BACKFLOW PREVENTER - We strongly recommend a backflow preventer or vacuum breaker on any shared water source. Backflow preventers block any water from negative or back pressure in your irrigation system at shutdown from entering back into your water source, thus reducing the risk of contamination of your water source. Many municipalities require the use of an approved backflow preventer in drip irrigation systems. You may want to check with your local municipality before installing your irrigation system. The backflow preventer is designed to relieve pressure when the watering cycle stops or the system is turned off, so you will see water dripping from it at system shutdown. Backflow preventers should not be dripping constantly. To troubleshoot leaking of a hose end backflow preventer, the first thing to check is the washer seal in the female connection. The rubber washer will deteriorate or dry out over time and need to be replaced periodically. Make sure the washer is seated all the way down in the fitting. Another cause for a leaking backflow preventer is incorrect placement in the head assembly. If it is placed upstream from a timer and the system is not being used but the hose is on, pressure will build up, which can damage the backflow preventer. To relieve some of the pressure, water will leak from the relief holes. If a timer is used, it must always be installed first, before the backflow preventer.

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 Backflow Preventer

dripdepot.com

Q. Do I need a backflow preventer?

A. We strongly recommend a backflow preventer or vacuum breaker on any shared water source.

    Backflow preventers block any water from negative or back pressure in your irrigation system at shutdown from entering back into your water source, thus reducing the risk of contamination of your water source.

    Many municipalities require the use of an approved backflow preventer in drip irrigation systems.

    You may want to check with your local municipality before installing your irrigation system.

Q. Why is my backflow preventer leaking?

A. The backflow preventer is designed to relieve pressure when the watering cycle stops or the system is turned off, so you will see water dripping from it at system shutdown.

    Backflow preventers should not be dripping constantly.

    To troubleshoot leaking of a hose end backflow preventer, the first thing to check is the washer seal in the female connection.

    The rubber washer will deteriorate or dry out over time and need to be replaced periodically.

    Make sure the washer is seated all the way down in the fitting.

    Another cause for a leaking backflow preventer is incorrect placement in the head assembly.

    If it is placed upstream from a timer and the system is not being used but the hose is on, pressure will build up, which can damage the backflow preventer.

    To relieve some of the pressure, water will leak from the relief holes.

    If a timer is used, it must always be installed first, before the backflow preventer.

    Remember to hand tighten these parts only and do not use teflon tape or pipe dope on hose thread connections.

    If you over tighten them, it can ruin the washer seal or cause cracks that will leak as well. 

https://help.dripdepot.com/support/solutions/articles/11000044573-backflow-preventer