How to Install a Irrigation Anti-Siphon Valve

Anti-siphon valves are the primary type of valve used for residential irrigation systems.   There are several very important rules that apply when you are using or installing an anti-siphon valve.  They MUST be installed in the correct way or they will not work!!

Quick & Dirty Summary

  • Anti-siphon valves must be installed 6″ higher than the highest sprinkler head or emitter outlet.
  • The anti-siphon valve inlet pipe must meet local plumbing code requirements for exposed pipe.  Generally that means using metal pipe such as copper, brass, or galv. steel.
  • No other on/off valves may be installed on the downstream side of an anti-siphon valve.
  • Anti-siphon valves may not be left “on” or “open” for more than 12 continuous hours at any time.
  • The optional “Flow Control” feature is worth paying more for.  It will probably save you more time and money than the extra cost it adds to the valve.  Plus most anti-siphon valves without flow controls are poor quality.
  • To avoid nasty surprises, avoid using a water supply for your irrigation system that passes through a house inside the walls, under floors, or through the attic.

Keep reading for in-depth details and answers to “why?”

If you don’t plan to use an anti-siphon valve most of this page will not apply to you, although some of the ideas and suggestions here apply to all types of valves.

Here’s a sketch of the installation of a typical anti-siphon valve:

anti-siphon valve
Anti-Siphon Valve Installation Sketch – Detail A

Note: Be very careful not to over-tighten the metal pipe where it connects to the plastic valve and pvc ell when using this detail. It is easy to crack and split the plastic if the metal pipe is over-tightened. Hand tighten it only, if you must use a wrench (not all of us are that strong) be extremely cautious! Use Teflon tape on the male threads as a sealant. You can buy Teflon tape at any plumbing store.

The Anti-Siphon Valve Installation Rules:

Install Above Ground Higher than All the Sprinkler Heads

The anti-siphon valve MUST be installed higher than any of the sprinkler heads or emitters it turns on and off.  It should be higher than any of them, not higher than most of them.  (Back many years ago some said it must be higher than the average height of the sprinklers, you may run across out-of-date information on this, the standard is now higher than ALL the heads.)  The normal rule is at least 6 inches above the top of the highest sprinkler head, some brands say it must be 12 inches.  If it isn’t higher, the built-in backflow preventer will not work.  The anti-siphon valve will also spill water from the air vent if not installed higher than all the sprinklers or emitters.  The anti-siphon valve must also be above ground.  You can put a box over it to hide it, but it must be above the ground level.

Metal Pipe on Inlet Side

In many areas you are required by code to use metal pipe on the inlet side of the anti siphon valve. This is because the Uniform Plumbing Code only allows plastic pipe installed outdoors to be used on pressure lines if it is at least 18″ deep. Anything less than 18″ deep must be metal. (A pressurized line is defined as any pipe which is under constant pressure, such as a mainline. Isolation valves don’t count unless they are automatic and closed after each irrigation cycle.) Keep in mind that PVC and PEX plastic pipe/tube should not be left exposed to sunlight for a long period of time (more than a few months, weeks in desert regions.) Most plastic deteriorates when exposed to light and will become brittle.  Poly tube will hold up better to sunlight than PVC which is one reason it is used for drip tubing.  If for some reason you feel you need to use PVC pipe above ground, paint it with several thick coats of paint (silver paint works well), or better yet, wrap it with aluminum foil held in place by a layer of black polyethylene tape. Another solution is to build a wood or metal box to go over the pipe to keep it in the dark. This is also a good way to hide the valves and protect them from vandalism. The Battery Operated Controller Detail (2batcntr.gif) shows a method of enclosing an above ground valve using a plastic valve box that works well with anti-siphon valves.

Anti-Siphon Valves Used with Other Valves

A anti-siphon valve may not be installed upstream of another on/off valve, like a zone control valve.  A lot of old timers and less informed folks think that a anti-siphon valve can be used as a backflow preventer for a manifold of valves.  However this is incorrect, an anti-siphon valve will not stop backflow when installed in that manner.   You are simply wasting your money.  I get a lot of arguments on this, so rather than argue I’ll just say read the box or instruction manual the anti-siphon valve came with, it will say the same thing.  If you use anti-siphon valves for backflow prevention all of your zone valves MUST be anti-siphon valves. When you use other valves after the anti-siphon valve the valves trap the pressurized water in the pipe when they close,  This pressure keeps the vent in the anti-siphon valve from opening.  If the vent doesn’t open then the siphon is not broken and backflow can occur, so the anti-siphon valve will not stop the backflow.

Valves you can use downstream of a anti-siphon valve:  You can use drain valves for winterization downstream from a anti-siphon valve.  Drain valves work different and won’t cause the back pressure that makes an anti-siphon valve not work.  You may also use check valves used for preventing low-head drainage provided the highest sprinkler head on the circuit zone does NOT have a check valve on it. See the next section below.

Anti-Siphon Valves and Sprinklers with Check Valves

Some sprinklers have built-in check valves for preventing water from draining out of the pipes each time the system stops running.  These are often used when irrigating a sloped area.  If you use this type of head then you must remove the check valve feature from the highest head on each valve circuit.  This releases the back pressure caused by the check valves.  You can remove the check valve feature by disassembling the head and removing the little rubber washer on the bottom of the spring assembly.

Must Not Be Continuously On

The anti-siphon valve must not be “on” continuously for more than 12 hours.  The reason for this is that when the anti-siphon valve is on a small vent is pressed closed by the water moving through the valve.  This vent must open immediately when the valve is shut off.  When left on for long periods of time this air vent often sticks in the closed position, which makes the anti-siphon not work.

Flow Control

This is not a requirement for anti-siphon valves, just something I suggest.  Most good quality anti-siphon valves have a “flow control” feature.   If the valve has this feature, the box or description will say it has flow control.  Price is a good clue, cheap valves do not have this feature and I very strongly recommend you avoid them.  The flow control feature allows you to fine tune the performance of your valve as well as troubleshoot operation problems.  A valve without a flow control is like a car without an accelerator or brakes.  Without a flow control it can be very difficult to get a valve to work outside of it’s performance “sweet spot” which means if the water pressure or flow is either are not perfect you will be tearing your hair out.  Since water pressure and flow are seldom perfect, a flow control is worth every penny you pay for it!


Anti-Siphon Valve
Anti-Siphon Valve Installation Detail B – A Common Installation Method Used On Homes in Temperate Zones



Does the Water Supply Run Through the House?

This section on water supplies running through the house is covered elsewhere in the Sprinkler System Design Tutorial that this page is part of, so it is repetitive, but I’m including it here because a lot of people pull up this page when searching for information on anti-siphon valves and haven’t read the rest of the design tutorial.  Of course if you don’t already know the following information, it probably means you are making a lot of other mistakes too.  You might want to read the tutorial before you waste a lot of money!

Some house builders provide a sprinkler connection on a pipe where the water is coming out of the house.  I strongly recommend that you NOT use this connection. Run a new supply pipe from the water source that goes around the house. There are two reasons why this is advisable. The first is that there may be, and often is, a restriction in the pipe running through the house that severely reduces the water flow.  If there is a restriction, each time a valve closes your house will shake like a bulldozer just ran into it. For more information on things that go bump in the night, check out my comments on water hammer.  There are a lot of things that can cause a restriction, and you can’t test for one by simply turning on the water and timing how long it takes to fill a bucket.  I know someone at the store or in another tutorial probably told you otherwise, but they are wrong and there is a pretty good chance you will regret listening to them!  Many people over the years have written me to say “I should have listened to you, now what do I do?”  Unfortunately the answer to that question is to either run a pipe around the house or completely re-valve your irrigation system so it uses a lot less water.

(If you really have to use water that goes through the house then you need to measure the flow using the “Wet Method” described in the tutorial on flow rate homes that use pumps.  The wet method does use a bucket, but it also uses a combination of gauges and a specific method.  Use this method even if you don’t have a pump.  The wet method described in that article compensates for flow restrictions.)

Before getting distracted I mentioned two reasons for not running water through the house.  The other reason is that when your sprinklers are running at night, all that water will be screaming through the pipes in the walls of your house! If you’re lucky you won’t hear it. If you’re not, and most people aren’t, then sweet dreams will be a thing of the past!  What sounds like a gentle gurgle in the afternoon sounds more like a freight train running through the next room at 2 AM.

 Installation Step-by-Step

  1. Flush the pipes.  Flush again!  Automatic solenoid valves have tiny ports (water passages) in them that can be easily blocked by a grain of sand or debris.  The #1 reason valves fail to open or close is something blocking or partially blocking one of these ports.  Simply flushing the pipes out well will eliminate many of these failures of “brand new valves.”
  2. Install the valve on the supply pipe.  Do NOT install the pipe on the outlet side of the valve yet.  If using pvc it is generally easier to screw the male adapter into the valve before you glue the pipe into the male adapter.  Use Teflon tape on the male threads.  Wrap at least 3 layers of Teflon tape.  Tip: the cheap Teflon tape sold in many hardware and home improvement stores is very thin.  If you are using this type of tape you will need more than 3 wraps.  Hold the tape up to the light , if it is translucent it is the cheap stuff.  When applied to the threads the Teflon layer should be thick enough that the edges of the threads no longer appear sharply defined, with most of the “valleys” between the threads filled with tape.  Pull the tape tight as you wrap it on, stretching it slightly.  Wrap the tape in the same direction as the threads so it doesn’t unwrap itself as you twist the male threads in.  (Which direction is that?  If you’re looking at the threaded end of the male adapter you would wrap the tape in a clockwise direction.)  If the valve is plastic hand tighten the threads into the valve.  If you don’t have a lot of hand strength use a wrench to make the last full turn.  Do not over tighten, you can split the valve!
  3. For electric valves, connect your wires from the controller to the wires on the solenoid.  One wire should be a “common” that connects all the valves together (normally this is a white wire.)  The solenoid wires are interchangeable.  Either wire on the solenoid may be used for the common.  The other solenoid wire connects to a “lead” wire which goes to the specific station connector for this valve zone on the controller.  The wire splices should be made using a special waterproof wire splice connector.  You can buy waterproof wire connectors at any hardware store, they are inexpensive, reliable, and easy to install.  These connectors have a waterproofing gel in them that seals the wires.  The #1 cause of solenoid failures is a wire splice that was not completely water proofed.  Water is sucked from the splice up through the gaps between the individual wire threads in the solenoid wire by capillary action.  Once in the solenoid it rusts it from the inside out.  I can’t over-stress how important this is: Use waterproof connectors!
  4. Now that the anti-siphon valve is installed it needs to be “primed” (priming is my name for this, I don’t know of an official term used for this process) by forcing the water into the small water ports inside the valve.  Often the valve just primes itself as soon as you turn on the water supply to it.  But some valves are just stubborn and need to be forced.  Most higher quality valves are factory tested so they are already primed (you will probably notice that the valve drips water left over from the factory testing/priming), the cheaper valves are not primed.
    Tip: most solenoid valves do not close very well when there is not a pipe installed on the valve’s outlet.  It can help with testing and make things a lot less messy if you install several feet of temporary pipe or hose to the valve outlet to create some back-pressure on the valve and redirect the water away from where you are trying to work.  A garden hose works fine for this.  To attach a garden hose you will need a short 3/4″ SCH 80 nipple.  If the valve is 1″ size you will also need a 1″ x 3/4″ threaded reducer bushing.  Your garden hose has “hose threads” rather than “pipe threads” so it will not screw tightly onto the SCH 80 nipple’s 3/4″ pipe threads, but it will go on.  You will get some leaking from the connection, if you put a second washer in the hose end it will leak less.
    The procedure is slightly different at this point if the valve has a flow control.
    Valves with flow control:  Close the flow control on the valve.  Turn on the water supply to the valve.  If the valve has a bleed screw use it now, there may also be an off/on lever but the bleed screw works better for priming.  Open the bleed screw to turn on the valve.  Water will squirt out.  Don’t remove the screw, just loosen it a full turn or until water streams out.  If there is not a bleed screw there will probably be a lever under the solenoid that has “on” and “off” (or “open”/”close”) positions marked on it, set it to “on.”  Now open the flow control.  The valve should open.  Let it run for a minute or two to stabilize and work the air out.  Next close the bleed screw or move the manual lever to the “off” position.  The valve should close, it may take a while before it closes.  If it doesn’t close, slowly close the flow control, the valve should close by itself as you reduce the flow through it.  Leave the flow control in the partially closed position until you have finished installing the irrigation pipes and need to flush the pipes.  Open it fully for flushing.
    Valves without flow control: Turn on the water supply.  Water will probably flow out of the valve outlet for a few seconds when you do.  It should stop by itself after 30 seconds or so.  If the water doesn’t stop flowing from the valve try placing your hand over the outlet to block the flow of water.  This may get you very wet but will probably also force the water into the ports and make the valve close.  Now that the valve is closed, manually open the valve using the manual open/close lever or the bleed screw.  The valve should open.  Now close the bleed screw or move the manual on/off lever to off.  If you are fortunate the valve will close.  If it doesn’t close turn off the water supply.  Now turn it back on.  The valve should stay closed.  If it doesn’t stay closed this time, do the hand over the outlet thing described above and it should close.  Tip: You will probably not need to do any of the messy “hand over the outlet” stuff if you attach a garden hose to the valve as described in the earlier tip above!
  5. Now if using automatic valves, test the valves using the same procedure as the step above, but use the controller to open and close them.  The controller has a manual mode setting that allows you to turn on and off the valve, use it to make sure the valve is wired correctly and works.  See the controller’s instruction manual.
  6. Now install the downstream pipe after the valve along with the rest of your irrigation system (sprinklers or drip emitters.)
  7. Adjust the flow control (optional for valves with a flow control.)  Once the sprinklers or drip emitters are installed fine tune the valve performance by partially closing the flow control.  You will need to play with the flow control a bit, testing different amounts of closure, until you get a setting you like.  If you close it too far the sprinklers will stop performing properly.  However, partially closing the flow control is a good way to reduce misting of the sprinklers, and excessive misting wastes water and results in poor coverage.  Also you will notice that partially closing the flow control will make the valve close faster.  A warning on this, don’t try to make the valve close too fast, it is normal and desirable for it to take a few seconds to close.  If the valve closes too fast it can send shock waves into the water supply pipe.  It is normal for it to make a whooshing noise as it closes along with a soft clunk.   Once you have the flow control adjusted leave it in that position.  The valve is designed to be operated with it partially closed.  Just like the accelerator in your car doesn’t need to be “floored” all the time.

Newly Installed Valve Won’t Close

This is a fairly common problem with newly installed valves.  It often happens when the water inside the valve is not being forced into the ports.  It also may occur when you have installed the valve but haven’t installed any of the downstream pipe after it.  An additional possibility is one of the tiny water ports inside the valve is plugged, possibly by a manufacturing defect, more often by dirt from not flushing the water supply pipe out before installing the valve.

Valves with Flow Control:  If you have a valve with the flow control feature, now is when it pays off! This is a lot of steps but they are easy:

  1. Turn the manual on/off lever to the “on” position.
  2. Next, if there is a bleed screw, open it one turn (water will dribble out.)
  3. Now close the flow control until the valve closes.
  4. Wait one minute.
  5. Close the bleed screw and turn the manual on/off lever to the “off” position.
  6. Finally reopen the flow control.   Most of the time the valve will stay closed and work normally.  Why did this work? Closing the flow control forces water into those small ports and opening the manual on/off lever and the bleed screw allows trapped air to escape.

Valves without flow control: If the pipe downstream of the valve has not been installed yet try one of the following:

  1. Install a temporary adapter on the valve outlet so you can attach a garden hose to the valve as described previously.
  2. Turn the manual on/off lever to the “on” position.
  3. Next, if there is a bleed screw, open it one turn (water will dribble out.)
  4. Wait… the valve should close within a minute or two.
  5. Close the bleed screw and turn the manual on/off lever to the “off” position.
  6. Hopefully the flow restriction in the hose may force the water into the ports and the valve should now start working correctly.

Or… fast but wet method:

  1. Turn the manual on/off lever to the “on” position.
  2. Next, if there is a bleed screw, open it one turn (water will dribble out.)
  3. Place your hand over the valve outlet to restrict the flow out of it.  (You will probably get very wet!)
  4. Wait… the valve should close within a minute or two. If the valve closes you can remove your hand.
  5. Close the bleed screw and turn the manual on/off lever to the “off” position.
  6. If the valve didn’t close slowly remove your hand, trying not to spray yourself with water.  Most of the time the valve will close when your hand is placed over the outlet.  Your hand forces the water into those small ports and opening the manual on/off lever and the bleed screw allows trapped air to escape.

If the valve still stays open then it may have gotten dirt in it or have a manufacturing defect.  You can remove and return it as defective, or attempt to clean or repair it.

Valve Won’t Open?

See the article on How to Fix an Automatic Valve that Won’t Open.

This article is part of the Sprinkler Irrigation Installation Tutorial Series
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