the bottom of the vacuum breaker, combination valve, or device is the critical level. Measure every "6 inches above" or "12 inches above" requirement from the critical level.
| Device or application | Minimum height of critical level |
|---|---|
| Flushometer valve AVB (603.5.1) | 6 inches above the bowl's overflow rim or the highest part of the urinal, or per listing |
| Tank ballcock (603.5.2) | 1 inch above the full opening of the overflow pipe |
| AVB, general (Table 603.2) | 6 inches above all downstream piping and the flood-level rim |
| PVB and SVB (Table 603.2) | 12 inches above all downstream piping and the flood-level rim |
| Deck- or equipment-mounted vacuum breakers (603.5.13) | 1 inch above the flood-level rim, per listing |
| VB on a water supply inlet to a tank or vat (603.5.5) | 6 inches, or per listing, on the discharge side of the last valve |
Specific Applications (UPC 603.5)
| Section | Application | Required protection |
|---|---|---|
| 603.5.3 | Water closet flushometer tanks | Approved assembly, device, or method |
| 603.5.4 | Heat exchangers | Protect the potable side. Double-wall exchangers need a space between walls vented to atmosphere; single-wall exchangers in indirect-fired heaters must meet 505.4.1 |
| 603.5.5 | Water supply inlets to tanks, vats, sumps, and pools | Air gap; a listed vacuum breaker on the discharge side of the last valve with its critical level at least 6 inches up; or a backflow preventer suited to the degree of hazard |
| 603.5.7 | Outlets with hose threads (except water heater drains, boiler drains, and clothes washer connections) | Nonremovable hose-bibb backflow preventer or vacuum breaker, or an AVB at least 6 inches above the highest point of use on the discharge side of the last valve. In freezing climates, use a listed self-draining frost-proof hose bibb with integral protection |
| 603.5.8 | Water-cooled compressors and degreasers | Backflow preventer per the chapter; protection against backpressure if the equipment can produce it |
| 603.5.9 | Aspirators | Vacuum breaker on the inlet; discharge through an air gap |
| 603.5.11 | Nonpotable lines that cannot be corrected | Treat as nonpotable; label outlets "CAUTION: NONPOTABLE WATER, DO NOT DRINK" |
| 603.5.15 | Health care and laboratories | Vacuum breakers for bedpan washer hoses at least 5 feet above the floor; hose connections at least 6 feet above the floor |
| 603.5.16 | Portable cleaning equipment, dental vacuum pumps, chemical dispensers | Air gap, AVB, SVB, or RP |
| 603.5.17 | Freeze-proof yard hydrants and stop-and-waste valves that drain into the ground | May not be installed underground |
Containment Devices Set by the Purveyor
Water purveyors sometimes install a residential dual check at single-family meters. It is not a testable assembly and is not in UPC Table 603.2, but it still closes the system. WA 608.3 then requires a thermal expansion tank on the building side (Section 3.4). Under WAC 246-290-490, a purveyor serving a single-family home may rely on UPC point-of-hazard devices. An example is a properly installed AVB on an irrigation system, provided the premises is not on the high-hazard list.
A 3/4-inch potable fill line discharges into an open process tank. The spout's inside edge is 1-1/2 inches from a single vertical tank wall. What minimum air gap does UPC Table 603.3.1 require?
1 inch
1-1/2 inches
2-1/4 inches
3 inches
A vacuum breaker has no critical level marking. From what point is its required height above the flood-level rim measured?
The bottom of the vacuum breaker or device
The highest downstream outlet
The top of the device
The centerline of the inlet
A hose bibb is installed on the exterior wall of a house in Spokane. What does UPC 603.5.7 require?
A removable screw-on vacuum breaker
A listed self-draining frost-proof hose bibb with an integral backflow preventer or vacuum breaker
A double check valve assembly in the crawl space
No protection, because hose bibbs are exempt
Sections you finish are checked off in the contents.