# Types of Backflow Preventers Compared (CA)

> Types of backflow preventers compared: air gap, RP, double check, PVB, SVB, and RPDA or DCDA fire assemblies, with hazard level, testing, and uses.

Source: https://www.exoduswater.com/post/types-of-backflow-preventers

Cross-connection & backflow October 30, 2026Exodus Water Services 

# Types of Backflow Preventers: AG, RP, DC, PVB, SVB, and Fire Line Assemblies

Types of backflow preventers compared: air gap, RP, double check, PVB, SVB, and RPDA or DCDA fire assemblies, with hazard level, testing, and uses.

**Short answer:** The main types of backflow prevention are the **air gap (AG)** , the **reduced pressure principle assembly (RP)** , the **double check valve assembly (DC)** , the **pressure vacuum breaker (PVB)** , and the **spill-resistant vacuum breaker (SVB)** , plus the detector versions used on fire lines, the **RPDA** and **DCDA** . In California, an air gap or RP is required for high hazard connections, a DC may only isolate low hazard connections, and a PVB or SVB protects only against backsiphonage, not backpressure. These rules come from the State Water Board’s Cross-Connection Control Policy Handbook (CCCPH), revised April 21, 2026.

The right type depends on two questions: how dangerous is what could flow back (the degree of hazard), and can backflow happen by backpressure, backsiphonage, or both? This guide answers both and compares every type side by side.

## Backsiphonage and backpressure: the two ways backflow happens

The CCCPH describes two causes of backflow (CCCPH 2.1):

- **Backsiphonage** happens when pressure in the water system drops below atmospheric pressure, for example during a main break, flushing, a pump failure, or heavy firefighting demand. Water on the property can be pulled backward into the public supply.
- **Backpressure** happens when pressure on the property side is higher than the water system’s pressure, for example from a pump, boiler, or pressurized system connected to the potable supply. Water is pushed backward.

Some preventers stop both. Vacuum breakers stop only backsiphonage. That single difference rules them out of many applications.

## High hazard vs. low hazard

The CCCPH sorts cross-connections by what they could put into the water (CCCPH 3.1.1):

- A **high hazard cross-connection** poses a threat to the potability or safety of the public water supply. What could enter is a contaminant or health hazard.
- A **low hazard cross-connection** does not threaten potability or safety but may affect the aesthetic quality of the water. What could enter is a pollutant or non-health hazard.

Your water supplier decides which applies through a **hazard assessment** of your property (CCCPH 3.2.1). The handbook’s Appendix D lists premises it treats as high hazard, from medical and dental offices to car washes, hotels, and sites with an auxiliary water supply. For high hazard premises, the water system must provide containment with an air gap or an RP (CCCPH 3.2.2(c)).

## Comparison table: types of backflow preventers

| Type                                           | Protects against               | Hazard level                                | Field testable?                          | Typical uses                                                             |
| ---------------------------------------------- | ------------------------------ | ------------------------------------------- | ---------------------------------------- | ------------------------------------------------------------------------ |
| Air gap (AG)                                   | Backsiphonage and backpressure | High or low                                 | No. Visually inspected at least annually | Tank and reservoir fills, recycled water and wastewater interconnections |
| Reduced pressure principle assembly (RP)       | Backsiphonage and backpressure | High or low                                 | Yes, at least annually                   | High hazard premises containment, medical, industrial, commercial        |
| Double check valve assembly (DC)               | Backsiphonage and backpressure | Low only                                    | Yes, at least annually                   | Low hazard services and many fire sprinkler systems                      |
| Pressure vacuum breaker (PVB)                  | Backsiphonage only             | High or low, where there is no backpressure | Yes, at least annually                   | Irrigation systems                                                       |
| Spill-resistant vacuum breaker (SVB)           | Backsiphonage only             | High or low, where there is no backpressure | Yes, at least annually                   | Irrigation and indoor uses where spillage matters                        |
| RP detector assembly (RPDA, RPDA-II)           | Backsiphonage and backpressure | High or low                                 | Yes, at least annually                   | Fire lines needing RP protection                                         |
| Double check detector assembly (DCDA, DCDA-II) | Backsiphonage and backpressure | Low only                                    | Yes, at least annually                   | Low hazard fire lines                                                    |

Hazard levels for the mechanical assemblies follow the CCCPH definitions and the California Plumbing Code’s Table 603.2, which rates PVBs and SVBs for backsiphonage at both pollution (low hazard) and contamination (high hazard) levels, but not for backpressure. Annual testing comes from CCCPH 3.3.3(b), and annual air gap inspection from CCCPH 3.3.3(c).

## Air gap (AG)

An air gap is a physical vertical separation between the end of a potable water pipe and the flood level of an open or non-pressurized receiving vessel. In the CCCPH it must be at least two times the effective opening, and never less than one inch (CCCPH 3.1.1). Because there is no connection at all, an air gap protects against both backsiphonage and backpressure.

Air gaps have no moving parts, so they are not tested with gauges. They must be visually inspected at least annually by a certified tester or cross-connection control specialist (CCCPH 3.3.3(c)). Some Appendix D premises, such as recycled water interconnected with potable piping, must be isolated by an air gap only, unless an alternative is approved under CCCPH 3.2.2(c). Read more in [air gap backflow prevention](https://www.exoduswater.com/post/air-gap-backflow-prevention) .

## Reduced pressure principle assembly (RP or RPZ)

An RP has two independently acting, internally loaded check valves, with a hydraulically operating, mechanically independent differential-pressure relief valve between them, plus shutoff valves at each end and test cocks for field testing (CCCPH 3.1.1). If either check valve leaks, the relief valve can discharge water to the atmosphere rather than let it flow backward. That is why an RP is acceptable for high hazard connections, and why the California Plumbing Code notes that an RP “may discharge water” (CPC Table 603.2).

RPs must be installed with the lowest point at least 12 inches above grade, and no more than 36 inches above finished grade unless the water system approves otherwise (CCCPH 3.3.2(b)). See our full guide: [RPZ backflow preventer explained](https://www.exoduswater.com/post/rpz-backflow-preventer-explained) .

## Double check valve assembly (DC)

A DC has two independently acting, internally loaded check valves with tightly closing shutoff valves at each end and test cocks (CCCPH 3.1.1). It has no relief valve. The CCCPH states that a DC may only be used to isolate **low hazard** cross-connections (CCCPH 3.1.1).

DCs installed or replaced after the CCCPH was adopted must follow the same above-grade installation rule as RPs, with below-ground installation possible only if the water system approves it because no alternative exists (CCCPH 3.3.2(c)). See [double check valve assembly explained](https://www.exoduswater.com/post/double-check-valve-assembly-explained) .

## Pressure vacuum breaker (PVB)

A PVB has one independently acting, internally loaded check valve and a loaded air inlet valve on the discharge side of the check, with test cocks and shutoff valves at each end (CCCPH 3.1.1). When supply pressure drops, the air inlet opens and breaks the vacuum, so water cannot be siphoned back. The CCCPH limits PVBs to backsiphonage protection and states they are not to be used to protect from backpressure (CCCPH 3.1.1).

A PVB must be installed at least 12 inches above all downstream piping and outlets (CCCPH 3.3.2(d)). For premises containment, a PVB may only be used for roadway right-of-way irrigation systems where there is no potential for backpressure (CCCPH 3.3.2(e)).

## Spill-resistant vacuum breaker (SVB)

An SVB works like a PVB, with a check valve and an air inlet valve downstream of it, but it has a test cock and a bleed or vent port designed to limit spillage (CCCPH 3.1.1). Like the PVB, it protects only against backsiphonage. It must be installed at least 12 inches above all downstream piping and outlets, and it **may not** be used for premises containment (CCCPH 3.3.2(d) and (e)).

## Fire line assemblies: RPDA and DCDA

Fire sprinkler lines often use **detector assemblies** . A detector assembly is an RP or DC with a bypass that contains a water meter and a smaller assembly of the same type. The bypass meter must accurately register flows up to two gallons per minute and show registration for all flow rates (CCCPH 3.1.1). That lets the water system detect small, unauthorized use or leaks on a line that should normally have no flow.

- **DCDA and DCDA-II** are double check detector assemblies. Like the DC, they may only isolate low hazard cross-connections (CCCPH 3.1.1).
- **RPDA and RPDA-II** are RP detector assemblies, for situations that require RP protection.
- The “type II” versions put the bypass around the second check only, with a single check valve and meter in the bypass (CCCPH 3.1.1).

Where the CCCPH requires a DC, a DCDA or DCDA-II may be substituted if appropriate, and where it requires an RP, an RPDA or RPDA-II may be substituted (CCCPH 3.2.2(a)).

**Fire system rules:** For user premises with a fire protection system, water systems must ensure no less than DC protection within ten years of the CCCPH’s effective date, with limited exceptions. Fire systems that are high hazard, including those that may use chemical additives such as antifreeze or foam, or that have an auxiliary water supply, need at least RP protection (CCCPH 3.2.2(e) and (e)(1)). Some low hazard residential fire systems on a single service with looped NSF/ANSI 61 piping do not need an assembly at all (CCCPH 3.2.2(e)(3)).

## Devices that are not testable assemblies

You will also see smaller **devices** in plumbing, such as atmospheric vacuum breakers (AVBs) on irrigation lines and hose connection vacuum breakers on outdoor faucets. The California Plumbing Code allows them in specific uses (CPC 603.5.6 and 603.5.7), but they have no test cocks, so they are not the field testable assemblies a water system relies on for containment. See [hose bib backflow preventers](https://www.exoduswater.com/post/hose-bib-backflow-preventer) .

## Who decides which type you need

Your water supplier does, through a hazard assessment reviewed or conducted by a certified cross-connection control specialist (CCCPH 3.2.1(h)). The assembly installed must be no less protective than the degree of hazard requires (CCCPH 3.2.2(b)). Every new or replacement RP, DC, PVB, and SVB must also be an approved assembly, evaluated under the USC Foundation for Cross-Connection Control and Hydraulic Research standards or ASSE International standards (CCCPH 3.3.1(b)).

Where the California Plumbing Code and the CCCPH set different minimum protection, the more protective one applies (CCCPH 2.3).

## Frequently asked questions

**What are the main types of backflow preventers?**Air gap (AG), reduced pressure principle assembly (RP), double check valve assembly (DC), pressure vacuum breaker (PVB), and spill-resistant vacuum breaker (SVB), plus the detector versions RPDA and DCDA used on fire lines.

**Which backflow preventer gives the most protection?**An air gap or an RP. They are the two methods the CCCPH accepts for containment of high hazard cross-connections (CCCPH 3.2.2(c)).

**Can a double check valve be used for a high hazard connection?**No. The CCCPH limits the DC, DCDA, and DCDA-II to isolating low hazard cross-connections (CCCPH 3.1.1).

**What is the difference between a PVB and an SVB?**Both have a check valve and an air inlet valve and protect only against backsiphonage. The SVB has a bleed or vent port designed to limit spillage. A PVB may be used for containment only on roadway right-of-way irrigation, and an SVB may not be used for containment at all (CCCPH 3.3.2(e)).

**What backflow preventer is used on fire sprinkler lines?**Usually a double check detector assembly (DCDA) for low hazard systems, or an RP detector assembly (RPDA) where RP protection is needed, such as systems using chemical additives or an auxiliary water supply (CCCPH 3.2.2(e)).

**Do all backflow preventers need annual testing?**Every testable assembly (RP, DC, PVB, SVB, RPDA, DCDA) must be field tested at least annually. Air gaps are visually inspected at least annually instead (CCCPH 3.3.3(b) and (c)).

**What is an RPZ?**RPZ is a common name for the reduced pressure principle assembly, or RP. The “zone” is the space between the two check valves, where the differential-pressure relief valve sits (CCCPH 3.1.1).

Last reviewed: October 30, 2026.

## Sources

- State Water Board, [Cross-Connection Control Policy Handbook, April 2026 revision (PDF)](https://www.waterboards.ca.gov/drinking%5Fwater/certlic/drinkingwater/docs/2026/cross-connection-control-policy-handbook-apr-2026.pdf) : sections 2.1, 2.3, 3.1.1, 3.2.1, 3.2.2, 3.3.1, 3.3.2, 3.3.3, and Appendix D
- State Water Board, [Cross-Connection Control Policy Handbook page](https://www.waterboards.ca.gov/drinking%5Fwater/certlic/drinkingwater/cccph.html)
- IAPMO, [2025 California Plumbing Code (read-only edition)](https://epubs.iapmo.org/2025/CPC/) : sections 603.3, 603.5.6, 603.5.7, and Table 603.2

Not sure which assembly your property needs? Call (805) 991-7333 or [request a review](https://www.exoduswater.com/contact-us?service=backflow) .

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