When it comes to managing water systems, many people assume that pressure reducing valves (PRVs) can handle everything—from lowering water pressure to stopping backflow. That’s not true. While PRVs play a key role in controlling pressure, they are not designed to prevent backflow. Mixing these two functions often leads to plumbing problems, safety risks and code violations.
Understanding the real job of a PRV and knowing why it can’t stop backflow is important. This helps homeowners, contractors and building managers avoid costly mistakes. It also ensures water systems stay safe, efficient and up to code.
In this blog post, we’ll explain the difference between pressure reduction and backflow prevention. We’ll look at how PRVs work, why they fail to stop backflow, and what devices you actually need to protect your water supply. If you’ve ever wondered why your pressure reducing valve doesn’t stop water from flowing backward, this guide will give you clear answers.
What Does a Pressure Reducing Valve Do?
A pressure reducing valve controls the pressure of water entering your home or building. City water systems often deliver water at very high pressure—sometimes over 100 psi. That’s too much for residential plumbing systems, which typically require pressure between 40–60 psi.
This is where a PRV comes in. It’s installed on the main water line, usually right after the water meter. The valve reduces the incoming pressure to a safer, manageable level. This protects your pipes, appliances and fixtures from stress and wear. It also helps save water and reduces the risk of leaks or burst pipes.
But here’s the thing: a PRV is designed to control pressure in one direction—from the city line into your plumbing system. It doesn’t monitor or control what happens if water tries to flow backward from your house into the public water supply.
What Is Backflow?
Backflow is the unwanted reversal of water flow. Instead of flowing into your home or building, water flows back out—into the city’s main water line. This can cause serious contamination if that backflow carries chemicals, bacteria or other pollutants.
There are two main types of backflow:
- Back pressure – This happens when the pressure in your system becomes higher than the supply pressure. It pushes water back into the public supply line.
- Back siphonage – This happens when the supply pressure drops suddenly, like during a water main break or heavy usage. That drop creates a vacuum, pulling water out of your system and back into the supply line.
Either situation can allow contaminated water to enter the public supply. That’s a health hazard and it’s exactly what backflow preventers are made to stop.
Why PRVs Can’t Stop Backflow
It’s a common mistake to think that a PRV can act as a backflow prevention device. The truth is, it can’t. Let’s look at why.
1. PRVs Regulate Pressure, Not Flow Direction
A pressure reducing valve controls how much pressure water has when entering your system. It doesn’t control which direction the water flows. That means if pressure changes, water can move in reverse—right through the PRV—without resistance.
The valve’s internal mechanism is made to slow water down, not block it. It doesn’t seal tight in the reverse direction. So if your home’s water pressure rises above the supply pressure, water can move backward through the valve.
2. PRVs Aren’t Check Valves
A check valve is built to allow flow in only one direction. It closes completely if water tries to reverse. PRVs do not have that feature. They have a spring-loaded diaphragm to control pressure but not a tight seal to prevent reverse flow. Some PRVs may slow down backflow slightly but they won’t stop it.
3. System Pressure Changes All the Time
Water pressure is always shifting. You might have backpressure from a boiler, irrigation pump or elevated tank. Or there could be back siphonage from a nearby water main break. In either case, the PRV won’t protect the public supply. The only thing that stops contaminated water from going back into the city line is a proper backflow prevention device.
What You Need Instead: Backflow Preventers
If you need to stop backflow, a backflow preventer is the right tool for the job. These devices are made to block reverse flow and protect drinking water from contamination.
There are several types of backflow preventers, including:
- Atmospheric Vacuum Breakers (AVB)
- Pressure Vacuum Breakers (PVB)
- Double Check Valve Assemblies (DCVA)
- Reduced Pressure Zone (RPZ) Assemblies
Each one is used in different situations, depending on the level of risk and local plumbing codes. For example:
- AVBs are used in garden irrigation or hose bibbs.
- PVBs are common in lawn sprinkler systems.
- DCVAs are used for low to medium hazard applications like fire sprinkler systems.
- RPZs are used in high-risk situations like hospitals, labs or chemical systems.
These devices are tested and approved to meet strict safety standards. They seal tightly when pressure changes and prevent contaminated water from re-entering the public system.
What Happens If You Don’t Use a Backflow Preventer?
If your system causes backflow into the public line and contaminates the water supply, the results can be serious. You might face:
- Health risks from bacteria, pesticides or chemicals entering drinking water
- Fines or legal issues from violating plumbing codes
- Property damage from backflow-related leaks or overflows
- System failures due to unregulated pressure changes
In many places, building codes require backflow prevention for certain systems. Ignoring that requirement can lead to inspections, fines, or a forced shutdown until proper equipment is installed.
Common Misconceptions About PRVs and Backflow
Here are some myths we hear often—and the truth behind them:
Myth 1: “My PRV lowers pressure, so backflow can’t happen.”
Fact: Lowering pressure doesn’t block reverse flow. If pressure behind the valve rises or supply pressure drops, backflow still occurs.
Myth 2: “I don’t need a backflow preventer because I have a closed plumbing system.”
Fact: Even closed systems can create backpressure. A water heater or pump can raise internal pressure and force water backward.
Myth 3: “One valve can do both jobs.”
Fact: PRVs and backflow preventers serve different purposes. Some combination units exist, but they must be rated and installed properly to handle both roles.
How to Protect Your System the Right Way
If you want full protection for your water system, follow these best practices:
- Install a PRV on the main line to manage incoming pressure.
- Install a backflow preventer if your system connects to irrigation, boilers or other high-risk setups.
- Check local plumbing codes. Many cities have strict rules about backflow prevention. Make sure your setup is compliant.
- Test your backflow preventer regularly. Most codes require yearly inspections by a certified technician.
- Avoid DIY fixes. Improper installation of PRVs or backflow devices can cause leaks, failure, or worse. Use a licensed plumber.
Final Thoughts
Pressure reducing valves do a great job of lowering water pressure, protecting your pipes and improving efficiency. But they don’t stop backflow. That’s not their job—and assuming they do can lead to serious risks.
To truly protect your water system and comply with plumbing codes, you need to install the right device for the right purpose. Use PRVs to control pressure. Use certified backflow preventers to stop contaminated water from flowing in the wrong direction.
Don’t confuse one for the other. And don’t skip either. When used together, these devices keep your system safe, efficient and reliable.
