backflow device overflow

I have recently put in an outdoor sprinkler system in and when I turn on the zones I am getting water coming out of the backflow preventer.

This only happens when the valves are first open and it does not happen on every zone.

The device is more than the 12" above valves and sprinklers.

My water supply lines are 1/2" and I convert to 3/4". I have 1/2" coming out of the backflow and down the wall and convert to 3/4" at the bottom where I make the bend.

My sprinkler lines do run downhill (we have a downhill slope so I don't see how Icould have avioded this) and the lines themselves drain after shutting off the zone.

What is happening and how might I correct the problem.

I am concerned because I am getting knocking in the lines when this occurs.

bfp

The most likely reason is that the supply line is small and the irrigation lines are large. When the valve opens, the water in the supply line cannot start moving fast enough, but the water downstream "loses" its pressure immediately, for a second or so. That the backflow preventer momentarily has no pressure and its spring opens the port. The water hammer is caused when the water finally starts moving and hits a "brick wall" caused by the outlet pipe being full of water.

Lenichr,

Because your lateral lines are draining themselves is where the problem is occurring. Everytime a valve opens the mainline is trying to re-fill the empty lateral line. The pressure on the inlet side of the backflow is dropping so low, the poppet inside is not sealing, and letting the backflow leak which also compounds the problem.

The best way to fix the issue is to install drain check valves in the bottom of every head on the zones that are draining themselves. To be safe, you may want to do this to every head that you have. The checks are cheap (around a dollar), so see you local distributor. The checks install at the base of the head. When the head retracts back down after running, it will seal all the water in the lateral pipe. It is important that you do not get any dirt in the bottom of the heads when you install these or else they may not seal. It is also a good idea to re-edge the heads so they retract cleanly. Your mainline wont be trying to play catch up to refill the lateral piping. The heads will pop up faster, allowing for a quicker seal, and the backflow wont leak. This will also eliminate the banging in the house.

If the elevation change is over 10 ft in difference, you may want to try RainBird SAM heads (Seal-A-Matic). These have stronger retraction springs and will hold better.

I have seen this many times and "checking" the heads seems to solve the problem for the least amount of expense.

Good Luck,
RainMaker