Yes. Every backflow assembly costs some pressure, because water has to push open spring-loaded check valves, and the loss grows with flow. An undersized assembly, a partly closed shutoff or a clogged strainer makes it worse. Flush-valve toilets and urinals need 25 to 45 psi flowing under IPC Table 604.3, so check the loss at peak flow.
Why an assembly costs pressure
A backflow assembly is a set of check valves that stay closed until water pushes them open. Phoenix's code defines a reduced pressure principle assembly (RP or RPZ) as "two independently acting approved checkvalves together with a hydraulically operating, mechanically independent pressure differential relief valve located between the checkvalves." A double check has the two checks without the relief valve.
Each check is held shut by a spring. Water has to overcome that spring loading to flow, and the faster it flows, the more friction it meets in the body. That's the pressure loss. Manufacturers publish it as a curve for each size: Watts' spec sheet for its LF009 RPZ includes a capacity chart of pressure drop against flow for every size from 1/4 inch to 2 inches, and lists "Large body passages provides low pressure drop" as a feature. The same chart marks the "typical maximum system flow rate" for each size, at 7.5 feet per second.
The takeaway: the loss isn't one number. It depends on the model, the size and how much water the building is pulling at that moment. The chart for your exact assembly is the place to read it.
Five reasons pressure drops more than it should
- 1The assembly is smaller than the service. Scottsdale requires that "The assembly shall have a diameter at least equal to the diameter of the service connection" (Revised Code 49-63), and Mesa requires the same "unless approved in advance in writing by the Department" (City Code 8-1-5). A 1-inch assembly on a 2-inch service will choke flow at peak demand.
- 2A shutoff valve wasn't fully reopened. The assembly has its own shutoff valves, and they get operated during testing and repairs. Phoenix Water lists "A partially closed house valve" among the potential causes of reduced pressure. Check that both shutoffs on the assembly, and the building main, are all the way open.
- 3A strainer is loaded with debris. Many assemblies are installed with a strainer ahead of them (Watts offers an "S" strainer option on the LF009). That screen can load up with sediment, and a loaded screen costs pressure; should a strainer go before a backflow preventer covers the trade-off.
- 4The building was already close to the limit. A tall building, long runs or old galvanized pipe can leave little margin. The assembly is just the last loss that tips it over.
- 5The problem isn't the assembly at all. Phoenix notes that pressure varies with demand and can drop "when there is construction in the area." Rule out the street before blaming the valve.
What the fixtures need
The plumbing code sets minimum flowing pressure at fixtures in Table 604.3 of the 2024 IPC. Commercial fixtures need a lot more than a home faucet:
| Fixture | Flow pressure (psi) |
|---|---|
| Water closet, blowout, flushometer valve | 45 |
| Water closet, siphonic, flushometer valve | 35 |
| Urinal, valve | 25 |
| Water closet, flushometer tank | 20 |
| Lavatory, public | 8 |
Phoenix's 2024 amendments change the fixture flow rates in Section 604.4, not this pressure table, so the numbers above apply in Phoenix. Section 604.6 adds that "Where street water main pressures fluctuate, the building water distribution system shall be designed for the minimum pressure available." And under 604.7, where supply pressure can't meet Table 604.3, "a water pressure booster system conforming to Section 606.5 shall be installed."
So the design math has to include the backflow assembly's loss at peak flow, subtracted from the lowest street pressure, minus elevation and pipe friction, and still land above those numbers at the farthest flush valve.
Whose problem it is
In Phoenix, it's the owner's. The city says it "does not regulate the water pressure beyond the meter on the customer's side," and that "issues after the water meter are the responsibility of the homeowner," which for a commercial property means the owner. Phoenix gives an acceptable range at the meter of "between 40 and 100 psi." A building near the bottom of that range has little room for an RPZ loss.
How to sort it out
- 1Read pressure upstream and downstream of the assembly with water flowing. A big gap at modest flow points to the assembly or a strainer.
- 2Check both shutoffs and the strainer before anything else. That's the cheap fix.
- 3Pull the model and size off the assembly and compare it to the service size and the maker's capacity chart.
- 4If it's undersized, the fix is a correctly sized replacement, which needs a permit in Phoenix, or two assemblies in parallel where the city allows it.
- 5If the building was marginal all along, a booster pump may be the real answer, per IPC 604.7.
HQ tests, repairs and replaces domestic backflow assemblies and can measure pressure on both sides during a visit. Free estimates, upfront pricing.
For the two main assembly types, read RPZ vs double check. If you're considering the parallel route, see parallel backflow preventers for continuous water. And if the drop came on suddenly with no new assembly, start with why water pressure suddenly drops.