Which Device for a Carbonator and a Photo Lab
You are reviewing backflow protection for a mixed-use building. Tenant A operates a restaurant with a carbonator connected to the potable line and a dish sprayer with a submerged inlet. Tenant B runs a photo lab with a fixed processing sink where hoses are routinely dropped below the rim into chemical baths. Tenant C has an irrigation system with pop-up heads and a chemical injection fertigation unit. The domestic service enters at 1-1/2 inches and feeds all three tenants through submeters. The owner wants one device at the service entrance to cover everything and asks you to specify it. There is a 4-inch floor drain nearby and limited space in the meter room.
1. For each tenant connection, identify the appropriate type of protection and justify it by hazard and by the nature of the connection.
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Carbonator: the connection is a direct potable-to-carbon-dioxide-pressurized vessel, a health hazard under pressure backflow, so it needs an air gap or an RP assembly; a carbonator is typically protected by a listed dual-check with atmospheric vent or an air gap per the manufacturer and code. Dish sprayer with submerged inlet: a low-hazard backsiphonage risk best handled by an air gap or a hose-connection vacuum breaker, since the sprayer head can be submerged. Photo lab sink with hoses dropped into chemical baths: this is a health hazard and a backsiphonage risk; the appropriate protection is an air gap at the fixture or a reduced-pressure assembly on the branch serving that sink, not a simple vacuum breaker, because chemicals are involved. Irrigation with fertigation: health hazard from injected chemicals, requiring an RP assembly or an air gap at the injection point, plus appropriate protection at the irrigation connection. Each connection's hazard and geometry drive the choice; a single generic device is not a substitute for matching protection to each risk.
2. The owner insists on one device at the service entrance. Evaluate that proposal and state what you would specify given the space constraint.
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One service-entrance device can protect the public main from everything in the building, and because the highest hazard is a health hazard, that device would have to be an RP assembly (or an air gap) sized for the full service demand. That satisfies protection of the main but does not protect one tenant's potable piping from another tenant's contaminant, and it does not remove the need for fixture-level protection where codes require it, such as air gaps at carbonators and sinks. So the proposal is incomplete rather than wrong. Given limited meter-room space, specify a properly sized RP at the service entrance for main protection, then add point-of-use protection at the carbonator, the photo sink, and the irrigation injection point. Confirm sizing against the combined peak demand and the device's pressure-loss curve, and verify the 4-inch drain can carry relief discharge. Document the layered approach and get AHJ concurrence.