Backflow prevention is perhaps the second most important component of an irrigation system after the water itself. Backflow prevention’s primary goal is to protect the potable water supply from harmful contaminants that can be found in the soil. Bacteria, fungus, fertilizers and other treatments are often present in the soil and these can all be harmful if ingested. Backflow prevention prevents these harmful contaminants from flowing back into the potable water supply, protecting not only yourself and your family, but your neighbors and community as well. In the article below, I will cover several different types of backflow prevention that are commonly found in use.
It is important to note that different municipalities will require different types of backflow prevention when it comes to lawn irrigation. Whether it be ASV, PVB, DCV, or RPZ (acronyms will be explained below), every town and city can be expected to have their own requirements when it comes to backflow prevention and you will want to reach out to them to find out what is approved for use in your area. The one common exception to this is hose-end backflow preventers, the type you thread on to an outdoor spigot. Almost universally, hose threaded AVB units (Atmospheric Vacuum Breaker) are acceptable (and required) for use on residential irrigation systems operating from an outdoor hose bib. The system still needs to be within specification for the unit used in regards to flow and pressure, but they are recognized across the states as sufficient backflow when running from a hose bib. Again, and this cannot be emphasized enough, check local regulations before purchasing and installing any type of backflow prevention.
Types of Backflow
There are two types of backflow we concern ourselves with: backsiphonage and backpressure. Backsiphonage is a reversal of flow caused by negative pressure in the supply line. Backpressure is a reversal of flow caused by downstream pressure exceeding system pressure. Both types can occur in residential water systems, however not all backflow devices protect from both types of backflow.
Out of the two types, a residential system is most likely to encounter backsiphonage, however backpressure is still a concern, particularly in water systems utilizing a pump, such as a well or similar.
Backflow is a concern at any cross connection. A cross connection in this context is defined as any direct connection between potable and non-potable water, such as an irrigation system. It does not matter if this cross connection is temporary or permanent, backflow prevention will be needed.
A Word About Constant Pressure...
Constant pressure, also referred to as static pressure, is a situation in which a unit, be it a filter, a backflow preventer or even a pressure regulator, is not relieved of pressure when the system is not in use. Typically this will be anything that comes before the zone or timer valve. Think about when you turn off the water at the hose bib; the internal valve inside the hose bib is now under constant pressure, as the water continues to push against it until the valve is opened again.
With non-constant pressure backflow preventers, one is needed for each and every zone; with constant-pressure backflow preventers, a single unit can pevent backflow for all zones. All hose end backflow preventers are not rated for constant pressure.
A Word About Regulations…
It will always be worth checking with local water regulatory officials. Each municipality has their own regulations; some regulate what you can not use, some regulate what you must use in regards to backflow prevention. Some municipalities will even require the installation be done by a licensed professional, or by the homeowner after receiving training and/or certification. Some municipalities will require different backflow protection for sprinkler systems than they require for drip irrigation systems.
Essentially, if you have not, you will need to look into local regulations and requirements. Installing the proper system now can save significant money down the road, re-doing an installation can be a very costly endeavor.
A Word About Acronyms…
If you have done any research on backflow prevention prior to locating this article, you’ve no doubt run into more acronyms than you know what to do with, they are particularly ubiquitous when it comes to backflow prevention. In this article I will try to use the popular/accepted acronyms for each type and hope to include as many alternatives as possible.
Hose-End Atmospheric Vacuum Breaker (AVB)
If you’ve ever purchased, or looked into purchasing, a drip irrigation kit you’ve probably used or seen one of these. When we think backflow prevention, these are usually the default that we turn to in the residential side of the drip irrigation industry. Common acronyms for these types of backflow preventers are AVB, HEVB and HBVB. They are not acceptable for use in sub-surface applications, and should be elevated above the highest point emitters when possible as this allows the unit to break the vacuum that can lead to conditions favorable to backflow. They are among the most affordable of all backflow prevention options but also amongst the most limited. For a residential drip system running from a hose bib they work great, but you won’t find many in use in lawn irrigation systems.
Note: not all municipalities approve the use of AVB units, be sure to check with local regulatory officials.
Our listings of of Atmospheric Vacuum Breakers and Hose End Vacuum Breakers can be browsed here: Backflow Preventers (AVB and HEVB).
Pressure Vacuum Breaker (PVB)
These are quite similar to atmospheric vacuum breakers, however they can withstand static pressure and are typically placed before the zone valves. Like AVB units these need to be installed higher than the highest point emitter on the zone and must be above-grade. These are notorious for leaking some when backflow occurs, so be sure in its installation you account for this in regards to drainage as they should not be submerged in water.
Pressure vacuum breakers are generally not recommended for irrigation applications, however they can be used under constant pressure and contain a test port for annual testing.
Anti-Siphon Valve (ASV)
Anti-Siphon valves can be handy in that they are both a control valve and backflow preventer in one. These are the most common types seen in residential irrigation systems. Like the two above, the backflow prevention portion of it is an atmospheric vacuum breaker, meaning they also need to be installed above the highest point emitters on the system and cannot be installed below-grade, even if the sprinklers or drip are below the valves. Anti-Siphon valves must be downstream of the zone valves in order to provide protection from backflow. When used for backflow prevention, every zone on the system must also use an anti-siphon valve.
Our listings of Anti-Siphon valves can be browsed at this link: Anti-Siphon Irrigation Valves
Reduced Pressure Zone (RPZ)
Sometimes also referred to as RP or RP Units or RPZA. These offer the highest level of protection available out of any backflow prevention device (except perhaps air gap systems) and are the go-to in high hazard conditions. Due to this, commercial projects use these exclusively, however they are not limited to commercial uses. Residential use, though not as common as the other options, is not uncommon. Homeowners wanting the strongest protection possible for their potable water supply will often opt for RPZ backflow protection.
Though they need to be installed above-grade, they do not have to be higher than emitters on the system. They must never be installed in a location where they may become submerged under water. Due to their efficacy, many professional irrigation designers will only design for systems that will use an RPZ unit. Their thought is that, other than cost, there is no detriment in over-protecting a water source but there can be in under-protecting one. Note, when functioning properly these can leak a lot of water, thus it will be necessary to plan for proper drainage.
Double Check Valve (DCV)
These are sometimes referred to as Double Check Assembly (DCA), or just DC. A double check valve is a backflow preventer that features a second check valve. The second one acts as a back-up should the first fail or otherwise get stuck open. The two check valves in these assemblies operate independently.
Similar to a double check valve is a dual check valve. Dual check valves should not be used in potentially toxic or hazardous systems, meaning they are not recommended for use in irrigation systems. Double check valves are often recommended in irrigation systems and can be used in hazardous systems (note, what qualifies as hazardous can vary significantly from location to location, check with local regulatory officials). The biggest difference between a double and dual check valve is in testing capabilities. Double check valves have test ports, dual check valves do not.
Technically, these two types should not be grouped together, any irrigation professional can (and will!) tell you that. Although they are extraordinarily similar, they are not the same thing. For the DIY’er, however, I’ve grouped them together for convenience. Essentially, a double check valve is the better option (when approved for use in an area) out of the two for irrigation purposes.
The chart below covers the primary features of each type of backflow preventer. Note, that "Hazardous Systems" does not necessarily include irrigation systems. Hazardous in this context is generally referred to applications such as car washes, launderers and others that use caustic chemicals.
With that said, irrigation systems can be considered hazardous depending on chemicals (fertilizers, water treatments) that are used with that irrigation system. Some municipalities will classify a simple sprinkler system as hazardous. Like other variables when it comes to backflow prevention, many of the specifics will vary significantly from location to location. Accept no final word of authority other than local officials in these regards -- what one location considers non-hazardous, another very well may consider a high hazard.
None of the backflow preventers mentioned in this article should be submerged, including those that can be installed below-grade. Regarding minimum install height, note that like others, the minimum installation height is likely to differ from location to location. The recurring theme of "check with local officials" applies here again.
|Type||Hazardous Systems||Static Pressure||Install Height||Backpressure||Backsiphonage||PSI Loss||Testable|
|AVB||N||N||6" Above Emitters||N||Y||Approx 2 PSI||N|
|PVB||Y||Y||12" Above Emitters||N||Y||Approx 2-5 PSI||Y|
|ASV||N||N||6" Above Emitters||N||Y||See Valve Specs||Y|
|RPZ||Y||Y||12" Above Grade, Drainage||Y||Y||Approx 8-12 PSI||Y|
|Double Check||N||Y||Any, Do Not Submerge||Y||Y||Approx 3-5 PSI||Y|
|Dual Check||N||Y||Any, Do not Submerge||Y||Y||Approx 3-5 PSI||N|
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