Winterizing a backflow preventer
CSU Extension: backflow devices are damaged by freezing unless drained of all water; hose connection vacuum breakers are best removed and stored.
Freeze protection
CSU Extension advises removing and draining hose connection vacuum breakers for winter.
CSU Extension: PVB and RP devices must never be blown out with compressed air, since rubber seals can melt.
CSU Extension: exercise ball valves and test cocks, then leave them half open for winter.
CSU Extension: ball valves left fully open or fully closed tend to trap water that expands and causes damage.
CSU Extension spring step: close test cocks and ball valves before pressurizing, then open the valves slowly to avoid water hammer.
Slow opening prevents water hammer damage when the supply is turned back on.
CSU Extension winterizing begins with turning off the water supply to the irrigation system, often at a stop and waste valve.
Sources: Freezing damages undrained devices (Colorado State Univ. Extension); Never blow out a PVB or RP with compressed air (Colorado State Univ. Extension)