So the building keeps water while one assembly is tested or repaired. Scottsdale requires two or more parallel assemblies when a user needs a continuous water supply, with their combined cross-sectional area at least equal to the supply line. Each assembly still needs its own annual test, permit and access, and each must be the type the hazard calls for.
Why a single assembly means an outage
Arizona's backflow rule requires each required assembly to be tested "at least annually" and "after installation, relocation, or repair," and says "An assembly shall not be placed in service unless it has been tested and is functioning as designed" (A.A.C. R18-4-215). Phoenix's code puts the same yearly duty on the owner: "The customer shall test all backflow prevention assemblies at least once a year" (City Code 37-143).
Phoenix can terminate water service when a customer "Removes or bypasses a required backflow prevention assembly without the prior approval of the Planning and Development Director" (City Code 37-146). So the building waits. A failed test makes it longer: parts, a repair and a retest before the assembly goes back in.
What Scottsdale requires
Scottsdale writes the parallel rule directly into its code. Revised Code 49-63(c) says: "When a user requires a continuous water supply, two (2) or more backflow prevention assemblies shall be installed parallel to one another at the customer supply line to allow a continuous water supply during testing of the backflow prevention assemblies."
It sets a sizing floor too: "When backflow prevention assemblies are installed parallel to one another, the sum of the cross-sectional area of the assemblies shall be at least equal to the cross-sectional area of the customer supply line."
Here's what that means in practice. Area grows with the square of the diameter, so two smaller assemblies can match one bigger line. Using nominal sizes, a 2-inch line has about 3.1 square inches of area, and two 1-1/2-inch assemblies add up to about 3.5. Two 1-inch assemblies, at about 1.6 combined, would not meet the rule for a 2-inch line. Ask the city to confirm the math for your sizes before you order.
Keep in mind the rule covers capacity with both assemblies open. With one out for testing, the other carries the whole load alone, so plan the test for a low-demand window.
How the rest of the Valley handles it
Phoenix's code doesn't spell out a parallel requirement the way Scottsdale's does, but nothing in it stops one, and the same rules apply to each assembly. In Phoenix:
- Each one needs a permit. "The customer shall obtain a permit from the Planning and Development Department for the installation of any backflow prevention assembly" (City Code 37-143).
- Each one gets tested every year, and each gets its own report.
- Each one needs room. Phoenix's amended IPC Section 608.15.3 requires "not less than 12 inches between the lowest portion of the assembly and grade, floor, or platform" and "A minimum 3-foot (914 mm) clear space shall be maintained for testing, maintenance, and repair" around secondary assemblies. Two assemblies side by side need that space for both. Where floor space is tight, a model listed for a vertical position can help, as long as the exact model and size carry that listing (can a backflow preventer be installed vertically).
- Each RPZ needs a drain. Under amended 608.15.2.1, the relief drain "shall be sized to drain the maximum discharge flow rate from the relief port as published by the backflow preventer manufacturer." Two RPZs can discharge at the same time.
A related rule covers buildings with more than one water service. Phoenix's amended Section 608.7.2 notes that "Multiple water services which are interconnected onsite shall be provided with not less than a Double Check Valve Assembly at each service connection." That's not the same as a parallel pair on one service, but it comes up on campuses with two meters.
Both legs must match the hazard
Water can take either path, so each assembly has to be the type the hazard calls for. If a medical building needs an RP, both legs are RPs. Putting a double check on the "spare" side to save money leaves an unprotected path every time it carries flow.
Is it worth it?
Parallel assemblies mean two tests a year instead of one, two sets of repair parts over time and more room at the service. They pay off where an outage costs more than that, such as:
- Patient care, assisted living and dialysis, where water can't stop.
- Hotels and large multifamily buildings, where a shutoff means notices to every unit.
- Restaurants and food plants that can't close for a test.
- Buildings where one assembly has a history of failing and staying out of service.
If an existing single assembly is coming up for replacement anyway, that's the cheapest time to re-pipe for two. HQ tests, repairs and replaces domestic backflow assemblies and can map out a parallel layout for a permit.
For how often testing is required, see commercial backflow testing requirements in Arizona. For multifamily properties, apartment complex backflow testing covers the outage problem for tenants, and what happens after a failed test explains the repair and retest timeline.