How to Select the Proper Backflow Preventer

Backflow Preventer: A device that allows water to go through it in one direction, but prevents it from going backwards in the opposite direction.

Here are links to the backflow preventer related topics below in case you come back and want to reread something. (Sorry this article is so long, it’s a complex topic and there are a lot of options available to you!)

Backflow Preventer Basics:

Backflow Preventer definition: A device that allows water to go through it in one direction, but prevents it from going backwards in the opposite direction.

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 guard.

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You should have a backflow preventer on your irrigation system if your water comes from a “potable” (drinkable) source (see next paragraph.) If your irrigation water source is considered potable, then in most places it is illegal to not have the proper local authority-approved type of backflow preventer on your irrigation system. If your water source is non-potable, you generally are not required by law to use a backflow preventer (but not always – some jurisdiction even require them for non-potable water like recycled, reclaimed, and gray water sources.)

There are many types of backflow preventers. Almost everywhere the local authorities will dictate that certain types of backflow preventers may NOT be used with irrigation systems within their jurisdiction. In some cases, the authorities will dictate the exact type of backflow preventer you MUST use. You may hear or see the term “Cross-Connection Control.”  This essentially is referring to backflow prevention.  A cross connection is a connection between a drinking water supply and a source of pollution or contamination.  A backflow preventer is almost always installed at a cross-connection to prevent contamination of the drinking water source.

What’s potable water? Definition: potable water means the water is suitable for drinking. Depending on local law, that may include drinking water for animals. If you would be willing to drink it without treatment, then it is probably going to be considered potable. Non-potable water is water that is not suitable for drinking. (Once water enters into your irrigation system it is considered to be non-potable, more on that later.) Examples of water sources that are often considered non-potable are lake and pond water, water from streams, and well water from a contaminated aquifer that is not suitable for drinking. Most other wells do require a backflow preventer, even if the well doesn’t provide drinking water. This is to protect the aquifer the well takes the water from, because even if you don’t get drinking water from the well, your neighbors may get drinking water from the same underground aquifer. If you plan to apply fertilizers or pesticides using your irrigation system, then in most cases you must have a backflow preventer- regardless of the water source. Nobody wants those chemicals going into lakes, streams or the water table!

why Why do you need a backflow preventer? All irrigation systems contain a chemical called dihydrogen monoxide. This odorless, tasteless chemical is known to be deadly if inhaled and has killed millions of people. OK, before you flame me, dihydrogen monoxide is, of course, the scientific name for ordinary water (h2o). Which points out that you have to be really careful about what you read and believe on the Internet. Unfortunately when it comes to backflow preventers there is a lot of questionable, and sometimes completely wrong, information. So what’s the deal with backflow prevention? Is it really necessary? The answer is yes!

Your landscape has all kinds of nasty things in it that will make you sick or worse if you drink them. Thus irrigation water is officially considered a contaminant (creates a health hazard) rather than just a pollutant (is objectionable in color or odor). What’s in irrigation water? How about toxic chemicals (fertilizers, pesticides, etc.) and animal waste? (Not that I want to gross you out, but every day millions of dogs lift their legs in a fond salute to their favorite sprinkler head!) These things can and WILL come back up your irrigation pipes and into your drinking water if you don’t stop them. If you have a well, they can go down your well and into everyone else’s drinking water. If you are on a community water system they could go back up into the pipes and poison your neighbors. The valves that turn on and off your irrigation system are not sufficient to stop backflow. The purpose of the backflow preventer is to protect you when the valve breaks or leaks, which all valves will do eventually. Saving a little money by skipping the backflow preventer will not seem so smart after you spend a small fortune on hospital bills (or funeral expenses) for a poisoned family member or pet!

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Now wait a minute, some people say, doesn’t the water pressure in the water system keep the irrigation water from going backwards? Yes, most of the time it does. However, there are times when the water pressure drops in the supply system, and this is when the backflow occurs. No, it is not a frequent occurrence, but it does happen more often than you think!  Often it occurs at night when you don’t notice the water was off for a few minutes.   An example is when the water company has to shut off the water to repair a water pipe, or hook up a new pipe.  This makes construction projects easily the most common cause of backflow problems.  Fire fighting is another common cause of backflow. Fire trucks use huge pumps to suck the water out of the fire hydrants. This often causes the water pressure in the surrounding areas to drop, and backflow will occur in the surrounding neighborhoods.

You can do a quick experiment yourself and create backflow in your home pipes. Simply tun off the water valve leading to your house. Next have someone turn on a faucet. Now turn on a different faucet that is higher than the first. You will hear air being sucked into the higher faucet. You just created backflow in your house piping. Pretty easy, wasn’t it?

Another common argument against the need for backflow preventers is that if all the valves are closed the water can’t go backwards through them, so the valves should prevent backflow. The obvious problem with this is that if the backflow occurs at a time when the valve is open, like when the sprinklers are on, the valve will not stop backflow!  What if the valve is closed?  A standard manually operated valve should stop backflow when it is closed – if the valve if fully closed, has good seals, and does not leak. However most of the automatic valves, such as the electric solenoid valves used for irrigation systems, will not stop backflow even when “off” and fully closed. This is because these solenoid valves are directional in design.  If you look on the valve you will see that it has an arrow on it showing the flow direction. If the flow is reversed, the valve will often open slightly (that’s why the valve has the arrow on it- to warn you not to install it backwards!) Thus when backflow occurs and the flow direction reverses, an automatic valve will not stop the backward flow.

Backflow Preventer Costs

Prices vary dependent on your situation, but as a general rule the costs of backflow preventers fall into the order that follows, from most expensive to least expensive. (Don’t panic, descriptions of each of the types are further down this page. Knowing prices will help with selecting which type to use.)

$$$$   Reduced Pressure Type Backflow Preventer (RP)
$$$    Double Check Type Backflow Preventer (DC)
$$    Pressure Vacuum Breaker (PVB)
$     Anti-Siphon Valve (ASV) – up to about 6 valves*

*Generally a PVB with standard globe valves becomes less expensive than anti-siphon valves when you have a lot of valves on the system.

How to Select a Backflow Preventer

There are several types of backflow preventers available, so let’s take a look at your options. Always check with the local water provider or government building department (whomever issues building permits) to be sure that the backflow preventer you plan to use is legal to use in your area. Regulations vary depending on location.  Backflow preventers are expensive, so you do not want to have to tear it out and install a different one!!!

The following questions will help you decide which type to use. Each type is described in detail further down on this page, a link on the name will take you to the description.

caution   WARNING: Be sure to read the description and related warnings about the type backflow preventer carefully before deciding which type to use.  Make sure your choice is legal in your community!

  1. Is this a commercial or a single family residential site?
    • Commercial: (This includes ANY business property, including apartment complexes and condominiums.) Use a Reduced Pressure Type Backflow Preventer. This is the industry standard. For commercial projects you don’t take chances.  A Reduced Pressure Type provides the highest level of protection. If something goes wrong a commercial property owner is likely to be held to a very high standard.  An hour of your attorney’s time in court costs you more than any backflow preventer!
    • Single Family Residential: Continue to the next question.
  2. Do you plan to use fertigation (fertilizer injected into the irrigation water), apply fertilizer, pesticides (such as for insect control), or apply anything other than pure water using your irrigation system?  This includes products labeled as “organic”,  “natural”, and “safe”.  This also includes the “safe” mosquito control products applied by misters and sprinklers.  Remember all these products are concentrated in the irrigation system water, so while they may be safe when dispersed into the air, they can be much more dangerous when concentrated in the water in the pipes.
    • No: Continue to the next question.
  3. Do you want the backflow preventer to be installed below ground in a valve box?
    • Yes: Consider a Double Check Type Backflow Preventer, which is the only type that can be installed in a box below ground.  Note: other types may sometimes be installed in a large vault with unblockable drainage.  (For example, you can generally put a Reduced Pressure Type Backflow Preventer in a large basement that has floor drains.)
    • No: Continue to the next question.
  4. Is it possible to install the backflow preventer in a location where it will be at least 6 inches (150mm) above all of the sprinkler heads or drip emitters/drippers?
    • Yes: Continue to the next question.
  5. Is the area to be irrigated reasonably small, such as a front yard or back yard of a tract home?
    • Yes: Continue to the next question.
  6. Would it be acceptable to you if one or more valves are installed on pipes, at least 6″ above ground? (Maybe you can hide them behind a shrub or put a decorative cover over them.)

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.)

Atmospheric Vacuum Breaker. The atmospheric vacuum breaker (AVB) is the least expensive backflow preventer. The AVB is installed on the pipe right after the control valve. If you use AVBs for backflow prevention you must install one AVB after EVERY control valve, no exceptions. 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 an 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.  (Exception:  drain valves for winterizing the system are OK and may be installed after an AVB valve, provided they are configured only to drain the pipe.  A properly installed drain valve should not create backpressure on the AVB.)

Atmospheric Vacuum Breaker Installation Detail

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.

  1. Never install an 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.  (Exception:  drain valves are OK installed after an anti-siphon valve.)
  2. 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 an 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. (Contractors and suppliers: before you flame me for being wrong, do some research. Don’t embarrass yourself! Most anti-siphon valves have a warning on the box or in the installation instructions about this.  Just read the instructions!)
  3. 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.  However, no valves may be downstream of them.   If you are paying attention you will note that essentially this is the same thing I said in #1 and #2 above! Hopefully you’re getting the message by now!
  4. Anti-siphon valves should never be installed below ground, even if buried in a box.
  5. You can build an enclosure around the anti-siphon valves to hide them, if they are above ground.  However they must be above ground, and the enclosure must allow water to freely drain out of it if the anti-siphon valves leak.

Anti-Siphon Valve Installation Detail

Pressure Vacuum Breaker. A pressure vacuum breaker (PVB) is similar to an 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.

Warning: 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.

Pressure Vacuum Breaker Type Backflow Preventer Installation Detail

Reduced Pressure Backflow Preventer. The reduced pressure backflow preventer (R.P. Unit) is the king of the backflow preventers, made for high-hazard uses. It is also an expensive piece of equipment. It is the standard for commercial irrigation installations. This is the type of backflow preventer that I use on most of my designs. The R.P. Unit must be installed 12 inches (300mm) above ground, but it does not have to be higher than any of the sprinklers.  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 R.P. Unit is installed upstream of all the valves. R.P. Units are used for many things other than irrigation systems. 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. R.P. 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 water drain.

Reduced Pressure Type Backflow Preventer Installation Detail.

Double Check Backflow Preventers. 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. I will attempt to present both sides of the argument.

Don’t be fooled! 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 (hopefully!) non-toxic, so a dual check is OK. The water in your irrigation system is not as likely to be non-toxic, so a dual check is NOT OK!  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. However, since not all do, 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.

Double-Check Type Backflow Preventer Installation Detail

why For an explanation see my separate article with an in-depth discussion of the pros and cons of double check backflow preventers.

Dual Check Device. Technically this one is NOT a backflow preventer. It is essentially a stripped down version of the Double Check Backflow Preventer, without the shut-off valves or test cocks. What are they made for then? Just to confuse people? It may seem like it! What they are is a flow control device rather than a backflow preventer. Now I admit there is a thin line of difference between the two. A typical use for a dual check is to install one after a water meter to prevent the meter from running backwards (gee, I wonder why the water company wouldn’t want that to happen?). OK, to set the record straight, I have heard that some authorities do suggest the use of these devices as backflow preventers. In most cases they are requiring them in locations where the general consensus is that no backflow preventer is needed at all. (Most municipalities do not require backflow preventers on water supplies to single family homes, provided water is only used for drinking. The authorities assume that your toilets, washing machines, bathtubs, and dishwashers all have built-in backflow preventers- which pretty much all of them do.) However, some authorities apparently are allowing, and even recommending(!!!), that dual checks be used for irrigation systems, and this is very risky. What if someone wanted to build a nuclear power plant a few blocks from your home, and they decided to build the cheapest model available, one that wasn’t even designed to be used as a power plant? Then to save even more money, they decide to leave out all the test equipment used to confirm that it is not overheating or leaking radiation? Only Homer Simpson could get excited about that! Why would anyone in their right mind use a flow control device that has been stripped of all it’s test equipment as a backflow preventer?

Pressure Losses in Backflow Preventers
(PSI Loss)

To find the pressure loss through the backflow preventer you will need to consult the manufacturer’s literature. All of the following backflow preventer types are available in several sizes.

  1. Atmospheric vacuum breaker. Although you may have several atmospheric vacuum breakers on your system, the water will only flow through one of them on its way to the sprinklers or emitters. Therefore, you only include the pressure loss for one vacuum breaker in your calculation. Most manufacturers don’t have pressure loss information for vacuum breakers, so assume a pressure loss of 2 PSI for a vacuum breaker if no manufacturer’s literature is available.
  2. Anti-Siphon Valves. If you plan to use an anti-siphon valve enter 0 as the backflow preventer pressure loss. The backflow preventer is part of the anti-siphon valve and the pressure loss is included with the valve so you will enter it later.
  3. Pressure Vacuum Breaker. You will need to obtain the manufacturer’s pressure loss information for the pressure vacuum breaker you plan to use. As a general rule pressure losses for pressure vacuum breakers range between 2 and 5 PSI, so using 5 PSI would be a fairly safe figure.
  4. Reduced Pressure Backflow Preventer. You will need to obtain the manufacturer’s pressure loss information for the reduced pressure backflow preventer you plan to use. As a general rule pressure losses for reduced pressure backflow preventers range from 8 and 12 PSI, so using 12 PSI would be a fairly safe figure. Pressure losses for reduced pressure backflow preventers are very high compared to other backflow preventers due to the method they use to prevent backflow. The pressure drop is used to detect the backflow and redirect the water out of the bottom of the backflow preventer. Yes, it will spit water from time to time so make sure you provide somewhere for the water to go!
  5. Double Check Backflow Preventer. You will need to obtain the manufacturer’s pressure loss information for the double check backflow preventer you plan to use. As a general rule pressure losses for double check backflow preventers range from 3 and 5 PSI, so using 5 PSI would be a fairly safe figure.
  6. Check Valves and Dual Checks. There are two other types of backflow preventers available that you may run into. The first is a check valve. The second is a “dual check”. It is NOT the same thing as a Double Check even though it may be marketed as one! Don’t get ripped off! Pressure loss through a check valve tends to be about 3 PSI. Dual Checks have losses similar to Double Checks (5 PSI).


All backflow preventers must be installed correctly. Follow the instructions that come with the units. Backflow preventers should be checked yearly for proper operation. In areas where it freezes, the backflow preventer should be protected from freezing. See the Irrigation System Winterization tutorial for details.

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