RPZ vs double check valve comes down to one part and one question. A reduced pressure zone assembly (RPZ) has a relief valve between its two check valves that dumps water if either check fails, so it is used where contaminated water could reach the public supply. A double check valve assembly has two check valves and no relief valve, so it is limited to low hazard connections. Which one your building needs is decided by your water supplier’s hazard assessment, not by preference or price.
You will also see an RPZ called an RP, an RPBA or a reduced pressure principle assembly, and a double check called a DC or DCVA. They are the same two devices.
What each assembly does
Both devices stop water in your building from flowing back into the public main. That can happen when pressure in the main drops, or when something in the building pushes harder than the supply.
A double check valve assembly is two spring-loaded check valves in a row, with a shutoff valve at each end and test cocks for a tester’s gauge. If water tries to reverse, both checks close. The second check is a backup for the first.
A reduced pressure zone assembly also has two check valves. Between them is a chamber kept at a lower pressure than the supply, with a relief valve that opens to the outside. California’s State Water Board describes it as two independently acting internally-loaded check valves, with a hydraulically operating, mechanically independent differential-pressure relief valve located between the check valves.
That relief valve is the difference. If the first check leaks or the second check fails, the relief valve opens and discharges water to the outside. The contaminated water leaves through the relief port. It does not reach the main.
A double check that fails does so silently, and nothing shows until the annual test. An RPZ that fails shows it, because water runs from the relief port.
RPZ vs double check valve at a glance
| RPZ (reduced pressure zone) | Double check valve assembly | |
|---|---|---|
| Check valves | Two | Two |
| Relief valve | Yes, between the checks | No |
| Hazard level it may protect | High hazard (health hazards) and low hazard | Low hazard only |
| How failure shows | Water discharges from the relief port | No outward sign until it is tested |
| Drainage needed | Yes. It can discharge a lot of water | No |
| Installation | Above ground, with clearance, never in a pit that can flood | More flexible, subject to local rules |
| Pressure loss | Higher | Lower |
| Cost to buy and install | Higher | Lower |
| Testing | Periodic test by a certified tester | Periodic test by a certified tester |
Reduced pressure zone vs double check: the hazard decides
Regulators sort cross-connections by what could flow backward.
California’s Cross-Connection Control Policy Handbook, which took effect on July 1, 2024 and was revised in April 2026, defines the two levels. A high hazard cross-connection is one that poses a threat to the potability or safety of the public water supply, involving contaminants or health hazards. A low hazard cross-connection may adversely affect the aesthetic quality of the water, involving pollutants or non-health hazards.
The handbook then sets the limit that matters here. Of the double check valve assembly it says: “This type of assembly may only be used to isolate low hazard cross-connections.” High hazard connections have to be protected with an air gap or a reduced pressure principle assembly.
So the choice is not a matter of taste. If the connection is high hazard, a double check is not permitted for it.
What counts as high hazard
The handbook lists types of premises that water systems treat as high hazard, including sewage handling facilities, mortuaries, car washes and sites with an auxiliary water supply. Water suppliers commonly add medical and dental buildings, laboratories, chemical and plating plants, and premises with boilers or cooling systems that use chemical treatment.
Individual connections inside a building are classified too. An irrigation system with chemical injection, a boiler with treatment chemicals and a process line are typical high hazard connections. A fire sprinkler system with no additives is often treated as low hazard, and one with antifreeze or other additives as high hazard.
These are general examples. The classification for your building is made by your water supplier after its hazard assessment, and two suppliers can reach different conclusions about similar buildings.
Who decides which one you need
Three bodies have a say, and all three have to be satisfied.
The water supplier assesses the hazard at your premises and tells you what protection is required at the service connection. In the City of Sacramento that is the Department of Utilities’ Cross-Connection Control Program. Other parts of the county are served by other suppliers with their own programs.
The building department enforces the plumbing code for protection inside the building, at individual fixtures and equipment.
The fire authority has requirements for backflow protection on fire sprinkler connections.
If you have received a notice, it normally states the assembly type required. If you are planning a tenant improvement, a change of use or new equipment, ask the supplier before you buy anything. A change of tenant can change the hazard level. A retail unit that becomes a dental office is the standard example.
Whatever is installed also has to be an approved model. Suppliers usually require assemblies from an approved list, such as the one kept by the USC Foundation for Cross-Connection Control and Hydraulic Research.
What the difference means on site
The two assemblies make different demands on the building.
Drainage. An RPZ can discharge a large volume of water from its relief port, and it does so without warning. It needs a drain or a location where that water can go without causing damage. This is the most common reason an RPZ cannot simply be put where a double check used to be.
Location. An RPZ must be installed above ground or above the floor, with clearance below the relief port. It cannot go in a pit or vault that could flood, because a submerged relief port becomes a cross-connection. Double checks are sometimes allowed in vaults, depending on local rules.
Pressure. An RPZ takes more pressure out of the system than a double check. In a building where pressure is already marginal on the upper floors, that has to be allowed for.
Freezing and damage. Above-ground assemblies outdoors need protection from frost, vehicles and vandalism. An enclosure must still let the relief valve drain freely.
Space and access. Both need room for a tester to connect a gauge and for a plumber to rebuild them.
Converting from a double check to an RPZ
This comes up when a supplier reassesses a building, or when the use changes. It is rarely a like-for-like swap.
The plumber has to find a location with drainage, raise the assembly above grade if it was in a vault, allow for the extra pressure loss, and often re-pipe the approach. The work needs a permit in most jurisdictions and a passing test on completion. Ask for a site visit and a written price before assuming it is a simple replacement.
What a plumber will check next
- What your notice or your supplier requires, in writing.
- The size and location of the existing assembly.
- Whether there is drainage for a relief valve discharge.
- Available pressure, and what the building can afford to lose.
- Whether the model is on the supplier’s approved list.
- Permit, test and filing requirements.
Testing is the same either way
Both types have to be tested when installed and periodically after that by a certified backflow assembly tester. The City of Sacramento’s Cross-Connection Control Program states that state regulations require property owners to have their backflow prevention assemblies tested annually by a certified tester.
An RPZ test checks both check valves and the relief valve opening point. A double check test checks that both checks hold. An RPZ has more parts, so it tends to cost a little more to test and to rebuild. We set out what moves the price in how much a backflow test costs in Sacramento.
What not to do
Do not choose a double check because it is cheaper if your supplier requires an RPZ. It will not be accepted, and you will pay twice.
Do not plug, cap or pipe a relief port to stop it dripping. A dripping relief valve means debris on a check seat or a worn part, and it needs servicing.
Do not install an RPZ in a pit, or anywhere its discharge would flood a finished space.
Do not let unqualified staff adjust, dismantle or bypass a backflow assembly. These are regulated devices, and repairs have to be followed by a test from a certified tester.
Do not assume the assembly that came with the building is still the right one after a change of tenant or use.
Frequently asked questions
What is the difference between an RPZ and a double check valve? Both have two check valves. An RPZ adds a relief valve between them that discharges water if a check fails, which makes it suitable for health hazards. A double check has no relief valve and is limited to low hazard connections.
Is an RPZ better than a double check valve? It gives a higher level of protection. It also costs more, loses more pressure and needs drainage. Where the hazard is low and the supplier accepts a double check, the simpler device is appropriate.
Can I replace an RPZ with a double check valve? Only if your water supplier agrees that the connection is low hazard. If the connection is classified as high hazard, California’s handbook does not allow a double check to protect it.
Why is water coming out of my RPZ? The relief valve is opening. Occasional spitting can follow a pressure change. Continuous discharge means a check valve is fouled or worn, or the relief valve itself needs service. Call a certified tester or plumber and do not block the port.
Does a fire sprinkler system need an RPZ or a double check? It depends on the system and on local rules. Systems with no chemical additives are often classed as low hazard, and systems with antifreeze or other additives as high hazard. Your water supplier and fire authority decide.
Ask the supplier first
The assembly you need is whichever one your water supplier specifies for the hazard at your premises. Every other consideration, cost, location, drainage and pressure, follows from that answer. Get it in writing before you buy an assembly or sign a lease that changes how the building is used.
We install, test, repair and replace both types for commercial properties across the Sacramento area. See commercial backflow services, or call (866) 981-1310 with the wording from your notice and the size of the existing assembly.