Last month, in Part 2 of this series, we covered air gaps, air breaks, atmospheric vacuum breakers, hose thread applied vacuum breakers and pressure vacuum breakers. This month, in Part 3, we continue with double check valve assemblies and reduced-pressure zone assemblies, and a few examples of cross-connection incidents.

Double check valve assembly 

A double check valve assembly (DCVA) features two separate, spring-loaded check valves in series and isolation valves with test ports on each side of the check valves to see if they are holding. If debris gets stuck under the first check valve and causes it to leak, the second check valve acts as a backup to stop the backflow. DCVAs have test valves, meaning they can be mechanically tested to ensure they work properly. They are typically used for low-to-medium hazard installations.

My good friend, the late John Matthews, PE, who served on many ASSE committees, including the ASSE Seal Control Board, the ASSE Standards Committee and the ASSE board of directors, would always say: “A backflow preventer is an assembly of components that makes the device. The assembly must be tested to the performance standards. If a manufacturer supplies the device without the shut-off valves, it cannot be tested. That is why backflow preventers must be supplied with shut-off valves and referred to as an assembly.”

Backflow preventers for fire protection systems

A backflow preventer (BFP) is not required by the National Fire Protection Association’s NFPA 13, Standard for the Installation of Sprinkler Systems, and a BFP serves no fire protection-related benefit. BFPs are there to protect the public water mains from the stagnant, bacteria-laden, rusty water in a fire suppression system from being siphoned into the utility or domestic water system when there is a water main break or other event that causes a loss of water pressure and reversal of flow. 

The 2025 edition of NFPA 13, Section 5.1.7.1, states, “The requirements of the public health authority having jurisdiction shall be determined and followed.” 

The requirement for a BFP for fire protection systems is in the 2024 International Plumbing Code (IPC) and described using the term backflow prevention assembly in Section 608.1, specifically 608.17.4, and other requirements of the IPC; and in accordance with the 2024 International Building Code and 2024 International Fire Code, Sections 903.3.5 and 912.6. 

When backflow preventers for fire protection systems are used, a detector check valve is typically used because the BFP is usually on a separate branch entering the building, and there is no water meter on the fire line. To prevent unauthorized use of the fire protection water and detect leaks, many municipalities require a detector-type BFP, which has a meter to indicate whether water is used in the fire protection system. The bypass also has a small backflow preventer with a spring that has less resistance than the spring in the main line backflow preventer, which allows flow through the bypass first. 

Double detector check valve 

A double detector check valve assembly is simply a double check valve assembly with a small 3/4-inch bypass, a double check valve assembly and a water meter. The springs in the bypass backflow preventer are a couple of pounds less than the springs in the main line, which will force water through the bypass meter and check valve first, so any unauthorized use of water or leaks in the fire protection system can be monitored by checking the water meter to see if there is any flow. 

Typically, since these backflow preventers go on fire sprinkler systems protecting large, high-risk facilities, in addition to code requirements to meet the ASSE standard titled ASSE 1048, Double Check Detector Backflow Prevention Assemblies, insurance companies require a listing to the FM standards for the device to ensure the backflow preventer can flow a high volume of water under extreme pressure and thrust conditions. 

This is to ensure the backflow preventer is robust enough to withstand fire flow conditions without coming apart and having internal parts break off and flow into the fire pump impellers.

Reduced pressure principle backflow prevention assembly 

The ASSE standard for this type of assembly is ASSE 1013, Reduced Pressure Principle Backflow Prevention Assemblies. The purpose of a reduced pressure principle backflow prevention assembly (RPZ) is to keep high-hazard contaminated water from flowing back into a potable water distribution system when some abnormality in the system causes the pressure to be temporarily higher in the downstream contaminated part of the system than in the upstream potable water supply piping. 

These assemblies consist of two independently acting check valves, internally force-loaded with springs to a normally closed position. They are separated by an intermediate chamber (or zone) with a hydraulically operated relief valve that vents the zone to the atmosphere and is internally force-loaded to a normally open position. These assemblies are designed to operate under continuous pressure conditions.

Reduced pressure zone-detector check valve backflow preventer 

These backflow preventers, RPZDCs, are designed to meet the backflow testing requirements of ASSE 1047, Reduced Pressure Detector Backflow Prevention Assemblies and FM, as described above.

The purpose of an RPZDC is to prevent contaminated fire protection system water from flowing back into a potable water distribution system when the downstream contaminated pressure in the contaminated part of the system is temporarily higher than in the potable water supply piping. 

Strainer and soft-seated check valve ahead of an RPZ or RPZDC

A strainer will protect the check valves and sealing surfaces of backflow preventers from fouling due to foreign matter and debris that may be flowing through the water line. This not only protects the valve but also eliminates nuisance fouling, possible flooding and subsequent maintenance and shutdown. 

RPZs or RPZDCs leak by design. Leakage from a backflow preventer is normally attributed to foreign matter lodged in the seat of either the first or second check valve. A leak can usually be resolved by simply flushing a large volume of water through the valve, which should dislodge any loose particles. It is, therefore, most important on new installations that the piping be thoroughly flushed before installing the unit. 

It should be noted that spillage serves as a warning signal that the line contains sediment or debris and that the valve needs maintenance, cleaning and testing. In one building, a valve was leaking, so the maintenance man replaced the entire backflow preventer. It cost more than $20,000 because he thought that if something leaks, it must be defective.

In another building, a large object got caught in the first check valve, causing the relief valve to dump full through a 2-inch relief port at about 100 psi. It overwhelmed the 3-inch drain located under the backflow preventer and flooded the basement of a hospital, including all the boilers, chillers, electrical panels, controls, compressors, etc. A strainer, soft-seated check valve and properly sized drain are important for basement mechanical room installations. 

When a reduced-pressure backflow preventer is fouled, the reduced-pressure zone between the two check valves has a relief valve, discharging hundreds of gallons of water, causing flooding. Make sure there is a floor drain of adequate size for the anticipated flow volume. 

The use of a strainer with a soft-seated check valve has been an accepted practice for years. The strainer limits debris, and the soft-seated check valve prevents nuisance spitting of the relief valve when the upstream pressure drops. This is typically at night when municipal pumps turn off, irrigation systems turn on or fire flows occur in the area, and there is no flow in the building. 

The reduced-pressure zone and upstream pressure can get close enough to discharge a small amount of water to maintain the required pressure differential between the zones. The pressure drop attributable to the strainer and check valve is negligible and far outweighed by the advantages provided by the strainer. However, in many fire protection installations, they often don’t want the strainer because they prefer to have a little more pressure and flow for the sprinkler system.

Backflow and back-siphonage incidents 

• Contamination of the water utility supply

In 1991, a water utility customer complained about the water quality on a water utility system in Perry County, Arkansas. A water utility staff member investigated and found that a nearby chicken farm had been administering antibiotics to the chickens through the metering pump, injecting antibiotics into the animal watering system. It was discovered that water was backflowing from the injection pump through two fouled single-check valves installed in series. 

• Farm chemical backflow incident contaminates drinking water

A back-siphonage incident occurred when a farmer added water to a chemical sprayer tank on his farm. He would typically pour about 3 gallons of a herbicide chemical into his 200-gallon sprayer tank, then add water to the tank. The chemical was 2,4-D, also known as 2,4-Dichlorophenoxyacetic acid, which is a herbicide that kills broadleaf weeds. It is from the same family of herbicides as Agent Orange, which was used to defoliate jungles in the Vietnam War. 

The farmer positioned the garden hose supplied from the artesian well to an overhead frame, allowing the hose to hang down over the tank to supply water into the top of the farm sprayer tank. (An artesian well is a well drilled into the ground where impervious rock layers have trapped water that rises up to the water table level. They typically have low pressure and low flow.) 

Typically, the farmer would maintain an air gap between the end of the fill hose and the top of the chemical tank. The day he was filling the tank, the wind kept blowing the fill hose away from the tank’s fill opening. He extended the hose into the tank near the bottom, so the end of the hose became submerged. As the tank slowly filled, he went on to other chores. 

Later, he went to the house where his wife told him that the water had become cloudy and salty-tasting. He told his wife not to use any more water, then he ran to the chemical tank, where it was still slowly filling. He learned that when a faucet was opened in the home, the house pump came on. It was connected to the line from the artesian well, creating negative pressure or a siphon in the well line and the connected garden hose. So, when the booster pump came on, it was also siphoning the chemical from the sprayer tank into the water line.

• Backflow incident at Holy Cross College

The Holy Cross College football team was stricken with hepatitis from a cross-connection between the irrigation system and a drinking fountain near the football field. The backflow incident was later determined to have been caused by a backflow of contaminated water (https://nyti.ms/4hezdy5). 

It took close to nine months for health and plumbing officials to determine that a severe fire in a building nearby lowered the pressure in the water mains near the football field to the point where a back siphon backflow condition was created, allowing contaminants from a sunken hose valve in a pit to siphon contaminated water into the drinking fountain where the football team got their drinking water. 

Ninety-three percent of the football team tested positive for hepatitis; the team canceled the remainder of their games that season. 

• Backflow incident in a commercial building

Customers of a bank building in Atlanta, Georgia, saw yellow water flowing from drinking fountains and green ice coming from the cafeteria ice dispensing machines.

It was later reported that a pump circulating the condenser water system quit working. A maintenance man, unaware of the danger, connected the condenser water system to the domestic water system to get water to the cooling tower. The result caused large amounts of sodium dichromate and biocides to backflow into the potable water supply during a power outage. 

The backflow caused yellow water to appear from plumbing fixtures and colored ice cubes. Bichromate of soda (sodium dichromate) was used as a corrosion inhibitor; biocides were used to control algae in the condenser water system between the cooling towers and chillers. 

The building’s maintenance man created a cross-connection when he unwittingly connected the cooling tower condenser water piping system to the domestic water piping system.