Double Check Valve Vs. Reduced-Pressure Zone: What's The Difference Between These Two Backflow Prevention Systems?

27 March 2020
 Categories: , Blog

Backflow prevention is an important part of ensuring that the city's water is safe to drink. Whenever pressure in the water supply line drops, it's possible that water from the building's plumbing can begin to reverse direction and flow right back into the supply line. Pressure drops can happen during extreme water usage—for example, when fire hydrants are in use.

Unfortunately, sometimes water inside a building's plumbing system or in an irrigation system can contain harmful contaminants. In order to keep them out of your city's drinking water, you need a backflow prevention system to prevent water in your pipes from returning to the supply line. Many cities now mandate that businesses install and test these devices.

The two types of backflow prevention devices that are commonly used today are the double check valve system and the reduced-pressure zone system. To learn the difference between the two and to find out which one is the ideal system for backflow prevention, read on.

Double Check Valve System

A double check valve backflow prevention system consists of two check valves placed as a barrier between your water supply line and your building's plumbing.

Check valves are simple devices that are made from a seal and a spring. When water is flowing from the supply side, the pressure of the water presses the spring upward, which lifts the seal and allows water to flow into your building.

The tension of the spring is set so that the expected pressure of the water in your supply line will cause it to lift. If pressure drops below that amount, the spring immediately relaxes, causing the seal to come down and block the supply line, and this prevents any of the water in your building's plumbing from backflowing into your water supply.

Unfortunately, a check valve can sometimes fail. This occurs most often when debris become stuck in the spring, which prevents it from lowering the seal and preventing backflow into the supply line. Two check valves are installed in a double check valve system in order to provide some redundancy. If one check valve fails and the other operates correctly, then the single check valve is still able to block the flow of water through the pipe.

What happens if both check valves fail? The flow of water isn't stopped, and the reduced pressure in the supply line allows the higher-pressure water to flow back into it.

Reduced-Pressure Zone System

Reduced-pressure zone backflow prevention systems were created to add another level of redundancy to the system, which further decreases the chance of failure. They're very similar to double check valve systems—the difference is that there's a relief valve located between the two check valves.

The relief valve keeps the water between the two check valves at a pressure that's lower than the supply side water pressure. If the water pressure between the two check valves becomes too high, then a spring inside the relief valve pushes a seal open that allows water to drain to the ground. This reduces the water pressure in the section between the two check valves below the supply side water pressure, which prevents backflow.

The downside of a reduced-pressure zone device is that it will discharge water whenever backflow occurs. However, the extra layer of redundancy makes it a much more reliable backflow prevention system. In addition to that, you'll also be able to know whenever a reduced-pressure zone system is malfunctioning by the water coming from the relief valve. A double check valve system, on the other hand, won't give you any indication that both of the check valves inside of it are stuck open and allowing backflow.

Which Should You Install?

Which is the better device for your plumbing? In general, it simply depends on regulatory requirements for backflow prevention in your area. Reduced-pressure zone systems are typically required when your plumbing can potentially discharge harmful contents into the drinking water, whereas double check valve system are sufficient for backflow that only affect the taste or appearance of water.

However, regulatory requirements often become more stringent over time. Future-proofing your plumbing by installing a reduced-pressure zone backflow prevention system is typically worth the added expense. You'll also be able to easily tell when the system needs to be repaired if it begins discharging water.

To learn more about these systems, contact a plumber with backflow prevention services such as Nivo Backflow today.