The Blowout Window Is Shorter Than You Think

As of September 1, 2026. Standing guidance for irrigation winterization timing in Spokane and Eastern Washington.

The average first fall frost at Spokane International Airport lands around October 7, based on National Weather Service records for the area.[^1] Eastern Washington's National Oceanic and Atmospheric Administration climate normal for the region puts the 30%-probability first frost a couple of days earlier, around October 5.[^2] Either way, that's roughly five weeks out from the date on this article, and a facilities team that waits for a hard freeze to schedule its blowout is scheduling it for the week the freeze is already in the ground.

Key Takeaways

• Spokane's average first fall frost is early October, but a 30% probability of frost occurring earlier than that is built into the number. "Early October" is a median, not a guarantee.[^1][^2]
• Water expands roughly 9% in volume when it freezes. That's what splits a pipe wall or cracks a valve manifold: not the cold itself, but the ice with nowhere to go.[^4]
• A blowout has two limits that matter equally: pressure and volume. PVC pipe tolerates up to 80 PSI; polyethylene pipe should stay at or below 50 PSI. Exceed either and the failure is often underground, invisible until spring.[^3][^5]
• The compressor has to move enough air, not just enough pressure. A common sizing method is the largest zone's gallons-per-minute divided by 7.5, which gives the cubic-feet-per-minute (CFM) the compressor needs to deliver.[^3]
• The backflow preventer is the single most exposed component on the property: mounted above ground, holding water, and the first thing to crack when a blowout is rushed or skipped.[^3]

When Should an Eastern Washington Irrigation System Be Blown Out?

Before the first hard freeze reaches the ground, not after. In the Spokane area, that means scheduling for mid-to-late September, ahead of the early-October average. A frost date is a median outcome across many years, not a promise about this year. Any given fall has roughly a 3-in-10 chance of frost arriving earlier than the average date.[^1][^2] For a single-family yard, that's an inconvenience. For an 80-zone campus with a shared service contractor, it's the difference between getting a scheduled slot and getting an emergency one, if a slot is available at all once the first cold snap hits and every property in the region calls the same handful of contractors at once.

Definition (hard freeze): A hard freeze is a period of air temperature at or below roughly 28°F, cold enough and sustained enough to freeze standing water in exposed pipe and fittings, as opposed to a light frost that may only affect exposed vegetation.

What Actually Happens Inside a Pipe When Water Freezes?

Water increases in volume by about 9% as it turns to ice, and that expansion pushes outward against whatever is holding it (PVC pipe, polyethylene pipe, valve bodies, and the brass fittings inside a backflow preventer) until something gives.[^4] Rigid PVC laterals tend to split lengthwise. Polyethylene pipe, more flexible, can survive some expansion but will still rupture if enough water is trapped in a low point or a dead-end run. Backflow preventers are the most exposed component on most sites because they sit above grade, hold water by design, and take the season's first cold snap directly. A cracked backflow body isn't a spring inconvenience. It's a component replacement and, on a commercial site, often a code-required inspection before it can be put back into service.

[Founder's note, outstanding: a specific property Jon has walked where a missed low point, a rented compressor, or a skipped backflow drain caused a spring failure belongs here. Not yet supplied; see publisher's notes.]

What PSI and CFM Does a Blowout Actually Need?

Keep air pressure at or below 80 PSI on PVC pipe and at or below 50 PSI on polyethylene pipe, and size the compressor's air volume, not just its pressure, to the largest zone on the property.[^3][^5] Manufacturer guidance is consistent on both limits: exceeding 80 PSI on rigid pipe risks a sudden underground rupture, and exceeding roughly 50 PSI on flexible polyethylene pipe risks ballooning or splitting the pipe wall.[^3][^5]

Pressure alone doesn't clear a system; volume does. A commonly used sizing method divides the largest zone's gallons-per-minute (GPM) by 7.5 to get the required cubic-feet-per-minute (CFM) at the point of connection.[^3] A large commercial site with big-GPM zones needs a compressor that can sustain that flow, not a small unit that hits the right pressure gauge reading but runs out of volume two heads into a zone.

Table: The Fall Collision

The scheduling crunch and the frost date are close enough together that a property calling for a blowout in the first week of October is often calling into an already-booked contractor base, on a system that may already be exposed.

Already past the point of waiting? Request a Water Use Analysis and get the property on a scheduled blowout before the crunch hits, not during it.

What Does the Correct Blowout Sequence Look Like?

1. Shut off the water supply at the main and relieve standing line pressure before connecting any air source.
2. Connect the compressor downstream of the backflow preventer, at a dedicated blowout fitting or valve. Never blow air through the backflow assembly or a pump itself.[^3]
3. Confirm pipe material for the zone and set the regulator accordingly: 80 PSI ceiling for PVC, 50 PSI ceiling for polyethylene, and never above either regardless of pipe type.[^3][^5]
4. Open the farthest zone first and introduce air gradually rather than releasing a fully charged tank at once.[^3]
5. Run each zone in short cycles, long enough to clear water, short enough that dry plastic gear-driven heads aren't spinning without water to cool them, which can melt internal components.
6. Watch for the transition from spray to mist to dry air at each head before moving to the next zone; a head still spitting water isn't done.
7. Drain the backflow preventer last, after every zone has been cleared, not before, and not by blowing air through it.[^3]
8. Leave backflow test cocks open at roughly a 45-degree angle through the winter so any residual moisture has somewhere to drain, and set the controller to "rain" or "off" rather than powering it down, so the season's programming isn't lost.

The 30-Day Checklist

☐ Confirm pipe material (PVC vs. polyethylene) for every zone before scheduling the blowout ☐ Confirm the compressor's CFM rating against the property's largest zone, not just its PSI capability ☐ Schedule the blowout for mid-to-late September, ahead of the average first frost ☐ Confirm the backflow preventer's drain and test-cock locations before the crew arrives ☐ Set the controller to "rain" or "off," not powered down, to preserve programming ☐ Flag any zone with a known low point or dead-end run for a second blowout pass ☐ Calendar the spring startup date now, tied to the next budget cycle's service window

How Irrigation Managers Handles Winterization

Irrigation Managers runs winterization inside the same Analyze → Implement → Manage framework used on every managed site.[^brand] Analyze means the zone-by-zone pipe material, GPM, and low-point map are already documented before the crew shows up, not discovered on-site with a pressure gauge and a guess. Implement means the blowout sequence is executed to the pipe-rated PSI ceiling and sized to the correct CFM, zone by zone, with the backflow preventer drained last. Manage means the following spring's startup and the following fall's blowout are already on the calendar, tied to the property's budget cycle rather than to whichever week the weather turns cold.

Managed winterization is available across Irrigation Managers' service tiers, from Team Support & Guidance (as low as $1 per zone per month) through Full Irrigation Management with monitoring and reporting (as low as $5 per zone per month), scaled to sites of 50+ zones with a remote-capable controller.[^brand]

Request a Water Use Analysis →

FAQ

Why did my system freeze even though I have automatic drain valves? Automatic and manual drain valves rely on gravity and are only effective at the low points they're actually installed at. A system with unmapped low points, a valve installed slightly above the true low spot, or piping that was modified after the original drain design can trap water that a drain valve never sees. That's one reason a blowout, which forces air through the entire line rather than relying on gravity, is the more reliable method for commercial sites with complex zone layouts.

Can I just wait until the forecast shows a hard freeze and blow out then? That approach assumes a service slot will be available on short notice, which is often not true once the first cold snap of the season hits an entire region at once, and it assumes the freeze won't arrive earlier than forecast, which happens in roughly 3 of every 10 years, by definition of the 30%-probability frost date.[^1][^2]

Does polyethylene pipe need to be blown out if it's more flexible than PVC? Yes. Flexibility reduces the risk of a clean split but doesn't eliminate freeze damage. Trapped water can still balloon or rupture polyethylene pipe, particularly at fittings, and manufacturer guidance still caps blowout pressure at roughly 50 PSI for that pipe type specifically because it fails differently, not because it's immune.[^3][^5]

What's the single most common mistake in a DIY or rushed blowout? Using a compressor with enough pressure but not enough volume: hitting the PSI number on the gauge while moving too little air to actually clear a large zone, then stopping before the line is dry because the gauge looked right.

References

[^1]: KREM 2 (Spokane), citing National Weather Service Spokane office data, "Spokane weather: First frost of the season," reporting an average first frost date of October 7 for Spokane. https://www.krem.com/article/weather/spokane-area-first-frost-season/293-8096d32e-81fc-4e32-a620-aaf2e2203442 [^2]: Almanac.com, "Frost Dates for Spokane, WA," calculated from NOAA 1991–2020 climate normals at a 30% probability threshold. https://www.almanac.com/gardening/frostdates/WA/Spokane [^3]: Hunter Industries, "Winterizing Your Irrigation System" and "Valves - Winterization Procedures," manufacturer guidance on blowout PSI limits, CFM sizing method, and sequencing. https://www.hunterirrigation.com/winterizing-your-irrigation-system and https://www.hunterirrigation.com/en-metric/support/valves-winterization-procedures [^4]: Widely documented physical property of water: approximately 9% volumetric expansion upon freezing at standard pressure. [^5]: Multiple irrigation-industry and manufacturer sources confirm the same PSI ceilings independently of Hunter Industries (e.g., trade guidance on compressor selection for winterization contractors). https://www.vmacair.com/blog/air-compressors-for-landscapers-sprinkler-winterization [^brand]: Irrigation Managers LLC brand facts and service tiers, per company materials (irrmgmt.com), not independently re-verified for this draft.