Water Hammer - Food for Thought

So 6 months or so ago I developed some pretty bad water hammer when two of my sprinkler zones shut off. These zones are on a second 8 zone Rachio on the property controlling a guest house yard area and fed off a second backflow device, plumbed off a hose bibb of the guest house. The water hammer was most obvious at the backflow device, but strangely enough, would cause the two backflow devices on the house to shutter as well.

I knew it was something that I needed to look into, but with a new baby coming any day, and the rest of life consuming my time, I flipped the switch in Rachio to limit water hammer and hoped for the best until I got a chance to look into it. Being that the zone cycle soak, it didn’t really help much and the water hammer would hit 4 times each time the grass was watered.

Fast forward a couple months, I had the fun of dealing with a small flood in the guest house. The plastic nut on the toilet supply line (less than 2 years old) cracked and was spraying a nice stream of water out the side. Since the guest house isn’t used everyday, it was about 3 days before we found it…Coincidence? Probably not…

Decided I needed to take the time and figure out the water hammer issue, and here is what I came up with. I had 2 zones of grass, each zone contained 8-9 sprinkler heads, and at some point had been outfitted with some Hunter Pro adjustable heads that can flow from .6 - 2.36 GPM. Based on the arcs, I figured I was flowing a minimum 14.4gpm for each zone. This portion of the system is fed off a 1/2" hose bibb, thru a 1/2" Febco unit. At the GPM rate, I figured the water was moving in excess of 13 feet per second thru the pipes, and when a valve slammed shut at the end of the run, BAM. Water hammer!

So, I replaced all the heads with the MUCH lower flowing Hunter MP-Rotators (which I admittedly have a small obsession with), and the drastic decrease in flow did away with the water hammer completely.

Moral of the story: Don’t let water hammer go. The Rachio solution does work for corresponding zones, but there will always be hammer at the end of the run on the last zone. Water hammer can cause big issues, as possibly evident by my gutted house.


The Rachio 2 and 3 have a water setting in the “more” settings.

Press More, then press “Controller Settings”.

I have run across many customers with water hammers. Many of them have been caused by Rainbird anti-syphon valves which tend to shut off too quickly and will slam. Others have been resolved by installing a Hammer Arrestor in your mainline.

Yes, so do Gen 1’s and those work for consecutive zones running back to back, but if cycle soaking or end of the schedule, the hammer is still there.

The arrester was my last resort, but I was going to troubleshoot the valves or diaphragm, and then the backflow device. Even with my Hunter valves being relatively slow close compared to others on the market, shutting off water moving at 13 ft/s in a split second is going to cause hammer. When talking to a buddy at Sprinkler World, he said it’s pretty common when a system is drawing off a hose bibb off the back of the house and not at tied it at the main supply since they are usually only 1/2 lines. He said you have a tendency to “over flow” the lines causing high velocity in the pipe.

1 Like

Ahhh. That is a term we call “Manifolding.” Commonly used in wet walls of public restrooms.

We use a smaller feeder pipe and at the end, a much larger pipe that can store the required flush volume.

1 Like