Every fall, somebody in the Treasure Valley finds a cracked backflow valve in April. It almost always comes down to timing: the system was blown out too late, or not at all because the irrigation water had already been shut off.
The short answer
Blow out your sprinklers after your irrigation water is turned off and before the first hard freeze. In Boise, Meridian and Nampa, that usually means mid-October into early November.
A light frost won't hurt buried pipes. What does the damage is a hard freeze — several hours in the mid-20s or below — that reaches the parts of the system above ground: the backflow preventer, exposed risers and the valve box lids.
"The district shut the water off. Am I done?"
No. This is the most common mistake we see.
Most subdivisions here water from pressurized irrigation, and the district shuts it off in October. That stops new water coming into your system. It doesn't take out the water already sitting in the lines, valves and backflow — and that's the water that freezes, expands and splits fittings.
A blowout pushes compressed air through each zone, one at a time, until only mist comes out of the heads. Then the backflow valves are opened to drain and left that way for the winter.
What if a freeze is coming and you haven't booked yet?
If a hard freeze is in the forecast before we can get to you:
- Turn off the water to the system at the main shutoff.
- Open the drain valve or test cocks on the backflow preventer so it can empty.
- Wrap the backflow in an old towel or an insulated cover.
That buys you a few days. It isn't a substitute for a blowout, because water still sits in the low spots of every lateral line.
Why not do it yourself?
You can, with a big enough compressor. A pancake compressor from the garage doesn't move enough air to clear a zone, so it pushes water around without getting it out. Too much pressure, on the other hand, damages heads and fittings. Commercial tow-behind compressors deliver a large volume at a controlled pressure, which is what clears a line without hurting it.
Book early
The calendar fills fast the week the first hard freeze shows up in the forecast. If you book in September, you get a slot before the rush. Book a blowout, or get a ballpark price in two minutes.