The automatic drain valve or “auto drain” protects irrigation pipes from freezing in colder climates where that might be an issue. The auto drain has 1/2-in. or 3/4-in. male pipe threads (MPT) which thread into the outlet of a tee or elbow at the low point(s) of the piping for a zone. The auto drain has a seal which closes when the valve turns on and pressurizes the zone and opens when the valve turns off. This allows the line to be empty when the zone is not in use. However, this also means that the water from the zone— about 4 gallons in a 100-ft. section of pipe—will drain into the ground after each cycle, so you should only use drain valves where risk of pipes freezing during irrigation season is high or the system cannot otherwise be properly winterized at the end of the season. For outdoor irrigation use with cold water only.