So ... last year the old backflow arrester on our sprinklers failed. As part of fall shut down, I replaced it with a new double vaccuum break backflow thing.

It says it's supposed to be higher than the highest sprinkler, so it is about 3 or 4 feet up (old one was just above the ground, maybe why it failed?) ...

SOOOOO ... turn on system and WATER HAMMER is aweful. Super loud ... shaking the whole house....

I have a water hammer arrester in the system from before ... mounted horizontal, but BEFORE the big upward pipe to the vacuum break and the big down pipe from the vacuum break to the valves. OBVIOUSLY that is not is the right place....

Where SHOULD the water Hammer arrester be located?

Easy places to mount are ....
On the DOWN pipe from the vacuum break to the valves.... And that could be 3 or 4 feet UP or right close to the ground...
AFTER all the valves
Way back at the tee from the house main to the line that goes to the sprinkler system

If it has to go somewhere, I'll do it, but I want to make sure that is the right place before trying to do some alot harder....