Winterizing your irrigation system
Every year, before the first freeze, the ritual of irrigation "blow out" becomes the priority for all irrigation systems that are in regions of the country where the frost level extends below the depth of the installed piping.
Even if you have drained some water out of the system, the remaining water can freeze, expand and crack the PVC piping (rigid, white pipe), usually from fitting to fitting. Polyethylene pipe (flexible, black pipe) is used in many freezing climates. Although polyethylene pipe is more flexible and can expand under pressure, water left inside could freeze and rupture the pipe walls. Freezing water in the backflow assembly will damage the internal components and could possibly crack the brass body.
To minimize the risk of freeze damage to your irrigation system, you'll need to "winterize" your irrigation system. Irrigation systems in areas where "winterization" is mandatory are installed using one of three types of water removal in mind: manual drain, auto drain, or blowout. If you don't know your system type, it would be best to use the blowout method.
Outdoor mounted controllers?
Leave the power on and the dial / switch in the "Off" position. The heat from the transformer will keep the enclosure warm enough to keep condensation from forming inside the controller enclosure. The dial in the "OFF" will keep the controller from activating the solenoids in the field.
Indoor mounted controllers?
You may either leave the power on and the dial/switch in the "Off" position OR you may remove the battery backup and unplug the transformer.
There is not much to do to prepare the rain sensor for the winter months. If your sensor is the type with a cup or bowl that catches water, you might want to remove the water and place a plastic bag over the sensor. This will keep any water from accumulating and freezing in the cup or bowl area. If your sensor is the type that uses wafers or discs, you might want to remove the wafers and store them in the garage for the winter months. This will keep damp wafers from freezing.