i just finished installing a sprinkler line for my lawn and i used anti siphon valves and 4quot; sprinkler heads. it says in the manual that it has to be at least 6quot; from the highest sprinkler. i think i made a mistake. i installed it 7quot; from the highest 'sprinkler line' . will i have a problem with that regarding the operation of the anti siphon valves and the sprinkler system itself?
If the antisyphon valve is not leaking maybe you should focus attention on the main valve ( assuming it is a solenoid valve) to see if it closes completely. Another thing, if the irrigation zone is on an incline then the water in the system will drain to the lowest point. Sometimes it drains slowly causing the impression of a leaky feed valve.
anti siphon valves are junk. just install regular valves and put in a backflow preventer on your mainline before all the valves, but right after the shut off vavles.
as you stated, you said the directions said at least 6 from the highest sprinkler 7 will be ok as well as 20. i don't know what region you are in, but if you are in a freeze zone though it is always a good idea to blow the lines when winter comes around.
