8.3 Double Check & Reduced Pressure Principle (RPZ) Assemblies

8.3 Double Check & Reduced Pressure Principle (RPZ) Assemblies

Quick Answer: A double check valve assembly (ASSE 1015) stops backsiphonage and backpressure but is approved for low hazard only; a reduced pressure principle assembly (ASSE 1013) stops both directions at high hazard, and its relief opening must discharge through an air gap and must never be submerged.

Reading the Scenario: Backsiphonage or Backpressure?

Exam questions rarely name the mechanism. You have to read it out of the fact pattern.

Backsiphonage is backflow produced by negative (sub-atmospheric) gauge pressure in the supply piping - the supply pulls. Look for a water main break, heavy fire-flow draft from a nearby hydrant, an undersized or partly closed service, a shutdown with the risers drained, or a pump taking suction directly on the service.

Backpressure is backflow produced by a downstream pressure higher than the supply - the downstream system pushes. Look for a booster or transfer pump, a pressurized boiler or hydronic loop, compressed air or carbon dioxide injection, thermal expansion in a closed system, or plain elevation.

Elevation is the one you can compute. A column of water exerts 0.433 psi per vertical foot.

Worked example. A chemical mixing tank sits on the fourth floor, 42 feet above the potable connection, and is charged by a transfer pump that adds 30 psi.

• Static head = 42 ft x 0.433 psi/ft = 18.2 psi
• Total potential backpressure = 18.2 + 30 = 48.2 psi
• Street pressure at the connection = 45 psi

Because 48.2 psi exceeds 45 psi, a backpressure condition exists. That single comparison disqualifies every vacuum breaker on the market, no matter how high it is mounted. This connection needs an air gap or a reduced pressure principle assembly.

Degree of Hazard: Pollutant vs Contaminant

IPC 2018 Table 608.1 sorts every device by degree of hazard, and footnote a of that table sends you to the Chapter 2 definitions:

• Pollution (low hazard, pollutant) - an impairment of the quality of the potable water that does not create a hazard to public health, but adversely and unreasonably affects the aesthetic qualities of the water for domestic use: taste, odor, color. Examples include food-grade dye, a plain fire sprinkler system with no additives, a heat exchanger using an essentially nontoxic transfer fluid, and a plain boiler make-up line.
• Contamination (high hazard, contaminant) - an impairment that creates an actual hazard to public health through poisoning or through the spread of disease by sewage, industrial fluids or waste. Examples include boiler treatment chemicals, pesticide or fertilizer injection, mortuary and autopsy equipment, dental vacuum, car wash detergents, photo-processing chemicals and any sewage connection.

One rule follows from this and is worth memorizing whole: a low-hazard device may never be substituted for a high-hazard one, but a high-hazard assembly is always acceptable on a low-hazard connection. Upgrading is legal; downgrading is a violation.

Double Check Valve Assembly (DCVA) - ASSE 1015

A double check valve assembly is two independently acting, internally loaded (spring-loaded) check valves in series, bracketed by two tightly closing resilient-seated shutoff valves, with four test cocks so a tester can isolate and read each check separately. Table 608.1 rates it low hazard, backpressure or backsiphonage, sizes 3/8 in. through 16 in., built to ASSE 1015, AWWA C510, CSA B64.5 or CSA B64.5.1. Section 608.14.7 confirms these assemblies operate under continuous pressure conditions.

The redundancy is genuine - one fouled disc does not open a path - but the assembly has no vent to atmosphere and no way of announcing that it has failed. That is exactly why the code will not let it stand between the public supply and a contaminant. Because it has no relief port to submerge, a DC may be set in a properly drained vault where the local authority allows it.

Reduced Pressure Principle Assembly (RP / RPZ) - ASSE 1013

An RP assembly adds the one thing a double check lacks: a way to fail safely. It is two independently acting, internally loaded check valves with a hydraulically operating, mechanically independent differential pressure relief valve located between the checks and below the upstream check, plus two tightly closing shutoff valves and four test cocks.

In normal flow the reduced pressure zone between the two checks is held roughly 8 psi below inlet pressure, which keeps the relief valve seated. If the first check fouls, or if inlet pressure collapses, zone pressure climbs toward inlet pressure; when the zone comes within about 2 psi of the inlet, the relief valve opens and dumps the zone to atmosphere. Backflow becomes a puddle on the floor instead of a contamination event, which is why an RP is the only mechanical assembly the code trusts against high-hazard backpressure.

Installation rules that are actually in the 2018 code

• 608.14.2 - RP assemblies may be installed under continuous pressure conditions; the relief opening shall discharge by air gap and shall be prevented from being submerged. That sentence is the code basis for the universal ban on installing an RP in a pit or below-grade vault: submerge the relief port and the assembly becomes the cross-connection it was installed to stop.
• 608.15 - access shall be provided to backflow preventers as specified by the manufacturer instructions.
• 608.15.2 - backflow preventers shall not be located where subject to freezing unless removable by unions or protected by heat, insulation or both.
• 608.15.2.1 - piping from the relief port or air gap fitting shall terminate at an approved indirect waste receptor or outdoors where it will not cause damage or create a nuisance. Never hard-pipe a relief port directly into a drain.
• Mounting height. IPC 2018 publishes no numeric mounting height for an RP or DC. The industry figure comes from the USC Foundation Manual of Cross-Connection Control, 10th edition, which recommends 12 inches minimum from grade or floor to the lowest part of the assembly (the relief valve on a horizontally mounted RP), with maximums commonly set at 30 to 36 inches by the local authority so the assembly stays testable and repairable. Confirm the figure your jurisdiction enforces rather than assuming one.

Field test acceptance values

IPC 2018 Section 312.10.2 requires reduced pressure principle, double check, pressure vacuum breaker, spill-resistant vacuum breaker and the fire-protection detector assemblies to be tested at the time of installation, immediately after repairs or relocation, and at least annually, using an approved procedure such as the ASSE Series 5000 standards. Section 312.10.1 requires annual inspection of backflow prevention assemblies, barometric loops and air gaps.

Fire Lines: Detector Assemblies

A fire service is a low-hazard connection in its ordinary state - stagnant and rusty, but not poisonous - so 608.17.4 permits a double check backflow prevention assembly, a double check fire protection backflow prevention assembly, or a reduced pressure principle fire protection assembly on automatic sprinkler and standpipe systems. Two exceptions remove the requirement entirely: systems installed as part of the water distribution system with no fire department connection, and deluge, preaction or dry pipe systems.

Raise the hazard and the code raises the assembly. Under 608.17.4.1, where a system under continuous pressure contains chemical additives or antifreeze, or connects to a nonpotable secondary water supply, protection must be a reduced pressure principle assembly or a reduced pressure principle fire protection assembly. Where such a system is not under continuous pressure, an air gap or an ASSE 1001 atmospheric vacuum breaker is accepted. Table 608.1 lists the two detector versions: double check detector fire protection assemblies to ASSE 1048 (low hazard, 2 in. through 16 in.) and reduced pressure detector fire protection assemblies to ASSE 1047 (high or low hazard).

A detector assembly is a full-size main assembly with a small metered bypass assembly plumbed across it. The bypass meter registers the low flows - a leaking sprinkler head, or a tenant tapping the fire line for irrigation - that the large main assembly would never record.

Containment vs Isolation

• Containment protection sits at the service entrance, immediately downstream of the meter, and protects the public main from everything inside the property line. This is the protection the water purveyor inspects and bills for.
• Isolation, also called fixture or point-of-use protection, sits at the individual hazard: the boiler, the lab bench, the mop sink, the photo processor. It protects the building occupants and the rest of the building piping.

A fully compliant job usually has both, and one does not substitute for the other. A containment RP at the service does nothing for the tenant on the second floor who drinks water siphoned backward out of a chemical dispenser down the corridor.

Selection Decision Table

Exam Trap: The two numbers candidates most often swap are the RP relief valve opening point and the double check tightness value. 2.0 psid is the relief valve. 1.0 psid is a check valve on a DC, PVB or SVB. 5.0 psid is check valve No. 1 on an RP. An option that reads the relief valve must open at 1 psid or each double check must hold 2 psid is built from that swap. The costlier trap is visual: a double check assembly carries two shutoffs and four test cocks exactly like an RP and looks just as serious in a photograph, but Table 608.1 confines it to low hazard. Installing one on a chemically treated boiler, an irrigation system with a fertilizer injector, a car wash recycle tank or a mortuary is a violation no matter how new the assembly is.