Short answer
Exposed brass backflow assemblies crack when water inside them freezes. In cold climates, shut off the irrigation supply and drain or blow out the system each fall. In warm climates, put an insulated cover over the assembly when a freeze is forecast, and on hard-freeze nights shut the supply and drain it per the manufacturer's instructions.
Why backflows break
Irrigation backflow assemblies sit above ground, fully exposed. In the North they're winterized every fall as routine. The bigger risk is in warm-winter places like Phoenix, Texas and Florida, where homeowners rarely think about freezing, so a single night 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 homeowners, not a diagnosis. Conditions vary by home, so a licensed plumber should confirm the cause before any repair.