Air Gap Plumbing: How an Air Gap Prevents Backflow
An air gap is an open vertical space between a water outlet and a receiver's flood level. Required size, where it is required, and inspection in CA.
Short answer: An air gap in plumbing is an open, unobstructed vertical space between the outlet of a potable water pipe or faucet and the flood level rim of the tank, sink, or vessel it fills. Because the water has to fall through open air, nothing in the receiver can be pushed or siphoned back into the supply. Under California’s Cross-Connection Control Policy Handbook (CCCPH), an air gap must be at least two times the effective opening of the supply, and never less than one inch (CCCPH 3.1.1).
The air gap is the simplest form of backflow prevention, and the most protective. It has no moving parts to wear out. This guide covers how big an air gap must be, where California requires one, the difference between a water supply air gap and a drain air gap like the one by your kitchen sink, and how air gaps are inspected.
What an air gap is
The CCCPH defines an air-gap separation (AG) as a physical vertical separation of at least two times the effective opening between the free-flowing discharge end of a potable water supply pipeline and the flood level of an open or non-pressurized receiving vessel, and in no case less than one inch (CCCPH 3.1.1).
The 2025 California Plumbing Code defines the water distribution air gap the same way in plain terms: the unobstructed vertical distance through the free atmosphere between the lowest opening from a pipe or faucet conveying potable water and the flood-level rim of a tank, vat, or fixture (CPC Chapter 2 definitions).
Two terms matter:
- Effective opening: the minimum cross-sectional area at the point of discharge, expressed as the diameter of a circle, or the diameter of an equivalent circle if the opening is not round (CPC Chapter 2 definitions).
- Flood level rim: the highest level the water in the receiver can reach before it overflows.
How an air gap prevents backflow
Backflow happens two ways: backsiphonage, when the supply pressure drops below atmospheric and pulls water backward, and backpressure, when pressure on the property side exceeds supply pressure (CCCPH 2.1). An air gap stops both. There is no pipe connecting the supply to what is below it, so a vacuum in the main has nothing to pull on, and pressure in the receiver has nowhere to push.
That is why the California Plumbing Code rates the air gap for both pollution (low hazard) and contamination (high hazard) conditions (CPC Table 603.2), and why the CCCPH accepts an air gap, along with an RP, for containment of high hazard cross-connections (CCCPH 3.2.2(c)).
How big does an air gap need to be?
The CCCPH requires each air gap used in a water system’s program to meet Table 1 of ASME A112.1.2-2012 (R2017), Minimum Air Gaps for Generally Used Plumbing Fixtures (CCCPH 3.3.1(a) and Appendix B). The California Plumbing Code points to the same standard (CPC 603.3.1 and Table 603.2).
| Effective opening of the supply | Minimum air gap, not affected by side walls | Minimum air gap, affected by side walls |
|---|---|---|
| Not greater than 1/2 inch diameter | 1 inch | 1 1/2 inches |
| Not greater than 3/4 inch diameter | 1 1/2 inches | 2 1/4 inches |
| Not greater than 1 inch diameter | 2 inches | 3 inches |
| Greater than 1 inch diameter | 2 times the diameter of the effective opening | 3 times the diameter of the effective opening |
Source: ASME A112.1.2 Table 1, as reproduced in CCCPH Appendix B.
Side walls change the answer. Walls, ribs, or similar obstructions near the outlet can affect how air reaches the gap. Under the table’s notes, side walls do not affect the gap if they are farther from the inside edge of the spout than three times the effective opening for a single wall, or four times for two intersecting walls (CCCPH Appendix B). Closer than that, use the larger “affected by side walls” column.
Where California requires an air gap
At high hazard premises
For high hazard cross-connections, the CCCPH requires containment by an air gap or an RP (CCCPH 3.2.2(c)). Appendix D goes further for a few premises, marking them as isolated by air gap only unless the State Water Board approves an alternative under 3.2.2(c):
- Wastewater treatment processes, handling, or pumping equipment interconnected with piping connected to the public water system.
- Recycled water interconnected with a piping system that contains water from the public water system.
- Drinking water storage tank overflows connected to a sump or storm drain.
Installation rules for an air gap at the service connection
When an air gap is used for premises containment, the CCCPH requires (CCCPH 3.3.2(a)):
- The receiving container must be on the water user’s premises at the service connection, unless the water system approves another location.
- All piping between the service connection and the discharge into the receiving container must be above finished grade and accessible for visual inspection, unless the water system approves another configuration.
- The water system must confirm that the required air gap size has been installed.
- Any new air gap installation at a user’s service connection must be reviewed and approved by the State Water Board before installation.
The water system may, after State Water Board review and approval, use an alternate containment method in place of a required air gap if it would not increase risk to public health (CCCPH 3.2.2(c)(1)).
Inside buildings, under the plumbing code
The California Plumbing Code uses air gaps in many fixture and equipment connections. For example, water supply inlets to tanks, vats, sumps, swimming pools, and other receptors must be protected by an approved air gap, a listed vacuum breaker, or a backflow preventer suited to the hazard (CPC 603.5.5).
Water supply air gap vs. dishwasher air gap
People searching “air gap plumbing” often mean the small chrome cap next to a kitchen faucet. That is a drainage air gap, which protects the dishwasher from the drain, not the public water supply from the property.
| Water supply air gap | Drainage air gap (dishwasher) | |
|---|---|---|
| Protects | The potable water supply from what is downstream | An appliance or fixture from backed-up wastewater |
| Code definition | “Air Gap, Water Distribution” | “Air Gap, Drainage” |
| Where you see it | Tank fills, reservoir and recycled water interconnections, fixtures | Dishwasher discharge at the kitchen sink |
| Inspected by a backflow tester? | Yes, when used in a water system’s program | No |
The California Plumbing Code states that no domestic dishwashing machine may be directly connected to a drainage system or food waste disposer without an approved dishwasher air gap fitting on the discharge side, installed with its flood-level marking at or above the flood level of the sink or drainboard, whichever is higher (CPC 807.3). A drainage air gap is defined as the vertical distance through free atmosphere between the lowest opening of a pipe, fixture, or appliance conveying waste and the flood-level rim of the receptor (CPC Chapter 2 definitions).
How air gaps are inspected
Air gaps cannot be tested with gauges, because there is nothing mechanical to test. Instead, the CCCPH requires each air gap to be visually inspected at least annually by a certified backflow prevention assembly tester or a certified cross-connection control specialist (CCCPH 3.3.3(c)). The inspector confirms that the separation still meets the required size and that nothing has been added to bridge or bypass it.
Water systems must keep, for each air gap installation, the associated hazard, location, owner, and as-built plans (CCCPH 3.5.1(a)(3)), and the results of air gap inspections for the previous three calendar years (CCCPH 3.5.1(a)(4)). Every air gap must also stay accessible for inspection and maintenance (CCCPH 3.3.2(h)).
Common problems an inspector looks for:
- A hose or pipe extension that drops the outlet below the flood level rim.
- A raised water level or blocked overflow that reduces the gap.
- Piping added that connects the supply directly to the receiver, bypassing the gap.
Each of these turns an air gap into a cross-connection. Our field testing and air gap inspection guide covers the inspection from the water system side.
Air gap vs. mechanical assemblies
An air gap is not always practical. It breaks pressure, so water downstream usually needs a tank and pump to be re-pressurized. Where the rules allow either, that can make an RP the more practical choice. See types of backflow preventers and RPZ backflow preventer explained for the trade-offs.
Frequently asked questions
What is an air gap in plumbing? An open vertical space between the outlet of a water supply pipe or faucet and the flood level rim of the receiver below it. Water falls through open air, so it cannot flow back into the supply.
How big does an air gap need to be? At least two times the effective opening of the supply, and never less than one inch (CCCPH 3.1.1). Larger gaps are needed where side walls are close to the outlet (CCCPH Appendix B).
Is an air gap better than a backflow preventer? An air gap gives the highest protection because there is no physical connection. The CCCPH accepts either an air gap or an RP for high hazard containment (CCCPH 3.2.2(c)).
Does an air gap need to be tested every year? It is visually inspected, not gauge tested. The CCCPH requires an inspection at least annually by a certified tester or cross-connection control specialist (CCCPH 3.3.3(c)).
Does California require a dishwasher air gap? The California Plumbing Code requires an approved dishwasher air gap fitting where a domestic dishwasher connects to a drainage system or food waste disposer (CPC 807.3).
Is a dishwasher air gap the same as a backflow air gap? No. A dishwasher air gap is a drainage air gap that protects the appliance from wastewater. A water supply air gap protects the drinking water supply.
Do I need State Water Board approval for an air gap? A new air gap installation at a user’s service connection used for a water system’s program must be reviewed and approved by the State Water Board before installation (CCCPH 3.3.2(a)(4)).
Last reviewed: October 30, 2026.
Sources
- State Water Board, Cross-Connection Control Policy Handbook, April 2026 revision (PDF): sections 2.1, 3.1.1, 3.2.2, 3.3.1, 3.3.2, 3.3.3, 3.5.1, and Appendices B and D
- State Water Board, Cross-Connection Control Policy Handbook page
- IAPMO, 2025 California Plumbing Code (read-only edition): Chapter 2 definitions, sections 603.3.1, 603.5.5, 807.3, and Table 603.2
Need an air gap inspected or a cross-connection reviewed? Call (805) 991-7333 or request a review.