When: before the first hard freeze. In the Puget Sound lowlands that usually means late October to early November, earlier on higher ground or in outlying areas.
Why freezing is such a big deal
Water expands as it freezes. Inside a backflow assembly (especially an above-ground pressure vacuum breaker) that expansion cracks brass bodies, bonnets and test cocks. The damage often isn’t obvious until you turn the water back on in spring and it sprays everywhere. A cracked body usually means a new assembly, which costs far more than ten minutes of fall prep.
Step-by-step: draining your assembly
These steps fit the most common residential setups: a pressure vacuum breaker (PVB) or double check valve assembly (DCVA) serving a lawn-sprinkler system.
- Turn off the irrigation controller or set it to “off/rain” mode so it doesn’t try to run.
- Close the irrigation isolation valve. This is the valve where the sprinkler line tees off your main water line, often in the basement, crawlspace, garage or a valve box near the meter.
- Open the drain valve downstream of that isolation valve if your system has one, and let the line empty.
- Open the test cocks on the assembly with a flat screwdriver (a quarter turn) to let trapped water drain out.
- Set the assembly’s shut-off valves to half-open (45°). This lets water drain out of the ball-valve cavities, where it would otherwise freeze.
- Insulate the assembly with an insulated backflow cover or pouch, especially if it’s above ground. Don’t block the air inlet or relief port.
About air “blowouts”: many irrigation systems are cleared with compressed air in the fall. That should be done by a professional, downstream of the backflow assembly. Blowing high-pressure air through the assembly itself can damage the check valves and seals.
Spring start-up
- Close the test cocks and set the shut-off valves fully closed.
- Close any drain valves you opened.
- Open the irrigation isolation valve slowly to repressurize gently.
- Open the assembly’s shut-off valves slowly, #1 (upstream) first, then #2.
- Look and listen for leaks at the body, bonnet and test cocks.
- Schedule your annual test. Spring start-up is a great time for it.
Signs of freeze damage
- Water spraying or weeping from the bonnet, body or test cocks
- A PVB that spits water continuously from the top
- Visible cracks, bulges or a lifted bonnet
- Low pressure at the sprinklers after start-up
If you see any of these, shut the isolation valve and book a repair. Any repaired or replaced assembly needs a new test before it goes back into service.
Frequently asked questions
Do I have to winterize if I live in Seattle or on the coast?
Yes. Even in mild areas, overnight lows below freezing happen most winters, and exposed assemblies are vulnerable. A few minutes of prep is cheap insurance.
Can you winterize it for me?
Yes. We can drain and insulate your assembly, and many customers pair it with their annual test. See our test + winterize visit.
Does winterizing count as a backflow test?
No. Winterizing protects the assembly from freezing, but your purveyor still needs an annual test report from a certified tester.