What is a double detector check assembly on a fire line?

All three are testable backflow assemblies. The difference is the hazard they're rated for and whether they carry a detector bypass.

A double check valve assembly (DCVA) is two check valves in a row between two shutoff valves, with test cocks. NFPA describes it as having "two spring-loaded check valves with two shut off valves and four test cocks." It covers low-hazard water, which on a fire line means only water sitting in the pipes with no chemicals added.

A double check detector assembly (DCDA) is the same body plus a bypass. NFPA's description is short: it "is the same as a DCVA, but it also includes a bypass for the installation of a water meter to monitor for incidental water use that is also protected with a smaller DCVA." Watts puts the purpose of its 757DCDA line the same way: the Watts series "is used primarily on fire line sprinkler systems when it is necessary to monitor unauthorized use of water." The Zurn Wilkins 350DA spec sheet says it "Incorporates metered by-pass to detect leaks and unauthorized water use."

A reduced pressure detector assembly (RPDA) is the high-hazard version. It starts with a reduced pressure principle assembly, which adds a relief valve between the two checks, then adds the same metered bypass. Zurn Wilkins rates its 375DA for places "where a potential health hazard exists." You see RPDAs on fire systems with antifreeze, foam or other additives, or with a second water source on site.

Here's a trap in older buildings. Some fire services still have a single "detector check" or alarm check valve and no true double check at all. Mesa's code says that an existing "detector or alarm check valve, which does not qualify as backflow protection" has to be upgraded to a DCVA once it's unserviceable or needs replacement. The word "detector" in a valve name doesn't mean you have backflow protection.

Most commercial fire services don't run through a billing meter. Scottsdale's code says water goes to a property "through a meter only, except as provided in section 49-49 and for sprinkler-type fire prevention systems." Mesa's code even has its own rule for "unmetered services such as fire lines." There, the customer's side starts just past the city's valve on the fire service.

That leaves a blind spot. A sprinkler system should use almost no water between fires and tests. If the underground pipe cracks, or someone ties a hose bib or a drip line into the fire service, the water leaves without ever touching a billing meter. The detector bypass closes that gap. Everyday small flows register on the bypass meter, so a meter that keeps moving on a quiet fire line is telling you something.

The usual causes of a spinning detector meter:

1. 1An underground leak. A cracked fire main, a failed joint or a corroded tap between the street valve and the building.
2. 2A leak inside the system. A weeping drain valve, a dripping inspector's test connection or a sprinkler head that's seeping.
3. 3An illegal connection. Someone tapped the fire line to water plants or feed a wash-down hose. Scottsdale counts taking water from "any City water main or fire line" without authorization as unauthorized use, including "By connecting, directly or indirectly, to the City water system without a meter."

Leaks on your side are your problem, legally and financially. In Scottsdale, fire line owners are responsible for "maintenance and repair of that line up to the main line including the tapping sleeve and valve." Tempe bans letting water escape "through breaks, leaks or malfunction" once the leak should reasonably have been found and fixed. A detector meter that nobody reads doesn't help much. Write down the reading each month, the same day, and compare. A steady climb is the signal to get the line checked.

City hazard tables name a DC or an RP for the fire line. A detector assembly is one way to meet that rule, and Phoenix lists detector versions among its accepted fire line devices. Whether yours has the bypass comes down to what was approved on the plans, so check them and ask the water provider before a replacement is ordered.

Phoenix. The city's 2024 plumbing code amendments, in effect since August 2025, name "reduced pressure detector fire protection, double check detector fire protection" assemblies among those that must be tested at installation, after repairs or relocation and at least once a year. Phoenix's older backflow guidance, TRT 00635, lists four testable devices for fire supplies, a list that traces back to the 2006 Uniform Plumbing Code; the 2024 amendment text above is the current rule. Two on the Phoenix list are the "Double check detector assembly" and the "Reduced pressure detector assembly." The guidance sums it up: "Essentially, all potable water supplies to fire lines serving fire protection systems require installation of backflow prevention assemblies." In City Code 37-144, Class 1 and 2 fire systems need a backflow assembly, and Classes 3 through 6 need an RP.

Mesa. City Code 8-1-8 calls for a DCVA on a fire sprinkler system with a direct connection to the public system and "No additives or auxiliary supplies of any kind." It calls for an RPA when there's an auxiliary supply on site, additives are used, or private hydrants are on site. Section 8-1-10 adds that a service feeding both fire and domestic lines "must be protected using a reduced pressure assembly."

Scottsdale. Revised Code 49-62 sorts fire systems by class: "Class 3, all systems: DC" and "Classes 4, 5 and 6, all systems: RP," with Classes 1 and 2 at DC. Section 49-64 requires a double check on new commercial construction. Scottsdale also lets the fire chief require two assemblies side by side where the sprinkler supply "may not be interrupted during testing."

Tempe. City Code 33-105 has a separate fire protection table. It sets the assembly by connection type: a direct noncontaminating connection, a direct contaminating connection, a system with a pump or storage tank, and a system with an auxiliary supply. Section 33-104 also says "No bypass shall be installed around backflow prevention assemblies." Don't confuse that with a detector bypass, which is part of the assembly and has its own small backflow preventer.

Statewide. Arizona's backflow rule, R18-4-215, lists "water that has entered a fire sprinkler system" as a hazard but says "A Class 1 or Class 2 fire sprinkler system is exempt from the requirements of this Section." The Valley cities above still require protection on those systems under their own codes, so the city rule is the one you follow.

A detector version of each assembly is how a DC or RP rule gets met when a detector meter is also wanted.

A DCDA or RPDA on a fire line is fire protection equipment, and testing it is fire work. In Phoenix, City Code 37-143 says that on fire lines, "the initial and annual test must be performed by a certified tester who is also permitted by the City Fire Marshal to test fire lines or fire sprinkler systems. The initial and annual test must include a full flow test." For Class 1 and 2 fire systems, City Code 37-144 adds that annual flow and valve confidence tests "must be performed by a tester who is permitted by the City Fire Marshal to test or maintain fire lines or fire sprinkler systems," and the results go to the Director and the Fire Marshal within 30 days. Other assemblies follow the Director's due date. Hire a fire-licensed contractor for that test, and in Phoenix confirm the Fire Marshal permit. For how testers qualify in general, see who can test backflow assemblies in Arizona.

Two quick scenarios show how this plays out.

The meter keeps moving at a strip center in Mesa. The fire contractor tests the assembly and it passes. The checks are fine, so the water is going somewhere downstream. The next step is finding the leak on the underground line between the assembly and the building, and that's pipe repair, not assembly work.

An old single check valve in Scottsdale starts leaking. Section 49-64 says a DC is required when "The existing single check valve shows signs of deterioration/defects." The owner's fire contractor designs and permits the new assembly. Scottsdale also requires the sprinkler system to be tested to show it can meet NFPA 13 minimum flow calculations before a new assembly goes on. That's a fire contractor's job from start to finish.

Next steps for an owner or manager:

1. 1Find the assembly and read the tag. DCDA, RPDA, DCVA or RP. Our guide to where your backflow preventer is helps if you can't find it.
2. 2Log the detector meter monthly. Any steady movement on a quiet system is worth a call.
3. 3Book the annual test with a fire-licensed contractor, and send the report where your city wants it. See where to send backflow test reports in the Phoenix area.
4. 4If the assembly passes but the meter still moves, have the underground fire line checked for a leak.

For the difference between the two base assemblies, read RPZ vs double check backflow preventers. For the wider annual rule, see commercial backflow testing requirements in Arizona.

HQ Plumbing & Air doesn't test fire assemblies or work on sprinkler systems, but we do repair underground fire lines, with free estimates.