Why does an RPZ valve need an air gap and how is it piped?

### Why does an RPZ relief valve discharge in the first place?

The relief valve is the safety part of the assembly. The EPA's cross-connection manual explains that the relief valve discharges to the atmosphere whenever the pressure in the central chamber gets within a few pounds of the inlet pressure. Water in that zone between the two check valves is the water that might have been contaminated. Dumping it to the air keeps it away from the drinking supply.

Watts adds a practical point in its manual. The drain and air gap sit where evidence of discharge is clearly visible, which signals the need to protect against water damage. That's why Watts says never to install the assembly in concealed locations.

Most of the time the valve stays quiet. When it does spit, the cause could be a pressure dip, a dirty check valve, or a worn part. The full breakdown is in why an RPZ backflow preventer spits water. What matters here is what happens next. The water comes out at the size of the relief port, and it needs a way to leave the room.

### Why does the discharge need an air gap?

A solid pipe from the relief port to a drain would link two systems. One holds drinking water. The other holds wastewater. Zurn Wilkins says the air gap has to be there "to prevent a cross-connection," and its instructions add a caution not to pipe the relief valve solidly to a floor drain, sewer or sump.

The air gap does the job with nothing but air. It has no moving parts. The EPA's manual calls air gaps non-mechanical backflow preventers, and ASME publishes a standard for them, A112.1.2, that covers air gaps in plumbing systems.

#### What is the air gap rule?

Different documents state it a little differently, so it helps to see them side by side.

Durham's interpretation makes one point that trips people up. The gap is two times the diameter of the outlet of the waste pipe, which in that case was the relief valve. It is not two times the size of the whole backflow assembly.

Two cautions apply. Durham is in North Carolina and quotes the 2018 code, so it shows how the model code reads, not what your city has adopted. And local rules win. Watts, Ames and Zurn all say to check local codes.

Also, keep the gap open. The EPA manual warns that an air gap is easy to defeat if the required gap is purposely or inadvertently compromised, for example by pushing the supply pipe into the receiving funnel. The same logic holds for a relief drain. If a pipe or fitting ends up touching the receptor, the gap is gone.

### How is the relief valve drain piped?

There are two common setups. Which one you use depends on where the assembly sits.

#### Indoors: air gap fitting, then a full-size line to a floor drain

Inside a building, the relief water has to be captured, because a failure can put a lot of water on the floor. Watts's manual says a drain line and air gap should be piped from the relief valve connection. A drain line with an air gap that ends above a floor drain, Watts says, "handles any normal discharge or nuisance spitting."

Here is how the pieces usually go, from the top down:

1. 1The relief valve port. This is the opening on the underside of the assembly. Leave it open.
2. 2The air gap fitting. The EPA manual says air gaps may be built from commercially available plumbing components or bought as separate units. Ames offers an Air Gap Drain that installs under the relief valve on its RP devices, catches minor discharges from pressure changes, and carries USC Foundation approval. Ames also says to check with local authorities first, because an air gap drain is not approved for all installations.
3. 3The drain line. Watts says not to reduce its size: "Do not reduce the size of the drain line from the air gap fitting. Pipe full line size."
4. 4The receptor. A floor drain or similar indirect waste receptor. Watts says to end the line above the floor drain rather than connect into it.

Whether it's a manufactured fitting or a built-up gap, the gap is the point. The pipe stops above the receptor and doesn't sit in it.

#### Outdoors: terminate in open air, above grade

Outdoors, Watts's outside-installation guidance says to pipe a discharge line from the air gap at the relief valve connection, and never to pipe it directly into a drainage ditch, sewer, or sump. Its manual also says the discharge should end approximately 12 inches above the ground, or run through an air gap to a floor drain. Point the discharge where it won't damage a wall, flood a parking space, or run across a walkway.

Phoenix's 2024 plumbing code amendment takes the same approach in one sentence. The relief port piping, or air gap fitting, has to discharge "to an approved indirect waste receptor or to the outdoors where it will not cause damage or create a nuisance." Both options are legal. The receptor has to be one that the code approves.

### How big does the drain need to be?

Bigger than most people expect. The relief valve can pass a lot of water in a failure, and the drain has to keep up.

Here's what the sources say:

• Phoenix (2024 IPC amendment, Section 608.15.2.1): the indirect waste receptor and the drain piping must be sized to drain the maximum discharge flow rate from the relief port, as published by the manufacturer.
• Watts: a floor drain sized for normal spitting may not handle a catastrophic failure. Its manual has a chart of maximum relief valve discharge by assembly size and zone pressure, and a list of typical floor drain flow rates.
• Watts and Durham: do not reduce the drain line below the fitting size. Durham adds that if two assemblies feed one line, the pipe doubles in size where the two lines meet and stays that size after.

Watts's manual lists these typical flow rates for floor drains, as sized by floor drain makers:

Use that as a sense of scale, not a sizing tool. The number that governs your job is the maximum discharge from the maker's published data for your assembly. Match a drain to that figure and you're covered for a worst case, not just an occasional spit. Flagstaff's city code shows the same idea. For a reduced pressure assembly on a fire line inside a riser room, it asks for a drain that can handle a "full port discharge" for the assembly size, into the sanitary sewer and not the storm water system.

Because either move defeats the device.

Plugging or capping the relief port. The EPA manual is blunt: "Under no circumstances should plugging of the relief port be permitted because the device depends upon an open port for safe operation." The manual also says a failing check valve or relief valve should always show up as a discharge from that port. Cap it, and the warning vanishes while the danger stays.

Piping it solid to a floor drain, sewer or sump. This is the direct connection the air gap is there to avoid. Watts says never to pipe the discharge line directly into a drainage ditch, sewer, or sump, and Zurn's caution says the same about floor drains.

Installing it where it can flood. If the relief port sits underwater, the air gap is gone. Watts says never to install the assembly where any part of the unit could become submerged in standing water. Ames says the RP assembly "shall not be installed in a vault or pit." Arizona's state rule, R18-4-215, says an RP assembly shall not go in a meter box, pit, or vault unless adequate drainage is provided, and Phoenix's amendment goes further and bars backflow devices from pits, underground vaults, or submerged locations. For height and clearance rules, see where an RPZ backflow assembly can be installed.

Building a drain that can't keep up. A small line that backs up puts water right back at the relief port. That's why the sizing rules above matter.

You don't need tools for a first look. Walk the assembly and ask:

1. 1Is there a visible gap between the relief port and the drain pipe or fitting, with air between them?
2. 2Does the drain line run full size from the fitting to the receptor, with no smaller reducer or flexible tube squeezing it?
3. 3Is the drain clear? A clogged floor drain or a blocked outdoor path means a discharge has nowhere to go.
4. 4Is anything stored around it, such as boxes, mops or supplies that hide the relief port or block the test cocks?
5. 5Has anyone plugged, taped or capped the port to stop a leak? If so, that has to be undone.

Then remember the rules that come with the assembly itself. Arizona's rule says each assembly must be tested at least annually, and after installation, relocation, or repair, and testing has to be done by a certified tester. For test schedules, see how often a commercial backflow preventer needs testing in Arizona, and for what happens next if an assembly does not pass, see what to do after a backflow assembly fails its test.

Not sure which valve you have? RPZ vs double check backflow preventers compares the two, and where is my backflow preventer helps you find it.

If the assembly sits outside where winters bite, the drain and the assembly also have to be protected from freezing. That's covered in what an RPZ hot box is and when you need one.

Call for help if you see any of these:

• A relief port that runs steady, or keeps dripping when no water is being used.
• A drain that overflows, backs up, or is smaller than the relief port.
• Missing, taped or capped relief piping.
• An assembly in a pit, or with its relief port near standing water.
• Any change to the assembly itself. Arizona requires a test after installation, relocation or repair, and testing has to be done by a certified tester.

For the plumbing around the assembly, such as a floor drain, a drain line, or piping that has to change so water can leave safely, HQ Plumbing & Air is licensed and insured, and gives free estimates and upfront pricing. Our team is on call 24/7 for emergency service if a discharge is flooding your space.