New York City’s water supply moves in one direction under normal conditions — from the water main into each property’s plumbing system, through pipes, and out of faucets, showers, and appliances. Pressure in the main line keeps that flow moving consistently in the right direction. When that pressure changes, the flow can reverse. Water that was inside a property’s plumbing system flows back into the public water main — potentially carrying whatever it picked up along the way.
That’s backflow. It’s a real public health risk, it’s the reason the NYC Department of Environmental Protection requires backflow prevention devices at a wide range of property types, and it’s something every property owner subject to those requirements needs to understand and stay compliant with.
How Backflow Happens
The physics of backflow are straightforward. The pressure in a water main is what keeps water moving toward your property rather than away from it. When that pressure drops — either in the main itself or relative to pressure on the property side of the connection — water moves toward the lower pressure, which means back into the main.
Several situations commonly cause the pressure differential that produces backflow. These include:
- A break or leak in the city water main that causes a sudden pressure drop throughout the system.
- Heavy use of fire hydrants in the neighborhood, which draws significant pressure out of the local main.
- Problems with city water supply that produce temporary pressure fluctuations across a broader area.
- A significant leak in a property’s own plumbing system that creates a localized pressure drop at the connection point.
Any of these can trigger backflow at a property that isn’t protected by a functioning prevention device. The pressure event can be brief — a few seconds — but brief is long enough for contaminated water to enter the public supply.
Why the Risk Is Serious
The severity of a backflow event depends entirely on what the water picks up inside the property’s plumbing before it reverses into the main. At a residential property without significant chemical or biological hazards, the risk is lower but still present. At a commercial kitchen, a property with in-ground irrigation, a laundromat, or any facility using chemicals or processing food, the contamination risk is substantially higher.
Human waste, cleaning chemicals, pesticides, fertilizers, industrial chemicals, and biological material from food processing can all enter the public water supply through backflow at an unprotected property. The consequences of contamination at that scale aren’t limited to the property where the backflow occurred — they extend throughout whatever portion of the water main the contaminated water reaches before detection.
This is why the DEP treats backflow prevention as a mandatory compliance requirement rather than an optional safety measure.
What a Backflow Prevention Device Does
A backflow prevention device is installed at the point where a property’s plumbing connects to the water main. Its function is to maintain the correct flow direction regardless of pressure fluctuations in the system — ensuring that water can move from the main into the property but cannot move from the property back into the main.
The most common device used at NYC properties is the reduced pressure zone device, or RPZ. An RPZ works by maintaining a zone of lower pressure between two check valves, with a relief valve that opens and discharges water to the exterior if the pressure differential collapses. The design ensures that even if both check valves fail simultaneously, the relief valve prevents backflow from reaching the public supply.
Installation of an RPZ requires a plan submitted to the DEP by a Professional Engineer or Registered Architect before work begins. The device itself must be installed by a licensed master plumber.
Which Properties Are Required to Have One
The DEP requires backflow prevention devices at any property where the plumbing system poses a contamination risk to the public water supply. The range of property types subject to this requirement is broader than most owners expect. Required properties include:
- Supermarkets and grocery stores
- Restaurants and commercial kitchens
- Schools
- Laundries and dry cleaners
- Properties with in-ground irrigation systems
- Properties with swimming pools
- Dwellings with treated water boilers
- Nursing homes and healthcare facilities
- Beauty salons and barber shops
- Properties involved in food processing or chemical use
This list covers the most common property types but isn’t exhaustive. Any property where the plumbing system could introduce contaminants into the public supply is potentially subject to the requirement. If there’s any uncertainty about whether a specific property needs a device, a licensed master plumber familiar with DEP requirements can make that determination.
The Annual Testing Requirement
Installing a backflow prevention device satisfies the DEP requirement for installation. It doesn’t end the compliance obligation. The DEP requires every installed backflow prevention device to be tested once per year by a certified tester. The test verifies that the device is functioning as designed — that the check valves are holding and the relief valve is operating correctly.
After the test, a Report on Test and Maintenance of Backflow Prevention must be filed with the DEP. Failure to complete the annual test and file the report can result in a $500 penalty per untested device. Properties with multiple devices face that fine for each one that goes untested.
A device that tests as failed needs to be repaired or replaced before it can be certified as compliant. A device that is overdue for testing represents an unprotected connection to the public water supply — which is precisely the condition the DEP requirement was designed to prevent.
Getting and Staying Compliant
For properties that don’t currently have a backflow prevention device and need one, the process starts with an engineer or architect submitting an installation plan to the DEP. Once approved, a licensed master plumber completes the installation. From there, annual testing keeps the property in compliance on an ongoing basis.
Empire Plumbing is a licensed master plumber in New York City providing backflow prevention device installation, annual testing, and repair services throughout Brooklyn, Manhattan, Staten Island, Queens, and the Bronx. For questions about whether your property needs a device or to schedule testing, call Gary at (718) 494-7301 or (917) 642-3041.