Turn Your Sprinkler Controller Into A Smart Controller

Landscape contractors have a blunt name for the ordinary sprinkler controller in a garage: the clock. It turns valves on at set times for set minutes on set days, and it has no idea whether it rained last night or whether the week ahead will be cool and cloudy. According to the EPA’s WaterSense program, American homes use more than 9 billion gallons of water a day outdoors, mostly on landscapes, and as much as half of it is wasted through overwatering caused by inefficient irrigation methods and systems. The clock is a large part of the reason.

Why a timer wastes water

A clock wastes water in two ways. The first is the starting schedule. Whoever sets it up has to guess how long and how often each zone should run, and the guesses tend to be generous and uniform. Look at a typical schedule and the lawn, the shrubs and the flower beds often run for similar times, whether they sit in full sun or shade, on a slope or on flat ground, with shallow roots or deep ones.

The second is the weather. Plants use far more water in a hot, dry, windy week in July than in a mild one in April, and almost none on a rainy day. A clock waters the same either way unless someone remembers to change it.

Accessories help a little. A rain sensor shuts off a cycle after a set amount of rain, and many controllers have a seasonal adjustment that scales every run time up or down by a percentage. Both depend on the same rough baseline, and neither knows what the plants are actually using.

What makes a controller smart

A smart controller schedules irrigation from conditions on the site rather than from the calendar. There are two main approaches.

Weather-based controllers

These work from evapotranspiration, or ET: the water that evaporates from the soil plus the water plants give off through their leaves. Weather stations calculate a reference ET each day from temperature, humidity, sunshine and wind, describing what a standard patch of well-watered grass would use. The controller multiplies that by a factor for each zone’s plants, so turf gets close to the full figure and established shrubs much less, and subtracts effective rainfall. In effect, it keeps a running account of the water in each zone’s root zone and waters when that account falls to a set level, often half of the water the soil can hold for the plants.

The weather data can come from three places: historical averages for the area stored in the controller, a signal from nearby weather stations sent daily over the internet or a wireless network, or sensors mounted on the property itself.

Soil moisture-based controllers

These measure the soil instead of estimating it. A sensor buried in the root zone of a representative area reads the soil’s water content, and when it is still above the threshold set by the user, the controller skips the scheduled cycle. Some systems only bypass cycles; others start irrigation when the soil dries to the threshold.

Upgrading an existing system

There are two routes. One is to replace the controller outright with a smart model, usually a straightforward swap of the wires on the same terminals. The other is to keep the existing controller and add a device that works alongside it, either a soil moisture sensor wired into the controller’s sensor terminals or an add-on that takes over scheduling decisions. Either way, the system keeps its valves, pipes and heads.

The EPA has labeled weather-based controllers that meet its efficiency and performance criteria under the WaterSense program since 2011, and it estimates that replacing a standard clock timer with a labeled controller can save an average home nearly 7,600 gallons a year. Many water utilities offer rebates on labeled models.

Getting the setup right

A smart controller is only as good as what it is told. For each zone it needs the plant type, the soil, the slope, the sun exposure, the kind of sprinkler and its precipitation rate, and ideally the root depth. The precipitation rate is the one most often guessed, and a catch-can test of each zone measures it properly. On clay or slopes, set the controller to split each run into short cycles with a pause between them so the water soaks in instead of running off.

Fix the system before automating it. Broken heads, misaligned nozzles and leaking valves waste water whatever the controller decides. A wet check of every zone, done before installation, catches most of these faults.

Most home controllers run their valves on low-voltage 24-volt current from a transformer, but the transformer itself is fed by household power. Unplug or switch off the controller before working on its wiring. A controller mounted outdoors needs a weatherproof enclosure and a GFCI-protected outlet, and any new wiring to the house supply is work for a licensed electrician.

Since then, the EPA finalized a WaterSense specification for soil moisture-based controllers in February 2021, so both kinds can now carry the label.