Do I Need Every Drop Out? (How Dry is “Dry Enough”)

Your driveway sprays (a realistic example)

You probably have Rain Bird 1800-series sprays along the driveway—corners with 90° nozzles, straights with 180° halves, all around 10′ throw at ~30 PSI. Typical flows at 30 PSI are ~0.39 GPM (quarter), ~0.79 GPM (half), and ~1.58–2.10 GPM (full), depending on model. Mix six sprays on one zone and you’re watering around 6–9 GPM. During a blowout we don’t want to over-pressurize those—just run enough air volume to sweep the water until it’s all mist. :contentReference[oaicite:1]{index=1}

Spray nozzles are happiest near their rated pressure (often ~30 PSI for 10′ patterns). Cranking way above that turns water to fog and can pop nozzles—so we don’t. :contentReference[oaicite:2]{index=2}

What must be dry (and what can be merely damp)

Why? When water freezes, it expands ~9%. Big slugs in rigid spaces do damage; a film doesn’t. :contentReference[oaicite:5]{index=5}

Why “CFM beats PSI” for getting to mist (not myths)

Extensions and manufacturer guides agree: use modest pressures and enough air volume (CFM) to create a moving front that pushes water all the way to the nozzles. With too little CFM, air rides over water and leaves pockets that slide back to low points when you stop—exactly where freezes begin. :contentReference[oaicite:6]{index=6}

Safe ranges (from published guidance): generally ~40–80 PSI max for blowouts; keep toward the low end on polyethylene laterals and never exceed the lowest-rated component in the zone. I aim around 50–60 PSI and let the airflow do the work. :contentReference[oaicite:7]{index=7}

Pipe volume math you can picture

Now visualize the driveway zone again: maybe 100′ of 1″ lateral (≈4.3–4.5 gal + risers + heads). We don’t “boil the ocean” with PSI—we sweep that volume to the nozzles and finish on mist. That’s what high CFM at modest PSI does well.

Backflow + shutoff: the sneaky freeze pocket

Close the #1 shutoff and walk away, and the short stub between the shutoff and the backflow can sit full. That little slug is outdoors, chills fast, and can split things. I open the test cocks to vent the backflow body, and I leave the drain plug off or partially threaded so if your shutoff weeps, it drips out instead of refilling the line. Check that area again in 24–72 hours. If it’s still dripping, the shutoff likely needs replacing. :contentReference[oaicite:8]{index=8}

After I leave: what you should see (& what a drip means)

• Mist, not slugs at the heads in the last pass.
• Backflow test cocks left open; shell can’t hold water. :contentReference[oaicite:9]{index=9}
• Drain area dries within 1–3 days. If not, the main shutoff is weeping—plan a replacement.
• You’ll find a paper Winterizing Report with anything I noticed for spring (pre-existing cracks, stubborn cans, timer quirks). Not every issue shows during air—spring is for final testing.

Real-world stories (with a grin)

Every fall I see the same scene: a brave homeowner, a tiny plug-in compressor, and a line of tall sprays daring them to “send it.” A week later, the little pancake sits at the curb with a hand-written “FREE” sign—smelled a bit… toasty. Meanwhile, Home Depot stays in the black on this zero-sum experiment.

Look—I respect the DIY spirit. If a zone is short and simple, you can sometimes limp to mist with a small unit and patience. But those tall spray bodies and check-valve heads hold water deep in the can. Without steady airflow, you get one head hissing while the others drop; pockets wait out winter and pop the body from the inside. Nothing ruins a spring Saturday like digging a cracked 12″ can out of rooty soil next to the driveway.

FAQ: drains, controller, timing, and “almost winter” panic

Exactly how I run a zone: bulk → mix → mist

1. Bulk displacement: Start farthest zone first at around 50–60 PSI with high CFM. We’re pushing a slug—steady airflow keeps it moving.
2. Mixed phase: It spits water + air. Keep heads extended so check-valves purge; don’t chase high PSI.
3. Mist phase: When every head is on a clean mist (no slugs), stop. Short, cool cycles prevent heat on dry parts—especially late season. :contentReference[oaicite:12]{index=12}