Backflow Prevention in a Mixed-Use Building
A mixed-use building has a ground-floor restaurant and four floors of residential apartments. The water supply enters the building through a 2-inch service line. The restaurant has a grease trap, a dishwasher, and a soda fountain. The building has a fire sprinkler system and a boiler for heating. The local code requires backflow prevention for all cross-connections. The contractor proposes to install a single reduced pressure zone (RPZ) backflow preventer on the main service line to protect the entire building. The water pressure at the street is 70 psi. The building's highest fixture is 50 ft above the service entrance. The RPZ has a pressure drop of 10 psi at design flow. The building manager is concerned about pressure at the top floor.
1. Evaluate whether a single RPZ on the main service line is sufficient for backflow protection, and discuss the implications for water pressure.
Read the worked answer
A single RPZ on the main service line provides protection against backflow from the building to the public water system, but it does not protect against cross-connections within the building. The code requires protection at each point of cross-connection. The restaurant has potential cross-connections (dishwasher, soda fountain) that require individual backflow preventers (e.g., air gaps or RPZ). The fire sprinkler system also requires a backflow preventer (often a double check valve assembly or RPZ depending on hazard). The boiler requires a backflow preventer on the make-up water line. Therefore, a single RPZ is insufficient; additional devices are needed at each hazard. Regarding pressure: the street pressure is 70 psi. The highest fixture is 50 ft above the service entrance, which requires 50 ft × 0.433 psi/ft = 21.65 psi to overcome elevation. The RPZ pressure drop is 10 psi. So the pressure at the top floor would be 70 - 21.65 - 10 = 38.35 psi, minus other losses (friction, meters). This may be adequate for most fixtures (minimum 8-15 psi required), but if the building has a booster pump, it might be needed. The manager's concern is valid if the pressure drops below the minimum required for proper operation of fixtures. The contractor should perform a hydraulic calculation to ensure adequate pressure at the top floor, considering all losses. If pressure is insufficient, a booster pump may be required. Also, the RPZ must be sized for the peak flow; a 2-inch RPZ may have a higher pressure drop at peak flow. The contractor should verify the RPZ's pressure drop at the design flow rate. Therefore, the single RPZ is not sufficient for internal cross-connections, and pressure must be evaluated.
2. What backflow prevention devices are required at specific locations, and what code requirements must be met?
Read the worked answer
The contractor must install: (1) An air gap or RPZ for the restaurant's dishwasher and soda fountain (depending on local code, typically an air gap for the dishwasher and an RPZ for the soda fountain). (2) A double check valve assembly or RPZ for the fire sprinkler system, as required by NFPA 13 and local code. (3) A backflow preventer (e.g., double check) on the boiler make-up water line. (4) Possibly a backflow preventer on the grease trap discharge if it connects to the potable water system (unlikely). (5) The main service line RPZ is acceptable for protecting the public water supply, but it must be tested annually. All devices must be installed by a certified tester, tested upon installation, and tested annually. The devices must be accessible for testing and maintenance. The contractor must obtain permits and have the installation inspected by the local authority. The code may require specific types of devices based on the degree of hazard. The contractor should consult the local plumbing code and the water purveyor's requirements. Also, the RPZ on the main line must be protected from freezing and have adequate drainage for relief discharge. The installation must comply with the manufacturer's instructions and local codes.