Backflow preventers play a very important role in protecting clean water from contamination. Whether installed on a residential irrigation system, commercial plumbing line, or fire sprinkler system, these devices stop polluted water from flowing backward into the main water supply. One of the most overlooked but essential maintenance tasks for a backflow preventer is draining it properly.
Draining a backflow preventer is especially important before winter, during repairs, or when the system is shut down for an extended period. If water is left inside, it can freeze, expand, and crack internal components, leading to expensive repairs or complete replacement.
In this article, you will learn how to drain a backflow preventer correctly, when it should be done, common mistakes to avoid, and helpful safety tips. This article is written for homeowners, property managers, and maintenance teams who want clear, step-by-step instructions without complicated technical language.
Introduction to backflow preventers
A backflow preventer is a plumbing device designed to protect potable water from contamination caused by reverse water flow. Backflow happens when water flows in the opposite direction than intended, usually due to pressure changes in the system.
There are two main causes of backflow. The first is back pressure, which occurs when downstream pressure becomes higher than the supply pressure. The second is back siphonage, which happens when the supply pressure drops suddenly, such as during a water main break or heavy water use nearby.
Backflow preventers are commonly required by local plumbing codes and water authorities because contaminated water can carry chemicals, fertilizers, bacteria, or debris back into the drinking water system. Proper maintenance, including draining, ensures the device continues working as designed.
Why draining a backflow preventer is important
Draining a backflow preventer is not just a maintenance recommendation; it is a critical step in protecting the device and the entire plumbing system. Water left inside the unit can cause serious damage under certain conditions.
During cold weather, trapped water can freeze and expand, cracking the valve body or damaging internal seals. Even in warmer climates, stagnant water can lead to mineral buildup, corrosion, and reduced valve performance over time.
Draining also helps during inspections, repairs, or seasonal shutdowns. It allows technicians to safely work on the system and ensures no pressure remains inside the device. Regular draining extends the life of the backflow preventer and helps avoid unexpected failures.
Basic components of a backflow preventer
Understanding the basic components of a backflow preventer makes the draining process much easier and safer. While designs vary, most backflow preventers include the following parts.
The shutoff valves are usually located at the inlet and outlet of the device. These valves control water flow into and out of the backflow preventer. They are often ball valves with handles that turn perpendicular to the pipe when closed.
Test cocks are small valves used for testing and draining. Most backflow preventers have four test cocks, labeled test cock 1 through test cock 4. These are used by certified testers, but also play a role when draining the unit.
Inside the device are check valves that allow water to flow in one direction only. Some models also include relief valves that release water if pressure becomes unsafe.
Knowing where these components are located helps prevent mistakes and ensures the draining process is done correctly.
When should you drain a Backflow Preventer
Knowing the right time to drain a backflow preventer helps prevent damage and unnecessary wear. There are several situations when draining is recommended.
1.) Before winter is one of the most common times. In areas where temperatures drop below freezing, backflow preventers should be drained before the first freeze to prevent internal damage.
2.) Before performing repairs or maintenance is another important time. Draining removes pressure and water, making it safer to work on the device.
3.) Shutting down irrigation systems for the season is also a good time to drain. This prevents stagnant water from sitting inside for months.
4.) After a system failure or contamination event, draining may be required as part of inspection and cleanup.
Tools and Materials Needed Before Draining
Before starting, gather the necessary tools and materials to make the process smooth and safe. Having everything ready helps avoid rushing or skipping steps.
You may need a flathead screwdriver or wrench to open test cocks, depending on the model. Some test cocks require a slotted screwdriver, while others can be opened by hand.
A bucket or hose is helpful to collect and direct water away from the area. Draining can release more water than expected, especially on larger systems.
Gloves are recommended to protect your hands, especially if the device has been exposed to outdoor conditions or debris.
Safety glasses are also a good idea, as pressurized water or debris may spray when opening valves.
Draining a backflow preventer is an important maintenance step, but professional servicing ensures everything is done safely and correctly. At Backflow Services Done Right, we offer expert backflow testing, repairs, and installations to keep your system fully compliant. Schedule your appointment today and let our certified technicians handle your backflow needs with precision.
Step-by-Step Guide: How to Drain a Backflow Preventer
Draining a backflow preventer should be done slowly and carefully to avoid damage. Follow these steps in order for the best results.
Step one: Shut off the water supply
Start by closing the upstream shutoff valve. This valve controls water entering the backflow preventer from the main supply. Turn the handle slowly until it is fully closed.
Next, close the downstream shutoff valve. This valve controls water leaving the backflow preventer. Closing both valves isolates the device and prevents new water from entering during the draining process.
Make sure both valves are fully closed. Partial closure can leave pressure inside the system.
Step two: Relieve internal pressure
Once both shutoff valves are closed, there may still be pressure trapped inside the backflow preventer. This pressure needs to be released slowly.
Begin by opening one of the test cocks slightly. Test cock number two or three is often recommended for relieving pressure, but any test cock can be used.
Open the test cock slowly to allow air to enter and water pressure to drop gradually. You may hear air entering or water dripping out.
Step three: Open all test cocks
After relieving pressure, open all test cocks fully. This allows water inside the backflow preventer to drain completely.
Open the test cocks in sequence, one at a time, to control water flow. Keep your bucket or hose positioned to catch the draining water.
If the device has a relief valve, water may discharge from that opening as well. This is normal during draining.
Step four: Allow complete drainage
Give the backflow preventer enough time to drain fully. Depending on the size and type of device, this may take several minutes.
Do not rush this step. Trapped water can remain in internal chambers if drainage is incomplete.
Gently rocking the shutoff valve handles or slightly adjusting the test cocks can help release remaining water, but avoid forcing any parts.
Step five: Leave valves partially open for winter storage
If draining for winter, leave the test cocks partially open after draining. This prevents trapped moisture and allows expansion if any water remains.
Some professionals also recommend leaving shutoff valves at a 45-degree angle during winter to reduce stress on seals.
Do not close everything tightly after draining if the system will remain unused in freezing conditions.
Common Mistakes to Avoid When Draining a Backflow Preventer
Draining a backflow preventer may look simple, but many homeowners and maintenance teams make small mistakes that can lead to serious damage, leaks, or even system failure. Understanding these common mistakes helps ensure the draining process is done safely, correctly, and without costly consequences. Below are the most frequent errors people make and why they should be avoided.
Forgetting to shut off both shut-off valves
One of the most common mistakes is shutting off only one valve instead of both the upstream and downstream shutoff valves. When only one valve is closed, water can still enter or remain pressurized inside the backflow preventer.
This mistake often results in continuous water flow while draining, making it difficult to fully empty the device. It can also create sudden pressure when test cocks are opened, increasing the risk of water spraying or damaging internal components. Always ensure both shutoff valves are fully closed before opening any test cocks.
Opening test cocks too quickly
Another frequent mistake is opening the test cocks too fast. Even when the main valves are closed, pressure can still be trapped inside the backflow preventer.
Opening a test cock quickly can cause a sudden release of pressure, leading to forceful water discharge. This can damage internal seals, loosen fittings, or cause injury. Test cocks should always be opened slowly and gradually to allow pressure to escape in a controlled manner.
Not draining all test cocks completely
Some people open only one or two test cocks and assume the device is fully drained. In reality, most backflow preventers have multiple internal chambers that require all test cocks to be opened for complete drainage.
Leaving one or more test cocks closed can trap water inside the unit. This trapped water is especially dangerous in cold weather, as it can freeze and crack the valve body or internal components. To avoid this, all test cocks should be opened and left open until no water is flowing.
Leaving the backflow preventer fully closed after draining in winter
After draining, many people close all valves tightly, thinking this will protect the system. During winter, this can actually cause more harm.
If any moisture remains inside the backflow preventer, tightly closed valves leave no room for expansion when temperatures drop. This increases the risk of freeze damage. For seasonal shutdowns, test cocks should be left partially open, and shutoff valves are often left at a slight angle to reduce stress on internal parts.
Forcing stuck or corroded valves
Backflow preventers installed outdoors are exposed to weather, dirt, and corrosion. A common mistake is using excessive force on stuck shutoff valves or test cocks.
Forcing these components can break valve stems, damage threads, or cause leaks that require full replacement. If a valve does not move easily, it is safer to apply a penetrating lubricant and wait, or contact a licensed plumber rather than forcing it open or closed.
If you’re unsure about draining your backflow preventer or notice leaks, pressure issues, or valve problems, don’t wait. Our experienced team is ready to answer your questions and provide fast, reliable solutions. Contact Backflow Services Done Right today to protect your water supply and avoid costly system failures.
FAQs about Draining a Backflow Preventer
Do I need to drain my backflow preventer every year?
Yes, draining a backflow preventer is recommended at least once a year, especially before winter in colder climates. Annual draining helps prevent freeze damage, reduces internal corrosion, and keeps the device functioning properly.
Can I drain a backflow preventer without professional help?
In many residential systems, homeowners can safely drain a backflow preventer by following proper steps. However, for large commercial systems, high-hazard devices, or if valves are damaged or stuck, it is best to contact a licensed plumber or certified backflow professional.
What happens if I forget to drain my backflow preventer before winter?
If water remains inside during freezing temperatures, it can expand and crack internal components or the valve body. This often leads to leaks, system failure, and expensive repairs or replacement in the spring.
Should test cocks be left open after draining?
For winter shutdowns, test cocks should usually be left partially open after draining. This allows any remaining moisture to escape and provides room for expansion if freezing occurs. Always confirm local recommendations or manufacturer guidelines.
Is draining the same as backflow testing?
No, draining and testing are different processes. Draining removes water from the device to protect it from damage, while testing checks whether the backflow preventer is working correctly. Most local authorities require annual testing by a certified professional, even if the device has been drained.