The function of an anti-siphon valve is to allow water to flow in only one direction, with the intent of preventing unsafe water from re-entering a clean water supply.

For example, when dispersing fertilizer through a lawn sprinkler system an anti-siphon valve would prevent the fertilizer from siphoning back into the household plumbing and contaminating the drinking water. Another application is in cars, where anti-siphon valves prevent theft of gasoline from the vehicle.

Most anti-siphon valves are simple: they have a spring-loaded one-way valve that only opens only when water flows in one direction; if water flows in the opposite direction, the valve seals and does not open.

Most building codes require anti-siphon valves in new construction, including toilet fill valves and external hose connections.

If a home inspection reveals the lack of anti-siphon valves in a residential irrigation system, they will need to be installed. You can do this as a DIY project. First, you will want to know the basics:

  • An anti-siphon valve must be installed above ground and at least 6” higher than the tallest sprinkler head (if your sprinkler head is 6” high when extended, the anti-siphon valve would need to be at least 12” high). Make sure you measure each sprinkler head; the anti-siphon valve must be 6” higher than all of them.
  • The inlet pipe must be metal: copper, galvanized steel, or brass, due to high pressures and the fact that PVC or plastic pipe deteriorates in UV light (sunlight). The only exception is if the inlet pipe is buried more than 18” deep.
  • To prevent malfunction, avoid installing other on/off valves on the downstream side of the anti-siphon valve.
  • Better safe than sorry: don’t use a water supply that passes through your house for your irrigation system.
  • If you’re purchasing a new valve, opt for one with a Flow Control feature that allows you to fine-tune the water pressure for optimal flow.
  • Install the anti-siphon valve directly downstream of the isolation valve, which is the main shut-off valve for your irrigation system. It is typically found directly after the coupling with the main water line.
  • Apply Teflon tape to all threaded pipe ends, to seal them.
  • Never over-tighten metal fittings when using PVC or plastic valves as they are easy to crack; for best results hand-tighten only (or use a light touch if you need to use a wrench), and use Teflon tape to seal the threads.

How To Install An Anti-Siphon Valve

  1. Install a dual-threaded metal inlet pipe into the isolation valve on one end and a metal elbow the same diameter as the inlet pipe on the other, and tighten.
  2. Insert the threaded end of the anti-siphon valve into the metal elbow connector. Note: if your anti-siphon valve doesn’t have a threaded end, you will need to install a short connector piece of pipe with two male threaded ends before installing the anti-siphon valve into the elbow connector.
  3. If your anti-siphon valve comes with a sprinkler head attached, you can skip this step. Otherwise, use a 3” piece of PVC pipe. Be sure to use PVC primer on the female ends before inserting the male ends and use PVC glue to secure the pipe to the sprinkler head and the anti-siphon valve.
  4. Once the first sprinkler is installed, begin running the pipe to the remaining sprinkler heads by installing a vertical piece of PVC pipe to the base of the sprinkler head. Make sure the first PVC elbow is at least 18” underground.
  5. Once all sprinklers are installed, test the system:
    1. Flush the entire sprinkler system to clear any debris such as dirt that may have gotten into the pipes or sprinklers (debris can cause clogging).
    2. Shut off the water supply.
    3. Locate the downstream (NOT inlet) pipe directly connected to the anti-siphon valve. Use a wrench to loosen the fitting and disconnect it.
    4. Detach the first sprinkler head.
    5. Check if water is coming out of the pipe.
    6. If yes, the anti-siphon valve is not working and needs to be replaced. If water does not flow, the valve is fine.
    7. Reattach the pipe: remember to reapply Teflon tape on the pipe threads, and tighten just a little more than hand-tight (careful not to crack PVC pipes when tightening metal fittings)
    8. Lastly, check for leaks by turning on the water supply and the sprinklers. You can use pipe sealant to seal minor leaks; major leaks require replacement of the pipe.

Safety tip:  Never leave an anti-siphon valve on (or open) longer than 12 continuous hours as it can “stick” and fail to close when needed.

Video on Anti Siphon Valve

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