Short answer
Valley freezes are rare but real, and exposed brass backflow assemblies crack when water inside them freezes. Put an insulated cover or wrap over the assembly when forecasts drop below freezing. On hard-freeze nights, shut the irrigation supply and drain it per the manufacturer's instructions.
Why backflows break here
Irrigation backflow assemblies sit above ground, fully exposed. Homeowners rarely think about freezing in Phoenix, so an overnight low in the 20s can crack a body or bonnet that has never been protected.
Before a freeze
- Fit an insulated backflow bag or box cover
- Wrap exposed pipe on either side
- Know where the irrigation shutoff is
If it cracks
Close the irrigation shutoff valve upstream of the assembly. The house can usually keep running normally. A cracked assembly needs repair or replacement, and some cities require a new test after replacement.
When to call a plumber
- Water sprays from the assembly after a cold night.
- There's no shutoff you can find.
- Your city requires a test after repair.
What the plumber does: see backflow testing & repair.
Common questions
Should I cover hose bibs too?
Foam faucet covers are cheap and worth putting on exposed hose bibs on forecast freeze nights.
Editorial note: this guide is general information for Phoenix-area homeowners, not a diagnosis. Conditions vary by home, so a licensed plumber should confirm the cause before any repair.