Grant Erwin wrote:
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.

Water hammering is usually caused by suddenly shutting off a fast flow.
e.g. You open a valve, the water gets to flowing and, especially if
the run is long, there is a fair bunch of kinetic energy in the moving
water. Now you quickly close the valve and the incompressible water has
to stop NOW. So where does all the energy go?

My suggestion is that what is happening is during the off time the water
is draining out of the irrigation system. When it first turns on there
is very little resistance to flow - just compressing air in the lines
and pushing out the nozzles. When the water hits the nozzle(s) there is
a sudden dramatic reduction in flow. See above.

The clue that this is correct is that there is no water hammer when the
system switches to the second zone since everything has had a chance to
fill up.

Suggested cu Install a bypass valve that, at first turn on, provides
a low flow for long enough to fill up the lines then open the main valve.

Ted