The Air/Vacuum Relief Valve (AVR) is designed to reliably discharge air during system start-up and allow air into the line during shutdown. The valve closes even with low water pressure and reduces water hammer.

• It is good practice to use an AVR for all subsurface installations. There may be unforeseen changes in zone height, and the AVR provides protection against debris ingestion or tubing collapse. 
• HDL-CV has a working check height of up to 6' (1.8 m). If the full zone has a height differential of less than 6' (1.8 m), it is safe to eliminate the need for air relief.
• For HDL-PC, HDL-R, or any product without check valves, AVRs should be used to prevent the back siphoning of debris into the emitter chamber after the irrigation cycle has completed.