Backflow Prevention Fundamentals

Water is supposed to flow one way: from the main, through the building, and out the drain. When it reverses, contaminated water can be pulled or pushed into the drinking water supply. It can happen in a house with a garden hose or in a hospital with a chemical line. Backflow protection makes sure that does not happen, and the choice of device depends on what the water could mix with and how the reversal could happen. Most jurisdictions enforce these rules through the plumbing code and the local water supplier.

What You Will Be Able to Do

• Explain back-siphonage and back-pressure
• Find cross-connections in homes, irrigation systems, and buildings
• Choose an air gap, vacuum breaker, double check, or RPZ for a hazard
• Install each device so it works
• Explain who tests assemblies and how often

Two Ways Backflow Happens

Back-siphonage happens when supply pressure drops so low that a vacuum forms. A main break or firefighters pumping from a hydrant can cause it. Water at the far end of a building gets drawn back toward the low-pressure point, like liquid going up a straw.

A classic example is a hose left in a bucket of pesticide. A main breaks down the street, the supply pressure collapses, and the hose becomes a straw that pulls pesticide into the house piping.

Back-pressure happens when pressure on the customer's side rises above supply pressure and pushes water the wrong way. Boilers, pumps, and elevated tanks can all do it. A carbonator in a restaurant can push carbonated water into the copper supply, where its acid dissolves copper into the drinking water.

Cross-Connections

A cross-connection is any actual or possible link between drinking water and something that could contaminate it. If a supply line ends below the flood rim of a vessel, you are looking at one. Common examples:

• Garden hoses in buckets, pools, or chemical drums
• Lawn irrigation, where heads sit in wet soil and fertilizer
• Boiler makeup lines with treatment chemicals
• Fire sprinkler systems, which hold stagnant water and sometimes antifreeze
• Dental chairs, lab sinks, and medical equipment
• Commercial dish machines and carbonated drink machines
• Fill lines to pools, fountains, and tanks

Hazard Levels

Devices are rated for the kind of hazard they protect against.

• High hazard (health hazard): Something that can make people sick or kill them, such as sewage, pesticides, boiler chemicals, and medical waste.
• Low hazard (non-health hazard): Something that may look or taste bad but would not cause illness.

If you are not sure, treat it as high hazard. The local code or water supplier has the final word.

Choosing the Device

Air Gap

An air gap is an open vertical space between the end of the supply pipe and the flood-level rim of the receptor. It must be at least twice the diameter of the supply opening and never less than 1 inch. A nearby wall can make the required gap larger. Nothing can outperform an air gap, because there is no physical path for water to go back up. It has no moving parts to fail. The downside is that you lose pressure, so you may need a tank and a pump after it.

Vacuum Breakers

A hose bibb vacuum breaker screws onto a hose spigot and costs only a few dollars. It stops the classic garden hose problem. Many codes require it on all outdoor spigots.

An AVB has a float that drops to let in air when pressure falls. It must sit at least 6 inches above the highest point downstream. No shutoff valve can be placed after it, and it must not stay under pressure more than 12 hours at a time, or it may stick shut.

A PVB adds a spring-loaded check valve, so it can stay under pressure all the time. The common rule is at least 12 inches above the highest downstream piping and outlets, but your local code and the maker's instructions decide. On a lawn sprinkler system, that means at least 12 inches above the highest head, wherever that head is in the yard. A PVB can spit water when it opens, so keep it out of places where that causes damage. Spill-resistant versions exist for indoor use.

Double Check Valve Assembly

A DCVA has two check valves in a row, with test cocks and shutoffs. It stops back-pressure and back-siphonage but has no way of telling you a check has failed. That is why it is only used for low hazards, such as many fire sprinkler systems without chemicals.

Reduced Pressure Zone Assembly

An RPZ has two check valves with a relief valve between them. The space between the checks is held at a lower pressure than the supply. If either check fails, the relief valve opens and dumps water out of the assembly. That water on the floor is the warning. It is the right choice for high-hazard connections, like boiler feeds with chemicals, labs, and irrigation with fertilizer injection, unless an air gap is used instead.

Worked example. A boiler with treatment chemicals takes makeup water from the city main. The chemicals are a health hazard, and the boiler can push back against the supply. Both risks are present, so a vacuum breaker will not work and a DCVA is not rated for it. You need an RPZ, or an air gap to a break tank.

Installation Rules

• Height: AVBs and PVBs must sit above everything downstream. Survey the yard before you pick a spot.
• No pits: PVBs and RPZs must stay above grade. They need to vent or drain to open air. Water in a pit could enter through the vent or relief port and cause the exact contamination you are trying to prevent. RPZs must have an air gap between the relief port and any drain.
• Drains for RPZs: When the relief valve opens fully, a 2-inch assembly can put out 40 to 60 gallons a minute or more. That can quickly overwhelm a small floor drain. Check the manufacturer's flow chart and size the drain for it.
• Access: Testers have to reach all test cocks. Leave clearance as your local rules require. Do not jam an assembly against a wall or up against a ceiling.
• Flushing: Flush a line before installing a device. Debris from new pipe is the top cause of fouled check valves.
• Freeze protection: A frozen assembly cracks its body and breaks internal parts, and that means replacing it. In cold climates, use an insulated, heated enclosure, or install indoors, or drain and winterize outdoor assemblies every fall.

Testing

Testable assemblies are PVBs, DCVAs, and RPZs. They are usually tested when installed, after any repair or move, and at least once a year. The test is done by a certified backflow tester using a calibrated test kit. ASSE 5110 is a common certification, and some states have their own. The test result goes to the water supplier. Skipping it can lead the water supplier to shut off service.

AVBs and hose vacuum breakers are devices with no test cocks, so they are not tested the same way. They should still be checked for damage.

When an RPZ Leaks

Do not plug a relief port. A leaking port is telling you something is wrong.

Common Mistakes

Check Your Understanding

1. A restaurant fryer hose is run through a dish sink, with its nozzle below the rim. What kind of problem is that?

A cross-connection. The supply can touch dirty water in the sink, and it could be pulled back. Add an air gap or the correct device.

2. A house sits on a slope. Its highest sprinkler head is 4 feet above the yard's lowest spot. Where can the PVB go?

Above the highest head, by at least 12 inches under the common rule. It cannot sit at the low spot.

3. A hospital lab line needs protection from a chemical analyzer, and you cannot use an air gap. Which device do you choose?

An RPZ. It is a high hazard, and back-pressure is possible.

4. An RPZ in a janitor's closet dribbles from the relief port all day. What is likely wrong?

The first check is probably fouled with debris, or its disc or spring is failing. It needs cleaning or rebuilding and a retest.

Key Takeaways

• Back-siphonage is a vacuum on the supply. Back-pressure is higher pressure downstream.
• An air gap is the most reliable protection: at least twice the supply diameter and at least 1 inch.
• Vacuum breakers stop back-siphonage only.
• DCVAs are for low hazards. RPZs handle high hazards and both kinds of backflow.
• Keep PVBs and RPZs above grade, with room for testers.
• Certified testers test assemblies at least once a year.