However, should the temperature fall below 15 degrees F for over a period of 4 consecutive hours, then there is a potential for the water to freeze in the main sprinkler line, which could cause the line, made of PVC (poly vinyl chloride), to crack. In the Houston area we don't usually have this problem, but we want you to be prepared in the event of a hard freeze.
To protect your automatic underground sprinkler system, you will need to block the supply of water to the sprinkler system's main header (pipe). This valve should be located near your water meter and is red (refer to figure 1 below).
allows you to block the supply of water to the main water sprinkler
header, which is usually water-full. By blocking this valve, even if
the freezing temperatures reach the main supply header (PVC pipe), the
pipe will not crack because it will not be water-full. Note: The other
lines that tie-in to this main supply line only contain water when your
system is running, so they are safe at all times. Refer to diagram below
(figure 2) to see overall layout.