When to Blow Out Sprinklers in Colorado

By Colton Brackett, Lead Plumber · Reviewed by Shane Pettigrew, Service Manager · Updated August 2026

Why the backflow assembly sets the schedule

The backflow preventer sits between the drinking water supply and the irrigation lines, and it exists to stop irrigation water siphoning backward into the house supply. That matters because irrigation water sits in soil among fertiliser and whatever else is on the lawn.

It is mounted above ground, which makes it convenient to test and vulnerable to weather. The body is cast brass or bronze, and those materials do not flex. Water trapped inside expands as ice, and the casting cracks.

Replacing it costs several times what an annual blowout does, which is the entire economic argument for autumn service. Everything else in the system can be argued about; this component cannot.

Why gravity draining does not work

People reasonably assume that turning the water off and opening a drain will empty the system. On most layouts it will not. Lateral lines run level or dip between heads, so water sits in the low sections regardless of grade.

Valve bodies retain water in their chambers independently of the pipe around them. Drip zones hold water in filters and emitters. All of those freeze separately, and none of them drain because the pipe above them did.

Compressed air is what actually clears them, pushed through zone by zone until the heads discharge air rather than water.

What freezing actually does to a lateral line

Water expands by roughly nine percent when it freezes, and irrigation pipe is thin walled polyethylene rather than the heavier material used for supply plumbing. It splits readily.

The split rarely shows itself in winter. It reveals itself in spring when the system is charged, and because the pipe sits underground the water runs into soil rather than across a surface where anyone would notice. Bills climb for weeks before a dry patch appears on the lawn.

That delay is why the autumn visit is worth more than it looks. Preventing the split costs a fraction of finding it, and finding it in April costs a fraction of finding it in July.

Doing it at the right pressure

This is where a well intentioned attempt causes the damage it was meant to prevent. Too little air volume leaves water in the laterals, which defeats the purpose entirely. Too much pressure damages heads, splits fittings underground, and produces a repair bill in spring.

A typical shop compressor delivers pressure but rarely the volume needed to clear a residential lateral properly. Volume, not pressure, is what moves the water out.

Working zone by zone rather than opening everything at once is the other half of it. Air follows the path of least resistance, so an open system clears the nearest zone thoroughly and the far ones barely at all.

What spring startup should look like

Recharging matters as much as draining. Opening the supply valve fully on a cold system sends a pressure surge through pipe that has sat empty all winter, which finds every weakness at once.

Charging slowly, with the system observed zone by zone, is how freeze damage gets found in April rather than after a month of water running underground unnoticed. A split lateral loses water continuously and shows on the bill long before it shows on the lawn.

It is also when the backflow assembly gets tested, confirming it still does the job it exists for. Both halves of the year are covered on the blowout and testing page.

What a blowout visit actually looks like

The supply is isolated at the irrigation shutoff first, separating the system from the house. Air is then introduced at working pressure and each zone is opened in turn, run until the heads discharge air rather than water, and closed again.

The backflow assembly is drained separately with its test cocks left open, so nothing remains anywhere inside the body over winter. The controller is shut down or set to its off position at the same time.

On a residential system of six to ten zones the whole visit generally takes forty five minutes to an hour and a half. Larger systems take longer purely because each zone is cleared individually rather than all at once.

Why commercial properties are on a different footing

Backflow assemblies serving commercial premises carry stricter testing obligations than residential ones, and the requirement is about the drinking water supply rather than about the irrigation.

Keeping a documented test record matters in that context, because an untested assembly is simply an unknown quantity rather than a passed one. The test itself is short; the documentation is the point.

Larger commercial systems also tend to have more zones, longer laterals, and more valve bodies retaining water, which is why they are scheduled with more time rather than squeezed into a residential slot.

Booking it without leaving it too late

The window is narrower than it looks. Blow out in early September and you give up irrigation through a warm stretch that frequently follows. Leave it until the forecast turns and every irrigation contractor in the metro is booked solid for the same week.

Late September through mid October balances both risks, and booking two or three weeks ahead of when you want it done is the practical way to land inside that window rather than chasing it.

If a hard freeze arrives before the appointment, shutting off the irrigation supply and opening the backflow test cocks buys some protection in the meantime. It is not a substitute for the blowout, but it is better than leaving the assembly charged.

The other systems on the same clock

Anything holding water outside the heated envelope is on the same schedule. Hose bibs with hoses still attached are the most common freeze point on a Denver house, and disconnecting them costs nothing.

Evaporative coolers are the other one people forget. The supply line running to a roof mounted unit sits fully exposed, and water left in it splits the line at the first hard freeze. Nobody finds out until they turn it on in spring and water runs across a roof, which is covered on the swamp cooler page.

Booking all of it in the same window is simpler than remembering each one separately when the forecast turns.