Backwater Valve vs. Backflow Preventer
At the BPNY office, after every storm or heavy rain, the phones ring with the same request:
“I need a backflow device installed.”
As always, our team gets to work collecting the necessary information—until the customer mentions why they need one:
“Well, when it stormed yesterday, water ended up in my basement.”
And that’s when we shift gears—literally.
Despite the similar-sounding names, a backflow prevention device and a backwater valve do two very different jobs. Understanding which one you need depends entirely on where the issue is happening—and what you’re trying to protect.
So What’s the Difference?
Let’s start with the basics:
A backflow prevention device (also called an RPZ device) is installed on your water supply line. Its job is to prevent water from flowing backward—protecting the city’s drinking water from being contaminated by anything from your property.
A backwater valve is installed on your wastewater line. Its job is to prevent waste water from flowing backward, back into your home, through drains, toilets and other waste-fixtures. Think of it as a one-way door that stops New York City’s overburdened sewers from forcing water (and everything in it) back into your home during heavy rains.
The key distinction? A backflow preventer protects the city’s water. A backwater valve protects your property.
So if your basement fills with water during a storm, it’s not a backflow preventer you need—it’s a backwater valve.
What Actually Happens During a Storm?
When a storm dumps large amounts of rain in a short period of time, that water drains from the streets into the city’s sewer system—a vast network of pipes that carries both stormwater and wastewater to treatment plants.
If too much water enters the system too quickly, the sewers can’t keep up. Pressure builds, pipes back up, and wastewater has nowhere to go—sometimes, it goes the wrong way: back into your home.
A properly installed backwater valve prevents this by closing off your waste line when the city’s system is overwhelmed, keeping sewage and stormwater where it belongs—out of your basement.
While not always required by code, a backwater valve can be a smart investment, especially if your property tends to accumulate more water after a storm.
What about backflow devices?
Backflow preventers are required by NYC code in certain buildings—particularly those with extra risk of contamination (like properties with sprinkler systems, boilers, or commercial kitchens).
These devices aren’t something homeowners often seek out on their own. But when the city orders you to install one, it’s important—and it must be tested and maintained regularly to stay compliant.
In Summary:
Backwater valve = protects your property from sewer backup
Backflow preventer = protects NYC’s water supply from contamination
They work on different lines (waste vs. clean water)
If you’re seeing water in your basement after a storm, you likely need a backwater valve, not a backflow device
Certain parts of NYC may require one, or both of these devices for specific building types.
Need clarity on which device is right for your building? Give BPNY a call—we’ll assess your situation and get the right solution in place.
