Should You Drain Backflow Preventer Assemblies on Your Sprinkler System?

Yes—if you expect freezing temps or you’re shutting down your sprinkler system, you should drain backflow preventer assemblies. Draining prevents burst brass, costly leaks, and possible contamination. In warm weather and normal operation, don’t drain; keep them pressurized and tested annually. Local codes matter, too, so follow manufacturer guidance or hire a pro.

What you’ll learn from this blog

• When to drain a sprinkler backflow preventer (and when not to)
• How to winterize a backflow preventer step by step
• Common mistakes that crack valves in cold snaps
• Simple freeze-protection upgrades that save money
• When it’s smarter to call a pro like Turfrain

Start Here: The Quick Answer You Came For 

If frost is in the forecast or you’re winterizing your sprinkler system, drain your backflow preventer assembly. That includes popular lawn setups like PVBs (pressure vacuum breakers) and RPZ/RP assemblies. Draining removes trapped water that expands into ice—exactly what splits brass bodies and blows out seams. During the watering season in mild temps, keep it pressurized and schedule your annual test.

A quick gut-check for homeowners:

• Expect overnight freezes (32°F/0°C or lower)? Drain it.
• Shutting the system down for winter? Drain it.
• Doing repairs downstream? Often yes—depressurize and drain per the manual.
• Live where it rarely freezes? Insulate and monitor, but don’t drain mid-season.

Why Draining Matters (A 15-Minute Task vs. a $600 Surprise) 

Water expands about 9% when it freezes. In a sealed brass cavity—like the heart of your backflow preventer—that’s a recipe for cracks. I’ve seen a neighbor ignore a single cold snap in October and wake up to a hairline split that didn’t show until spring—cue the geyser and a soaked driveway. Beyond damage, a failed assembly can risk contamination entering your home’s potable water. So yes, winterize before the first hard freeze, not after the first expensive repair.

When Not to Drain (And What To Do Instead) 

There are plenty of times you should leave the valve alone:

• Active watering season in non-freezing temps: Keep it pressurized so it operates correctly.
• Mild climates with occasional chill: Use insulation and a freeze cover; add a heat cable for exposed piping; keep vents clear.
• During backflow testing: A certified tester will handle opening/closing valves properly.

Think of it this way: draining is for freeze protection or full shutdown. Otherwise, maintain, test annually, and protect it from sun, wind, and lawn equipment mishaps.

How to Drain a Backflow Preventer Safely (Simple, homeowner-friendly steps) 

Always check your specific assembly’s manual and local codes. If anything feels off, stop and call a pro—pressurized water can surprise you.

• Turn off the irrigation water supply. This is the shutoff valve upstream of the backflow preventer.
• Open a downstream zone valve or a test cock to relieve pressure.
• Open the test cocks on the backflow preventer. They’re the small screw ports—open them in order, top to bottom, to let air in and water out.
• For PVBs: Gently lift or open the vent cap (if serviceable) to ensure it drains. Don’t force anything.
• For RPZ/RP assemblies: Ensure the relief valve drains freely; it’s designed to discharge, so keep it clear.
• Crack open the downstream shutoff (if present) briefly to release trapped pockets, then return it to closed.
• Leave test cocks partially open for winter so any residual moisture can escape.
• Insulate exposed piping and the body (but never block the relief port on RPZs). Use a breathable cover designed for backflow preventers.

Bonus tips:

• Blowouts: If you use compressed air to clear sprinkler lines, use regulated, oil-free air and safe pressures—often 40–50 psi for residential systems. If you’re unsure, let a pro handle it. Over-pressurizing can ruin valves.
• Tag it: Add a little note with the date you winterized. Spring-you will thank fall-you.

Freeze Protection You’ll Actually Use 

Picture this: a $25 insulated cover vs. a crack that costs hundreds and delays your first mow-and-water weekend. Easy choice, right? Keep things simple:

• Wrap exposed pipes with foam insulation; tape seams closed.
• Add a backflow cover sized for your assembly. For RPZs, make sure the relief opening stays clear and not sealed.
• Consider a short “freeze bypass” setup with unions—so future winterizing takes five minutes flat.
• Use a smart controller’s freeze skip if you have one; it’s not a full solution, but it avoids running water during a cold snap.

Conclusion and a Friendly Nudge 

You don’t have to overthink it: drain your backflow preventer assemblies before freezing weather or during a full system shutdown; protect and test them the rest of the year. A few small moves now prevent cracked brass, surprise leaks, and water-quality headaches later. If you’d like a quick, done-right winterize or spring startup, Turfrain is here to help. Contact Us and we’ll treat your lawn—and your plumbing—the way we would our own.