If you’ve been told your property needs a backflow preventer, the next question is almost always: which kind? Understanding the differences between a double check valve vs RPZ backflow preventer is essential for choosing the right device. The two most common devices installed in residential and commercial properties are the double check valve (DCV) and the reduced pressure zone (RPZ) backflow preventer. They both protect your drinking water, but they work differently and are approved for different situations.
Choosing the wrong one can mean a failed inspection, a code violation, or worse, contaminated water making it back into the public supply. This guide breaks down exactly how each device works, where each is required, and how to figure out which one your property actually needs.
What Is a Double Check Valve Assembly?
A double check valve assembly (also called a DCVA or double check backflow preventer) uses two independently operating spring-loaded check valves arranged in sequence. If one check valve fails, the second one serves as a backup. Water can only flow in one direction, and if the pressure reverses, both valves close to block the backflow.
Double check assemblies are considered a moderate-level protection device. They are approved for use where the risk of contamination is considered low to medium, specifically where the substance that could backflow is non-hazardous to health.
Common applications include:
- Residential irrigation systems with no chemical injection
- Fire sprinkler systems using potable water only
- Commercial properties with low-hazard connections
- Lawn sprinkler systems in single-family homes
Double check valves are generally more compact, easier to install indoors, and less expensive than RPZ devices. They do not discharge water during normal operation, which makes them easier to manage in finished spaces.
What Is an RPZ Backflow Preventer?
A reduced-pressure zone (RPZ) backflow preventer is a more advanced device designed for high-hazard applications. It contains two check valves like a double check assembly, but it adds a third critical component: a relief valve located in a pressure-monitored zone between the two checks.
If either check valve fails or if backpressure or back-siphonage occurs, the relief valve opens and discharges water to the atmosphere rather than allowing contaminated water to flow back into the supply line. This makes it a fail-safe device.
RPZ backflow preventers are required for high-hazard connections, including:
- Irrigation systems with fertilizer or pesticide injectors
- Commercial boilers and heating systems
- Medical or dental facilities
- Car washes and chemical processing
- Any connection where toxic or health-hazardous substances could enter the water supply
Because the relief valve discharges water when it activates, RPZ devices must be installed above ground in a location where water discharge will not cause damage. In Virginia, they cannot be installed in pits or vaults.
Double Check Valve vs. RPZ Backflow Preventer: Side-by-Side Comparison
| Feature | Double Check Valve | RPZ Backflow Preventer |
| Protection Level | Moderate (low-to-medium hazard) | High (health hazard applications) |
| Components | Two check valves | Two check valves + relief valve |
| Water Discharge | None during normal use | Discharges if a check valve fails |
| Installation Location | Indoor or outdoor; can be in a vault | Must be above ground, never in a pit |
| Common Applications | Residential irrigation, fire lines (potable) | Chemical injection, boilers, and medical facilities |
| Cost | Lower upfront and maintenance cost | Higher upfront and maintenance cost |
| Virginia Code Approval | Yes, for low/medium hazard | Yes, required for high hazard |
| Annual Testing Required | Yes | Yes |
| Feature | Double Check Valve | RPZ Backflow Preventer |
|---|---|---|
| Protection Level | Moderate (low-to-medium hazard) | High (health hazard applications) |
| Components | Two check valves | Two check valves + relief valve |
| Water Discharge | None during normal use | Discharges if a check valve fails |
| Installation Location | Indoor or outdoor; can be in a vault | Must be above ground, never in a pit |
| Common Applications | Residential irrigation, fire lines (potable) | Chemical injection, boilers, and medical facilities |
| Cost | Lower upfront and maintenance cost | Higher upfront and maintenance cost |
| Virginia Code Approval | Yes, for low/medium hazard | Yes, required for high hazard |
| Annual Testing Required | Yes | Yes |
Which One Does Virginia Require for Your Property?
In Virginia, backflow preventer requirements are governed by the Virginia Waterworks Regulations and local water authority rules (such as those set by Fairfax Water, WSSC, or Prince William County Service Authority). The hazard level of your connection determines which device you need.
Low-to-Medium Hazard Connections
If your property has a standard irrigation system without any chemical injectors, a double check valve assembly is typically the approved and required device. It satisfies code for residential lawn sprinkler systems throughout Northern Virginia.
High-Hazard Connections
If your system involves any substance that could be harmful to human health, an RPZ is required by code. This includes irrigation systems with fertilizer injectors, commercial HVAC connections, and any facility handling chemicals, pharmaceuticals, or biological materials.
If you are unsure whether your connection is classified as low or high hazard, a certified backflow technician can perform a hazard assessment. You can learn more about what properties are legally required to have backflow protection at our page on backflow preventer requirements.
Can You Upgrade from a Double Check to an RPZ?
Yes, and in some cases your water authority may require it. If you add a chemical injector to an existing irrigation system that previously only needed a double check valve, you are now operating a high-hazard connection. Continuing to use the double-check assembly would put you out of compliance.
Upgrades are also sometimes triggered by water authority reassessments or when a property changes its use, such as converting a residential property to a commercial space.
Our backflow installation service
covers both new installs and device upgrades, and our technicians can pull the required permits in Northern Virginia jurisdictions.
How Often Do Both Devices Need Testing?
Both double check valves and RPZ backflow preventers must be tested annually in Virginia. Testing must be performed by a licensed and certified backflow preventer tester, and results must be submitted to your local water authority.
Annual testing verifies that:
- Check valves are seating properly and holding pressure
- The relief valve (on RPZ devices) opens at the correct differential pressure
- No internal components have worn or corroded
Failing to test on schedule can result in a notice of violation from your water authority. Both device types can fail silently, meaning your water supply could be at risk without any visible signs. Routine testing is the only way to confirm your device is working.
Frequently Asked Questions
Can a double check valve be installed underground?
Yes. Unlike RPZ devices, double check valve assemblies can be installed in below-grade vaults or pits, which makes them a common choice when aesthetics or space are a concern.
Is an RPZ backflow preventer more reliable than a double check valve?
An RPZ offers a higher level of protection because of its relief valve, but both devices are reliable when properly maintained. The right choice depends on the hazard classification of your connection, not on which device is “better” in general terms.
What happens if the wrong device is installed?
If a double check valve is installed where an RPZ is required, your property will fail its backflow inspection, and you may receive a compliance notice from your water authority. In some cases, the water authority can require disconnection of service until the correct device is installed.
Do I need a backflow preventer if I only have a single-family home?
In most Northern Virginia jurisdictions, yes. If your home has an irrigation system, a secondary water connection, or a fire suppression line, a backflow preventer is required.
Learn more about whether backflow preventers are required for your property.
How much does each type of device cost to install?
Double-check assemblies are generally less expensive, with installed costs typically lower than RPZ devices. RPZ units cost more because of the added component complexity and the requirement for above-ground installation with proper drainage. Exact pricing varies based on pipe size and site conditions.
Who can test or install backflow preventers in Virginia?
Testing and installation must be performed by a technician certified by the Virginia Department of Health or an approved equivalent certification program. Always verify credentials before hiring.
Not Sure Which Device Is Right for You?
Choosing between a double check valve and an RPZ backflow preventer is not always straightforward. Hazard classifications can be nuanced, local water authority requirements vary, and an incorrect installation can lead to failed inspections or compliance issues down the road.
Our certified technicians can assess your property, identify the correct device for your application, and handle installation and permitting from start to finish. We serve homeowners and commercial property managers throughout Northern Virginia.
Explore our backflow services or schedule a backflow preventer installation today.
Not sure which device is right for you? Our certified technicians can assess your property. Contact us today.