Double Check Valve Assembly: How It Works (CA)

Short answer: A double check valve assembly (DC, also called a DCVA) is a backflow prevention assembly made of two independently acting, internally loaded check valves in series, with a tightly closing shutoff valve at each end and test cocks for field testing. It protects against both backsiphonage and backpressure, but California’s Cross-Connection Control Policy Handbook (CCCPH) allows a DC to isolate low hazard cross-connections only (CCCPH 3.1.1). For high hazard connections, an air gap or a reduced pressure principle assembly (RP) is required.

The double check is one of the most common assemblies on commercial services and fire sprinkler lines. This guide covers how it works, where it is allowed, the California installation and testing rules, and when you have to replace it with something more protective.

What is a double check valve assembly?

The CCCPH defines a DC as an assembly of two independently acting internally loaded check valves, with tightly closing shutoff valves upstream and downstream of the two checks, fitted with test cocks that enable accurate field testing (CCCPH 3.1.1). The California Plumbing Code describes the same parts: two check valves, four properly located test cocks, and two isolation valves (CPC 603.3.4).

A few terms worth knowing:

Check valve: a valve that lets water flow in one direction and closes when flow tries to reverse.
Internally loaded: each check is held closed by a spring or similar loading until supply pressure opens it.
Independently acting: each check works on its own, so if one fails, the other can still close.
Test cocks: small valved ports that let a tester connect gauges and check each valve.

How a double check works

Under normal flow, water pushes through the first check, then the second, and into the property. If pressure on the supply side drops, or pressure on the property side rises, both checks close. Because there are two, one fouled check does not by itself let water reverse.

What a DC does not have is a relief valve. An RP adds a differential pressure relief valve between its checks that can discharge water if the checks leak (CCCPH 3.1.1). A DC has no such outlet, so if both checks are held open by debris, nothing signals the problem until the next test. That is the reason the DC is limited to low hazard uses. The California Plumbing Code’s Table 603.2 notes that a DC “Does not discharge water” and rates it for pollution (low hazard) under both backsiphonage and backpressure, but not for contamination (high hazard).

Where California allows a double check

A DC “may only be used to isolate low hazard cross-connections” (CCCPH 3.1.1). A low hazard cross-connection is one that does not threaten the potability or safety of the public water supply but may affect its aesthetic quality; materials entering through it are pollutants or non-health hazards (CCCPH 3.1.1).

Your water supplier decides whether your property is low or high hazard through a hazard assessment (CCCPH 3.2.1). Common places you may see a DC include:

Fire sprinkler systems that are low hazard. 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 (CCCPH 3.2.2(e)).
Low hazard commercial or domestic services where the hazard assessment calls for containment but finds no health hazard.
Boiler connections under the plumbing code, where the CPC allows a DC for potable water connections to steam or hot water boilers unless chemicals are introduced, in which case an RP is required (CPC 603.5.10).

Where a double check is not enough

High hazard premises. The CCCPH requires an air gap or an RP for containment of high hazard cross-connections (CCCPH 3.2.2(c)). Appendix D lists examples, including medical facilities, dental offices with water-connected equipment, car washes, hotels and motels, laboratories, and sites with an auxiliary water supply.
High hazard fire systems. A fire protection system that may use chemical additives, such as antifreeze, foam, or corrosion inhibitors, or that has an auxiliary water supply, needs no less than RP protection (CCCPH 3.2.2(e)(1)).
Irrigation with pumps or backpressure. The California Plumbing Code calls for an RP where a sprinkler or irrigation system has pumps or can create backpressure (CPC 603.5.6.1).

The pre-2024 double check exception

Some high hazard premises still have a DC installed before the CCCPH took effect. The handbook lets a user premises with an existing DC installed prior to July 1, 2024, keep using it for premises isolation at sites with an auxiliary water supply if all of these conditions are met (CCCPH Appendix D):

Backflow does not occur at any time at the premises.
The DC passes all field tests within the testing and repair timeframe in the water system’s Cross-Connection Control Plan.
The water system approves the DC in writing and records its location.
No other high hazard activities are identified during a hazard assessment.

Once that DC is replaced, the exception ends: an air gap, RP, or both is required for all new or replacement installations after July 1, 2024 (CCCPH Appendix D). If an older DC at your site fails and cannot be repaired, plan for an RP. See RPZ backflow preventer explained.

Double check detector assemblies (DCDA)

Fire lines often use a double check detector assembly (DCDA): a DC with a bypass containing a water meter and a smaller double check assembly. The bypass meter must accurately register flows up to two gallons per minute and show registration at all flow rates (CCCPH 3.1.1). A DCDA-II puts the bypass around the second check only, with a single check valve and meter in the bypass. Both may only isolate low hazard cross-connections (CCCPH 3.1.1).

Where the CCCPH requires a DC, a DCDA or DCDA-II may be substituted if appropriate (CCCPH 3.2.2(a)).

Double check vs. RPZ vs. PVB

Feature	Double check (DC)	RP (RPZ)	PVB
Main parts	Two check valves	Two check valves and a relief valve	One check valve and an air inlet valve
Stops backpressure	Yes	Yes	No
Stops backsiphonage	Yes	Yes	Yes
Hazard level allowed	Low only	High or low	Backsiphonage-only uses
Discharges water	No	May discharge	May discharge

Sources: CCCPH 3.1.1 and CPC Table 603.2. For all types side by side, see types of backflow preventers.

California installation rules for a double check

Above grade by default. A DC installed or replaced after the CCCPH was adopted must follow the RP height rule: lowest point at least 12 inches above grade and, unless the water system approves otherwise, no more than 36 inches above finished grade. Below-ground installation can be considered only if the water system approves it where no alternative is available (CCCPH 3.3.2(b) and (c)).
Clearance. At least 12 inches of side clearance, and 24 inches on the test cock side, unless the water system approves alternate clearances that still allow testing and maintenance (CCCPH 3.3.2(f)).
Location and access. As close as practical to the service connection, and accessible for testing, inspection, and maintenance (CCCPH 3.3.2(g) and (h)).
Approval. Each new or replacement DC must be approved under the USC Foundation for Cross-Connection Control and Hydraulic Research standards or ASSE International standards, including ASSE 1015-2021 for the DC and ASSE 1048-2021 for the DCDA, with the 1YT mark where ASSE is used (CCCPH 3.3.1(b)).

How a double check is tested

A DC must be field tested by a certified backflow prevention assembly tester after installation, repair, winterizing depressurization, or permanent relocation, and at least annually (CCCPH 3.3.3(a) and (b)). The tester connects a test kit to the test cocks and confirms that each check valve holds and that the shutoff valves close tightly. The California Plumbing Code requires the test kit to comply with ASSE 1064 (CPC 603.4.2).

If the DC fails, it must be repaired or replaced within 30 days of notification, then retested (CCCPH 3.3.3(e)). See what happens if a backflow test fails.

Frequently asked questions

What is a double check valve assembly? A backflow prevention assembly with two independently acting, internally loaded check valves, a shutoff valve at each end, and test cocks for field testing (CCCPH 3.1.1).

Is a double check valve the same as a backflow preventer? It is one type of backflow preventer. Others include the air gap, RP, PVB, and SVB. The DC is limited to low hazard connections in California.

Can a double check be used for high hazard connections? No. The CCCPH says a DC may only isolate low hazard cross-connections. High hazard containment needs an air gap or RP (CCCPH 3.1.1 and 3.2.2(c)).

What is the difference between a double check and an RPZ? An RPZ adds a differential pressure relief valve between the two checks that can discharge water if they leak. That makes the RPZ acceptable for high hazard connections; the double check is not.

Can a double check valve be installed below ground in California? Only if the water system approves it where no alternative is available. Otherwise, a DC installed or replaced after the CCCPH was adopted must sit 12 to 36 inches above grade (CCCPH 3.3.2(b) and (c)).

How often does a double check need testing? At least once a year, and after installation, repair, relocation, or winterizing (CCCPH 3.3.3).

What is a DCDA? A double check detector assembly: a DC with a metered bypass containing a smaller double check, used mostly on low hazard fire lines (CCCPH 3.1.1).

Last reviewed: October 30, 2026.

Sources

State Water Board, Cross-Connection Control Policy Handbook, April 2026 revision (PDF): sections 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
IAPMO, 2025 California Plumbing Code (read-only edition): sections 603.3.4, 603.4.2, 603.5.6.1, 603.5.10, and Table 603.2

Need a double check tested or a hazard review? Call (805) 991-7333 or request a review.

Double Check Valve Assembly Explained: How a DC Backflow Preventer Works