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.

They have had the water department guys out to check his pressure reducing
valve, his backflow prevention valve, and the water pressure everywhere, and
they say everything checks out OK. He has had two different irrigation
consultants come out and other than suggesting things like reprogramming to try
a different zone valve to come on first (didn't help) they were similarly unable
to come at the root cause of the problem, nor could they suggest a workaround.

I don't really understand the phenomenon of water hammering, but I do understand
that it is a pressure oscillation which is characteristic of an underdamped
mechanical system. I suggested that he try adding resistance (some kind of flow
reducer, maybe a gate valve) or capacitance (one of those bulb thingys) but he
is on fire to find the actual root cause and solve it, not just find a
workaround. I suggested he try shutting the gate valve, the main water shutoff
valve to his house, about 95% tonight to see if the added resistance might do
the trick.

Anyone got any bright ideas? This guy is no dummy; he was an engineering manager
and has a EE degree from Rensellaer Polytechnic ..

Grant Erwin
Kirkland, Washington