Grant Erwin wrote:

I just ran into my old boss. He is now retired, and an avid gardener.
He put in a 12-zone automated watering system, and there is a problem
in it which is driving him bonkers. I'm posting this because you guys
know everything worth knowing.

His water system starts at the meter, then it runs to a tee, and off
of one leg goes his entire outside watering network. The other leg of
the tee goes to his house. Where the water enters his house, he has a
pressure reducing valve. On the outside watering leg, there is a
backflow prevention valve to ensure that even in the event of negative
city water pressure his gardening water can never wind up in his
neighbor's water glass.

At 4AM his watering system turns on. When the first zone valve cuts
on, the water pipes *in his house* begin hammering loudly. Eventually
it dies down and then the first zone valve cuts off and the second one
cuts on, no more problems. But the water hammering wakes him and his
family up every morning, and they worry about it damaging their house
piping because it sounds so violently loud.

I had water hammering at my old house, and it burst the hot water hose
to the clothes
washer twice, in the middle of the night. Lucky I was home both times
to turn the
valves off. (The hammering weakened the hoses, which popped at some
random time.)
I built air chambers above both the hot and cold valves that fed the
washing machine,
and never had a problem after that. The pipes also didn't rattle any
more when the washer cut
off. I got 1" copper pipe, and made the chambers reach from the valves
to near the ceiling.
Just a couple sweat reducers and a tee.

I would suggest he builds one of these from either copper or PVC pipe
and place it somewhere
along the line that feeds the irrigation system.

My guess is that the long pipes on the irrigation system make a
resonator, and a pressure
wave is reflecting back and forth on that pipe. These pressure waves
are hitting the pressure
regulator and making it blip little bursts of pressure into the home
branch of the pipes.
After the first valve cycles, it may be that the home lines have been
bumped up in
pressure enough that no more blips get through the regulator.

Jon