How to Winterize a Sprinkler System

Find the controller, usually in a garage or mounted on an outside wall, and switch it to off. Not a rain delay, not a skipped cycle. Off.

If the system tries to run once the water is shut off, you gain nothing and the zone valves spend the winter cycling against no pressure.

Some controllers have a dedicated off position on the dial. If yours does not, pull the power.

If the sprinklers run off your house supply, there is a curb stop somewhere in the lawn under a small capped hole, often near the sidewalk.

Lift the cap, drop a shutoff key down the hole until it seats on the valve, and turn clockwise until it stops. Put the cap straight back on so you can find it again in spring.

On a secondary or shared irrigation supply the shutoff will be somewhere else entirely. Find it before you start rather than halfway through.

The backflow preventer is the brass assembly standing above ground on the supply line. It is often behind a shrub or, as here, hidden under a fake rock.

Everything else in this job happens at this one fitting, so pull the cover off, clear whatever has grown around it over the summer, and give yourself room to get two hands in.

If you cannot find it, follow the pipe up out of the ground nearest your water meter.

Close the incoming shutoff on the backflow preventer itself. The water is already off at the street, and closing it here as well means you are working on a properly isolated section.

Leave the downstream side, the one facing the sprinkler system, open. That is the direction the air has to travel, and closing it is the most common reason a blow-out does nothing at all.

Lift the lid on the valve box. There may be one box or several scattered around the garden.

Inside are the zone valves, each feeding one line of sprinkler heads. Work out which valve is which zone now, while there is no pressure in the system, because you are going to open them one at a time and you do not want to be guessing with a running compressor.

Opening each valve briefly at this stage also bleeds off any pressure still sitting in the lines.

Put the compressor as close to the backflow preventer as the power will allow. Near an outlet saves running an extension cord.

Switch it on and wait. Filling the tank is the longest part of this whole job, and a small tank means both more waiting and more repeat passes per zone.

Pressure matters more than anything else here. Stay in the 40 to 80 psi range. 80 psi is the ceiling for rigid PVC pipe and 50 psi for polyethylene. Higher pressure does not clear the lines any better, it just splits fittings and blows seals out of the heads.

The backflow preventer has small test valves that sit closed by default. If you leave them closed, the air has nowhere to go and the compressor does nothing.

Put a flathead screwdriver in the slot and turn it until it lies parallel with the pipe. Parallel is open, across is shut. Go all the way, not most of the way.

The adapter is a short brass nipple, threaded on one end to match the backflow preventer and a quick-connect on the other to take the compressor hose. It costs a few dollars and there is no substitute for it.

Thread it onto the fitting hand tight. It does not need a wrench and overtightening brass into brass is how threads get stripped.

Then pull the collar back on the air hose quick-connect, push it onto the nipple, and let go. It clicks and locks.

Open one zone valve and only one, then send the air through.

The compressed air pushes the water out of that line and you will see it coming out of the heads as spray. Watch until the spray turns to a dry mist and then stops.

No more water does not mean the line is empty. On a small tank, do every zone at least twice, letting the compressor refill in between. When a zone runs dry, close it, open the next one, and work through every zone in the system.

Never run a zone with all the valves open. The air takes the path of least resistance and one line gets cleared while the rest stay full.

Do not close everything back up. Set the test valves on the backflow preventer to about halfway, roughly 45 degrees, and take the small rubber caps off the bleed ports.

Trapped water expands when it freezes and cracks brass, and leaving the assembly open gives any last drops somewhere to go and lets air move through. This is the step people skip and it is the one that costs them a backflow preventer.

Put the valve box lid back on, slide an insulated bag over the backflow preventer, and put a foam cover on every outdoor faucet once you have taken the hoses off. A hose left connected over winter is enough on its own to freeze and split the line behind it.