In the Valley, an RPZ backflow assembly is installed above ground, close to the water meter, where it can drain. Phoenix bans them in pits, vaults, meter boxes and basements, and wants at least 12 inches from grade to the bottom of the assembly, 3 feet of clear space, and a safe path for relief discharge.
The height and clearance rules
Phoenix's 2024 plumbing code amendments set the numbers. Section 608.15.3 of the International Plumbing Code, as Phoenix amended it effective August 1, 2025, requires "not less than 12 inches between the lowest portion of the assembly and grade, floor, or platform." Phoenix put the same 12-inch and 3-foot rules into Section 603.4.3 of its Uniform Plumbing Code amendments.
There's no hard ceiling in that section, but going high costs you. Phoenix's IPC amendment says "Elevated installations that exceed 5 feet above the floor or grade shall be provided with a platform capable of supporting a tester or maintenance person." So an RPZ hung high on a wall in a mechanical room needs a platform built under it.
Then there's working room. For secondary assemblies (the ones protecting the city main from the whole building), Phoenix adds: "A minimum 3-foot (914 mm) clear space shall be maintained for testing, maintenance, and repair." That's the space a tester needs to kneel, hook up a gauge kit, and pull parts.
Manufacturers draw a similar box. The Ames manual for its 4000RP series shows a minimum clearance of 12 inches and a maximum of 30 inches, for both indoor and outdoor installs, measured from the flood level to the bottom of the assembly. It also tells installers to leave enough room around the assembly for testing, service and inspection. Watts's 009 manual tells installers to check local codes for "the proper height above the ground."
Mesa sets a limit for indoor equipment. Mesa's point-of-use assemblies "must be installed no more than five (5) feet above finished floor level," though you may have to remove an access panel or door to reach one (City Code 8-1-9). Scottsdale asks for "an accessible location as close as possible to the service connection with proper clearances above the ground," following the city's standard details (Revised Code 49-63).
Keep it low and reachable. That's the whole idea.
No pits, vaults or buried boxes
This is the rule that trips up the most retrofits. An RPZ in a hole in the ground is a problem waiting for rain. If the pit floods above the relief port, dirty water sits right at the opening that is supposed to vent to the air.
Each level of rule handles it a little differently:
- Arizona (the state floor): Arizona's backflow rule, R18-4-215, says "An RP assembly shall not be installed in a meter box, pit, or vault unless adequate drainage is provided." The same rule says any required assembly must be installed to the maker's specifications.
- Phoenix (stricter): "A reduced pressure principle assembly or double check valve assembly may not be installed in a basement, meter box, pit, or vault" (City Code 37-144). Phoenix's IPC amendment goes further for all devices: "Backflow prevention devices shall not be installed in pits, underground vaults, or submerged locations" (Section 608.8.1). Its UPC amendment also bans backflow preventers in any area with fumes that are toxic, poisonous or corrosive (Section 603.4.9).
- Tempe: "Reduced pressure principle assemblies shall be installed above ground." A double check is different: it "may be installed below ground in a vault, if approved in writing, on a case-by-case basis by the department" (City Code 33-104).
- The manufacturers: The Ames manual is blunt: "The RP assembly shall not be installed in a vault or pit." Watts adds: "Never install the assembly where any part of the unit could become submerged in standing water."
So even where a state rule would allow a drained vault, Phoenix and Tempe don't, and the maker's instructions point the same way. Plan on above ground.
Where the relief water goes
An RPZ will discharge sometimes. A fouled check valve, a pressure dip on the street, or a failed part can each send water out the relief port. The site has to handle it. For what that discharge means when it happens, see why an RPZ backflow preventer spits water.
Phoenix's IPC amendment (Section 608.15.2.1) says the relief port piping must "discharge to an approved indirect waste receptor or to the outdoors where it will not cause damage or create a nuisance." It also says the receptor and drain piping must be sized for the most water the relief port can put out, as the manufacturer publishes it. That matters indoors, where a failure can flood a room.
Watts gives the practical version. The drain should end about 12 inches above the ground, or run through an air gap to a floor drain. It must never be piped solidly into a drainage ditch, sewer or sump. And "Do not reduce the size of the drain line from the air gap fitting."
Mesa writes the indoor rule into its code. A point-of-use RPZ needs an air-gapped drain line at least as large as the assembly, routed to a floor sink, mop sink or other wastewater outlet, and flexible hose is not allowed (City Code 8-1-9).
Outdoors, think about where the water runs. A relief discharge onto a sidewalk, into a parking space, or toward a building wall is the kind of damage or nuisance the code warns about. Grade the pad so water drains away.
Location, orientation and enclosures
### Close to the meter, before the first branch
Every Valley city puts the main assembly near where the water enters the property. Arizona's backflow rule, R18-4-215, says required assemblies "shall be installed as close as practicable to the service connection." Phoenix measures from the customer's side of the meter, and where a driveway or sidewalk is in the way, the assembly goes just past the obstacle, "but in no event beyond the first tap" (City Code 37-144). Mesa and Tempe both require the assembly before the first branch line off the service (Mesa 8-1-5, Tempe 33-104).
Phoenix adds site limits in its location guidance, TRT 00634: "Assemblies must NOT be located in traffic visibility triangles," and a spot behind the front yard setback line or just around a building corner is usually fine. Phoenix's current code also lets a customer ask to place the protection inside the facility, if they can show it still protects the public supply (City Code 37-144).
Scottsdale's design manual steers assemblies away from building entrances unless they're screened, away from spots where they block car doors, and away from high-visibility areas.
### Which way it faces
Ames states that "RP assemblies are approved by national approval agencies to be installed in a horizontal position." Any other layout, such as a vertical install, needs approval from the local authority. Double checks are more flexible, but the flow arrow has to point the right way. Ames says to flush the lines thoroughly before installing, and Watts recommends a strainer ahead of the assembly, except on seldom-used lines such as fire sprinkler supplies.
### Covers, cages and freeze protection
Phoenix's IPC amendment says outdoor enclosures for backflow devices must meet ASSE 1060, and that assemblies can't sit where they could freeze unless they can be removed with unions or are "protected from freezing by heat, insulation or both" (Sections 608.15.1 and 608.15.2). Watts says an assembly in an area that can freeze should go above ground in an insulated enclosure. Indoors, Watts says to "never install the assembly in concealed locations," because you want to see any discharge right away.
An enclosure is fine, as long as it still drains and still lets the tester in. In its landscape section, Scottsdale's design manual asks designers to "Completely screen backflow preventers using a screening wall, cage or dense plant materials."
Placement check, by city
The core rules line up across the Valley, but the details differ. Here they are side by side.
| Question | Phoenix | Mesa | Scottsdale | Tempe |
|---|---|---|---|---|
| RPZ in a pit, vault or meter box? | No (37-144; IPC 608.8.1) | Not addressed in Ch. 8-1; state rule and maker apply | Per city standard details | No, above ground only (33-104) |
| Minimum height | 12 in. to grade, floor or platform | Not set in Ch. 8-1 except for point-of-use (8-1-9) | "Proper clearances above the ground" | Per city standard details |
| Tall installs | Platform over 5 ft | Point-of-use: 5 ft max above floor | Per standard details | Per standard details |
| Working space | 3 ft clear | Not stated in Ch. 8-1 | Accessible location | Accessible, approved location |
| How close to meter | As close as practicable, not past first tap | Before first branch | As close as possible | Before first branch |
### Two Valley scenarios
A Phoenix restaurant replacing an old RPZ in a meter box. The replacement can't go back in the box. It has to come up above grade near the meter, with 12 inches of clearance, room to test, and a drain path. That usually means a new riser and a pad.
A Tempe office with a double check in a vault. If Tempe approved that vault in writing, it may stay. But if a new tenant's use needs an RPZ, the new assembly has to go above ground.
### Next steps before you set one
Permit first, then placement, then a test.
- 1Get the permit first. In Phoenix, installing, altering or replacing any backflow assembly needs a plumbing permit from Planning and Development, and the plans must show its location, type and size (TRT 00634). Fire line assemblies need Fire Department permits too. Scottsdale requires a separate permit for each assembly, including replacements (Revised Code 49-68).
- 2Pick the spot on the site plan. Near the meter, above grade, out of driveways and visibility triangles, with a drain path.
- 3Test it before use. Arizona's backflow rule, R18-4-215, says an assembly "shall not be placed in service unless it has been tested and is functioning as designed." Then it's tested every year.
Not sure which assembly you need? See RPZ vs double check backflow preventers. If you're trying to find an existing one, where is my backflow preventer helps, and new tenant or change of use covers when a new use triggers a new assembly.
HQ Plumbing & Air tests, repairs and replaces backflow assemblies on domestic water and irrigation lines, with free estimates and upfront pricing. Fire line assemblies are tested by a fire-licensed contractor, and HQ can handle underground fire line repair.
Location also depends on drainage; see why an RPZ needs an air gap and how it is piped. For enclosures, see what an RPZ hot box is and when one is needed.