Question
When a cycle is begining or when I manualy turn on a zone (at the controler or the zone valve itself)the watar hammer starts, and it keeps on until I shut it down or I turn off the main supply and turn it on again very gradualy. The water hammer seems to originate at the backflow preventer, and water is spiting out of the preventer relief with the frequency of the hammering. At times this will not occure at all.At times this will occure in the cycle at some time.At 95% of the time it occures.I have changed the BP and controle valves. This problem began early in the season and has increased in frequency since.
Please, old wise one, inlighten your student.   Greg M.

Answer
Hey Greg, I don't know how wise I am, but I'm definetely getting older, I hope this helps.  Usually when a backflow preventer starts to port water out the top, it means that the pressure going to it is low, and that is what it is designed to do. Why is the pressure low? It could be where the BP is installed. Is your BP installed on the mainline running from the meter to the house, or is it installed on a smaller line like a hose faucet coming off the side of the house. If it isn't installed on the mainline, you might need to move it to a new location. If it is installed on the mainline, but you are still having the problem, then you might have too many heads on one valve. One way to remedy this, is to add more valves, and take a few heads off of the other valves. This could get costly and take a lot of time. Your other option would be to change the heads or the nozzles on the existing heads, to a low volume nozzle. Let me explain how this works. Lets say you have an existing system that has 5 heads on it, and each of these heads have a 12f nozzle (12f means 12 foot radius and throw a 360 degree circle). A normal nozzle would use about 2 gallons per minute (GPM). So if we had 5 heads with 5 12f nozzles on them, when the valve goes on, it would use 10 gallons per minute (5 times 2 GPM=10 GPM)Maybe your system can only handle 6GPM. To remedy this, some manufacturers now offer lower volume nozzles, so the new 12f nozzle might only use 1GPM insted of 2GPM. This way if you changed all the old heads on this valve to the low volume nozzles, it would only use 5GPM instead of 10GPM, and now the system might be able to handle the lower volume of water going through the valves. Without knowing exactly how the system is set up, it is hard for me to tell you exactly what the problem is, but hopefully this will give you a starting point. Good luck and have a great day.