How often should you blow out your irrigation system?

Why bother

Water left in the lines splits them over winter. Standard upkeep. Dull and cheap, and a lot less dull than whatever it's quietly preventing.

What the job involves

1. Shut the water off to the system at the main irrigation valve, then turn the controller off or set it to 'rain' mode so it isn't trying to run a dry system all winter.
2. Sort the backflow preventer and the pump before you touch the compressor: drain the pump for its type and leave the isolation valves half-open, so any water that stays put has room to freeze without splitting the brass.
3. Hook up an air compressor and blow the zones out one at a time, starting with the one farthest from the compressor. Run each until the heads are spitting mist, not water. That's the zone cleared.
4. Keep the air at or below 80 PSI for rigid PVC and 50 PSI for flexible black poly pipe. Push it harder and you blow nozzles off and crack fittings, which is the exact damage you're out there to prevent. Wear eye protection and don't stand over the heads or valves.

Do it yourself, or pay someone?

Yourself, if you own or can rent a compressor with enough air (roughly 30-50 CFM, which a little pancake unit won't manage) and you hold under the PSI limits. Hiring it out runs about $60-$130, averaging $85-$91, plus $5-$10 a zone past a dozen or so. At that price, it's cheap insurance against one cracked pipe.

What skipping it costs

$150-$800 for a single freeze repair (a burst line or a split valve); a frozen backflow preventer, the priciest common casualty, runs about $300-$1,600 to replace; a bad winter that takes out the backflow plus several lines can reach roughly $1,000-$2,500. Set against a ~$60-$130 blowout, leaving water in the lines is the expensive bet. The water freezes, expands, and splits things open, and you don't find out until spring start-up, when the zones won't pressurize and the digging starts.

Signs it's overdue

• A hard freeze is in the forecast (around 28F overnight for a few hours), the lines still have water in them, and you haven't blown the system out this fall. That first cold snap is when the damage happens.
• At spring start-up a zone won't build pressure, heads dribble or won't pop up, or water bubbles up from the lawn over a buried lateral. Something split over winter.
• The backflow preventer above ground is cracked, weeping, or has a split brass body. That's the classic sign water sat in it through a freeze.

When to start thinking about it

Give yourself about 7 days' notice. That's when Grumpty starts grumbling, so it doesn't land on you the day it's due. Mark it done and Grumpty sets the next one, every 12 months, so you can go back to forgetting it exists.

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