Backflow preventers
Once the water enters your home or business the water companies are no longer in control of what happens to it. Once it enters your home or business it can become contaminated from sources inside and outside your home or business. How is this possible? If the water pipes are hooked up to a divisive contains contamination the potential for contamination entering the water system exists.
Types of backflow
This connection is referred to as a cross connection. When these contaminants flow back into the water system it is called backflow. To better understand this problem let's review the two basic types of backflow that occur in water supply systems.
Basksiphonage
The first is called basksiphonage. Basksiphonage is the reversal of flow in a system caused by a negative pressure or vacuum and the supply piping. If you ever used to siphon to transfer liquids from one container to another then you have experienced basksiphonage. Place a container of water on your kitchen counter and an empty container on the floor. Place a piece of tubing into the upper container. Suck some water into the tubing and pinch it off. Now lower the end of the tubing into the lower bucket and release the clamp. The water flows up out of the higher container and down into the lower container. You've just made a siphon. How does this relate to the water system in your house? Let's say you have a hose attached to the faucet on your laundry tub and your fill in the laundry tub with the soapy water. If everything is working correctly the pressure of the public water system forces the water out of the hose into the laundry tub. What happens however if elsewhere in the public water system a water main breaks and the pressure in the system drops or is lost. If that break occurs at a point below your laundry tub in altitude aback siphoning situation can occur. Now when the water main is repaired and the first few glasses of water drawn they could contain bubbles. What would happen if instead of bubbles a deadly chemical back flowed into the system? People could become sick. Another location around your home were basksiphonage could occur is your lawn sprinkler system. If any of your sprinkler heads are below flood level the puddle of water that lays around the head can be siphons back into your water system.
Back pressure
The second type of backflow is back pressure. A back pressure situation occurs when the pressure in the downstream piping system rises above the pressure in the supply piping. If you have a water pressure of thirty five pounds per square inch in your residential system and you attach another source with a greater pressure, the pressure differential will force the attached system liquid to flow back into the public water system. So how do you stop these dangerous situations from occurring? The easiest ways is by eliminating the cross connection. If these cross connections cannot be eliminated and approve device must be installed to prevent backflow from occurring. Approve backflow prevention devices use in water systems come in a variety of configurations. They are one-way valves. In this devices water is allowed to pass in only one direction. If water returns to reverse flow the valve will close and the flow will be stopped. Just as the water supplier has the responsibility of supplying you with safe water to drink, you the consumer have the responsibility of protecting that water against contamination. Federal state and local laws require that you have backflow prevention device is located at all cross connections. They must be installed by a licensed plumber and tested upon installation. Some models codes require annual testing by a state certified backflow tester. Water companies have begun to require that all commercial consumers have approved backflow devices installed in their water systems. Some communities are currently requiring residential customers to also comply and eventually all customers will need to comply.
Backflow preventer definition
A backflow preventer is like a one-way gate for water. Most backflow preventers are used to keep unsafe water from reversing flow and entering the clean water supply. Backflow preventers can be as simple as a single check valve that closes when water flow reverses. Using a simple check valve as a backflow preventer might be considered the equivalent of a turnstile at a store entrance, it is not very reliable, even a small amount of effort will overcome it. A more elaborate backflow preventer can be a complicated device that consists of multiple check valves, water release valves, air vents, and/or systems to allow it to be tested to assure it is working properly. This kind of backflow preventer might be the equivalent of an airport exit security checkpoint with one-way gates and a armed guard.
Types of Backflow Preventers
Following is a list of the various types of backflow preventers. All of the following backflow preventer types are available in all sizes. You may need to contact a specialty irrigation store or plumbing supplier to obtain some of these backflow preventers. A “control valve” is the valve that is used to turn on and off a group of sprinklers or a drip system. Typically an irrigation system will have several control valves, each turning on the irrigation in a different area of the yard. A control valve may be automatic (turned on and off by a timer) or it may be manual (turned on and off by hand).
Backflow prevention devices
- Air gap
- Atmospheric vacuum breaker (AVB)
- Check valve (usually not a legally approved method of backflow prevention)
- Chemigation valve (primarily used in agriculture)
- Double check valve, or double check valve assembly (DCVA)
- Pressure vacuum breaker assembly (PVB)
- Reduced Pressure Principle Valve (RPPV or RP Device)
- Reduced pressure zone device (RPZ)
- Spill Resistant Pressure vacuum breaker assembly (SPVB)
- Vacuum breaker
Air gap
The simplest, and a very effective way to provide backflow prevention, is to provide an air gap. An air gap is simply an open space between any device that connects to a plumbing system (like a valve or faucet) and any place where water can collect or pool. Alternatively, a specialized backflow preventer valve may be installed at strategic locations in the plumbing system wherever there is a risk of contaminated fluids entering the water supply pipes.
Atmospheric Vacuum Breaker
Atmospheric vacuum breakers (AVB) are the simplest design of backflow preventer. If you have less than six control valves, they are also the least expensive option. These devices are not designed as a whole-system backflow prevention valve; one must be installed at every control valve. It must be installed at least 6 inches (150mm) higher than the highest sprinkler head, bubbler, or drip emitter outlet that is turned on and off by that control valve (some AVB manufacturers, and in some areas local officials, require that AVBs be installed 12 inches (300mm) higher than the sprinkler heads). It may not be installed in any location where it might ever be submerged under water, like in a underground box. As a general rule AVBs are not economically practical if you have more than 6 or so valves. In this case you would want to consider a pressure vacuum breaker. Some municipalities do not allow the use of AVBs. Most people use a anti-siphon valve (see next item) rather than a valve and a separate AVB. An anti-siphon valve is generally less expensive and less work to install. If you install any valves, of any type, on the pipes downstream of the AVB, the AVB will not work! The downstream valve creates “back pressure” on the AVB which causes the vent in it to jam in the closed position. If this vent can’t open, the AVB will not prevent backflow.
Pressure Vacuum Breaker
Pressure vacuum breaker (PVB) are the most commonly used and least expensive type of whole-system backflow prevention valve. The benefits of this type of backflow preventer valve are its low cost and simplicity. These assemblies are easy to install, maintain, and repair. Some models of pressure vacuum breaker assemblies include built-in freeze protection valves; however, pressure vacuum breakers only protect the assembly itself, not the water lines. These breakers will also occasionally eject some water, so it is important to purchase a spill-resistant model for indoor installation, or for any area where water spillage would create a nuisance. Air gap drains are also required when installing indoors. A pressure vacuum breaker (PVB) is similar to a atmospheric vacuum breaker except that you only need to install one of them and it is installed on the mainline leading to the control valves. Like the AVB it must also be installed above ground and it must be 6 inches (150mm) higher than the highest sprinkler head or drip emitter controlled by any of the valves. In a sloped yard it would typically need to be installed at the highest point in the yard, with a mainline pipe running up to it from the water source, and then another mainline running back down to the control valves. A few local authorities require that the PVB be installed within 18 inches (450mm)of the connection to the water source, in which case you can’t use a PVB unless the water source is at the high end of the irrigated area. It may not be installed in any location where it might ever be submerged under water. Some municipalities do not allow the use of PVBs with drip irrigation systems. Some don’t allow the use of a PVB at all, so check with your water provider. A PVB backflow preventer may spit or spill water out from under the cap when backflow occurs, so it should be installed in a location where water spillage would not cause problems. If used on a water system where a pump and pressure tank supplies the water (like is used on most rural homes that have a well), the PVB may spit water each time the pump shuts off. This is because the pressure variations caused by the pump and pressure tank system can cause backflow from the irrigation system back into the water system. The likelihood of water spitting, and the amount of water that spits out, both increase with a longer mainline on the irrigation system. So if you have 10 feet (3m) of mainline between the PVB and the farthest valve there is less likely to be water spitting than if you have 500 feet (150m) of mainline pipe. One way to stop, or at least reduce, this water spillage is to install a spring-loaded check valve right after the PVB. The PVB may still spill a little water with the check valve installed, however in most cases it should be much less water.
Anti-Siphon Valve
A anti-siphon valve is a manual or automatic control valve with a built-in atmospheric vacuum breaker. Like the AVB it must be installed 6 inches (150mm) higher than the highest sprinkler head or drip emitter outlet. If you install any valves, of any type, on the pipes downstream of the anti-siphon valve, the anti-siphon valve will not work! The downstream valve creates “back pressure” on the anti-siphon valve which causes the vent in it to jam in the closed position. If this vent can’t open, the anti-siphon valve will not prevent backflow. It may not be installed in any location where it might ever be submerged under water. (Exception: drain valves for winterizing the system are OK and may be installed after an anti-siphon valve, provided they are configured only to drain the pipe. A properly installed drain valve should not create backpressure on the A/S valve.) Anti-siphon valves are the most common type of backflow preventer used on residential irrigation systems, primarily because they are simple and inexpensive. Some municipalities do not allow the use of anti-siphon valves, so it is best to check with the water company first. Generally you would install the anti-siphon valves in one or more groups, at the highest point in the area to be irrigated. A mainline pipe is run to the anti-siphon valve location(s) from the water source. Pipes then extend from each anti-siphon valve to the sprinklers or emitter tubes. Because anti-siphon valves must be installed at least 6 inches (150mm) above ground, it is a good idea to put a small planting of shrubs around them to help hide them from view. They are not particularly attractive! Water may come out of the anti-siphon valve periodically, so make sure you install them someplace where a little spilled water will not be a problem. The water will come out of the vent, which is under a cover on the top of the downstream side of the valve (you can see the vent holes under the cover if you turn the valve upside down and look for them.) If water does come out of the anti-siphon it means something is wrong that needs to be fixed. In most cases it means either a stick or rock got into the anti-siphon seal and jammed it open, or the anti-siphon valve was not installed higher than all the sprinkler heads or emitters. Never install a anti-siphon valve upstream of any other valve. If you do the anti-siphon valve will not prevent backflow and you have wasted your money buying it. Never use an anti-siphon valve as a backflow preventer installed on the mainline upstream of other valves. This is a common error that a lot of people make. I have heard employees at home improvement stores recommend installing a anti-siphon valve as a backflow preventer with standard electric globe valves installed after it for each of the sprinkler zones. I have seen many contractors do this also. Both should know better! Don’t you do it!!! It will damage the anti-siphon valve. Plus the anti-siphon valve will not prevent backflow when installed this way. You are no better off than if you didn’t use a backflow preventer at all. If you plan to use anti-siphon valves, every one of your sprinkler or drip zone control valves must also be an anti-siphon valve. You can use a ball valve upstream of the anti-siphon valves for an emergency shutoff. But no valves may be downstream of them. Anti-siphon valves should never be installed below ground. You can build an enclosure around the anti-siphon valves to hide them. But they must be above ground, and the enclosure must allow water to freely drain out of it if the anti-siphon valves leak.
Double or Dual Check Backflow Preventers
Double check valve assemblies (DCVA) are the most commonly approved type of device for inline underground sprinkler systems, and a good choice for interior or underground installations. These backflow preventers are less susceptible to frost damage because they are usually installed below ground or in a basement. This makes sprinkler system winterization much simpler. Double check assemblies are also less expensive than reduced pressure zone assemblies. However, double check valve assemblies are not approved as a backflow prevention device for water systems containing hazardous substances. There is some debate as to whether irrigation systems contain ‘hazardous’ substances (chemicals or toxins which pose a serious health risk), or water which is merely ‘objectionable’ (looks, tastes, or smells bad, but doesn’t pose a health risk). Double check assemblies are generally considered safe for sprinkler systems which do not have chemicals injected into the water. But depending on who you ask, double check backflow preventers may or may not be appropriate for irrigation systems. In many communities they are legal to use, and even recommended by local officials. Other communities do not allow them to be used on irrigation systems. We will attempt to present both sides of the argument. A “Dual Check” is not the same thing as a “Double Check Backflow Preventer”! They sound very similar, and they are “relatives”, but they are not the same. Dual check backflow preventers are for use with non-toxic materials. A typical use for a dual check is to install it where your house water supply connects to the water district’s pipe. The water in your house is non-toxic, so a dual check is okey. The water in your irrigation system is not as likely to be non-toxic, so a dual check is not okey! So how do you recognize which is which when you see them? A Double Check will always have two manual valves, one on the inlet and one on the outlet. These manual valves are used as emergency shut-offs and are also necessary to properly test the operation of the backflow preventer. A Double Check will also have test cocks (small outlets sticking out of the side of the backflow preventer) for connecting to test gauges. If it doesn’t have those shut off valves and test cocks it is not a Double Check Backflow Preventer! Many communities that allow double check backflow preventers do not allow the use of dual check backflow preventers. Don’t mistakenly buy the wrong thing! For more on dual checks, click here. First let’s take a look at what a double check is and how it works. A double check backflow preventer is simply two spring-loaded check valves in a row, with a shut-off valve on either end and test cocks to allow the unit to be tested for proper operation. The double check backflow preventer is the only true backflow preventer which does not have a vent to allow air to enter the lines or to allow water to escape when backflow occurs. It relies entirely on the tight seal of the two check valves to prevent backflow. In most places where double check backflow preventers are legal, local officials will allow them to be installed underground in a vault. But not all do, so you should always check with local officials before installing the unit underground. Double check backflow preventers can be installed lower than the irrigation system and often they are installed in basements in order to protect them from freezing. Regardless of where they are installed they must be readily accessible for maintenance and testing. Even in areas where double check backflow preventers are approved for use they may not be used on any irrigation system where chemicals (fertilizers, pesticides, fungicides, pipe cleaning agents) are injected into the irrigation water.
Reduced Pressure Backflow Preventer
The reduced pressure backflow preventer (RPZ), also sometimes called reduced pressure principle assemblies, are the industry standard for any type of commercial property, including apartment buildings and condominiums, because they provide the highest level of protection against pollutants and toxins. This is the king of the backflow preventers, made for high-hazard uses. Reduced pressure zone assemblies are the most expensive and complicated backflow valves to install; however, when installed correctly, these are the safest and most reliable backflow valves available. They are also usable for any type of plumbing or water system which requires a backflow deviceIt is also an expensive piece of equipment. It is the standard for commercial irrigation installations. It may not be installed in any location where it might ever be submerged under water. If installed in a structure or basement there must be a drain located near the backflow preventer. A single Reduced Pressure backflow preventer is installed upstream of all the valves. Drive through any commercial business area and you will likely spot a lot of these units, most often sitting right out by the street. Many times you may see several grouped together, each used for a different purpose. RPZ units may spit out water if they detect backflow, they also spit water if they are broken. So don`t install them inside a building without providing a floor drain.
