How Does a Sprinkler System Work? Zones, Valves and Pressure

An irrigation system looks complicated and is conceptually simple: a timer opens one valve at a time. Everything else is plumbing.

The part worth understanding is why it is one at a time — because that single constraint explains zones, run times, and most of what goes wrong.

The parts

Controller (timer). The brain. Sends 24-volt signals telling each valve when to open and for how long.

Backflow preventer. Sits between your house supply and the irrigation, and stops water flowing back the wrong way.

Valve manifold. A cluster of valves, usually in a buried box. Each valve has a solenoid — a small electromagnet that, when energised, releases pressure above a rubber diaphragm, letting water push it open.

Lateral lines. Buried pipe from each valve to its heads.

Heads. Pop up under water pressure, spray, then retract when pressure drops.

Drip lines, in beds — emitters delivering water slowly at the root zone rather than through the air. This is the efficient option wherever you are not watering a lawn.

Why zones exist

Your house supply delivers roughly 10–15 gallons per minute. A single rotor head can use 3 GPM or more.

The arithmetic is unforgiving: four or five rotors is a zone. Twenty heads running together means every head gets a fraction of the pressure it needs, and none of them throw water where they were aimed.

So the system waters in batches. That is the entire reason for zoning, and it is also why a system takes an hour or more to run through.

The useful side effect: zones can be programmed differently. A shaded bed, a south-facing lawn and a row of established shrubs all want different amounts of water, and separate zones let you give it to them.

Backflow prevention, which is not optional

If supply pressure drops suddenly — a burst main, a fire hydrant opening — water can be siphoned backwards through the pipes. Irrigation water has been sitting in soil, among fertilizer and animal waste.

A backflow preventer physically prevents that reversal. It is required by plumbing code almost everywhere, and in many jurisdictions must be tested annually by a certified tester. If you inherited a system with the house, find out when it was last tested.

Setting run times

The goal is deep, infrequent watering — it drives roots down, and shallow daily watering does the opposite.

• Roughly 1 inch of water a week, including rain, for most lawns.
• Spray zones: ~15 minutes. Rotor zones: ~40–45 minutes. Same amount of water, very different clock.
• Early morning, 4–8 am. Evening watering leaves foliage wet overnight and invites disease.
• Cycle and soak on clay or slopes: two or three shorter runs with gaps, rather than one long run that mostly becomes runoff.

Common faults

Winterizing

Anywhere the ground freezes, water left in buried pipe expands and cracks fittings, valves and heads.

The standard method is a blowout: compressed air pushed through each zone in turn until only mist comes out. It is worth paying for, because too much air pressure damages exactly the components you are trying to protect — and the pressure limits are lower than most people assume.