Low Pressure in the Sprinkler System: Causes and How They Are Found

What does low sprinkler pressure look like?

Heads that do not pop up fully, streams that fall short of the next head, rotors that turn slowly or stall, and dry crescents at the edges of a zone. Rain Bird's troubleshooting guide for contractors calls low pressure one of the most common problems in irrigation systems, and notes that sprinklers that do not pop up fully lead to uneven watering. EPA WaterSense's Sprinkler Spruce-Up adds that heads that do not pop up fully or are tilted can spray the wrong areas.

The opposite problem is worth ruling out at the same time. Colorado State University Extension says a mist cloud around a spray head is a common symptom of excessive pressure, and EPA notes that leaking joints can signal too much pressure. Misting heads and weak heads call for different fixes.

Is it the whole system, one zone or one head?

That question does most of the diagnosis. If every zone is weak, look upstream: the supply, the main shutoff, the backflow assembly, the pressure regulator, or (on a well) the pump. If one zone is weak, look at that zone's valve, its pipe and its heads, and count how much it is asking for. If one head is weak, look at that head: a clogged screen or nozzle, debris in the stem, or a cracked riser below it. Rain Bird lists all of these, and the fix is quick once you know which level the problem lives at.

Could a valve be partly closed?

Often, especially in spring. Rain Bird lists valves that are shut or only partly open as the first cause of low pressure and advises confirming that valves are fully open. On a Montana system that includes the main shutoff (sometimes a stop-and-waste valve that CSU notes can sit about 5 feet underground and needs a long key), the ball valves on the backflow assembly, and the flow control on each zone valve.

The backflow valves are a common culprit after winter. CSU's winterization guide advises leaving the ball valves on a pressure vacuum breaker half-open for winter and setting an RP assembly's isolation valves at about 45 degrees. If nobody opens them fully in spring, the whole system runs on a partly closed valve. Our spring startup guide covers the correct sequence.

Are there too many heads on the zone?

This is the classic design cause. Every head uses a certain flow at a certain pressure, and Rain Bird's homeowner guidance on how many sprinklers a zone can run gives an example: at 35 psi, one of its rotors with a given nozzle uses 3.11 gallons per minute, so a home with 10 gallons per minute available could run three of them on a zone. Add a fourth and the pressure at every head drops. Rain Bird lists too many heads on one zone and undersized piping among the causes of low pressure.

Systems that were extended piecemeal are where we see this most: a new bed or lawn strip got a few heads tied into the nearest zone. The fix is splitting the zone, changing to lower-flow nozzles, or moving the bed to drip. Our zone design guide explains how zones should be sized.

How do you measure pressure and flow at home?

Rain Bird describes two simple tests. For pressure, attach a gauge to the faucet nearest the water meter, make sure no water is running inside or outside the house, open the faucet and read the psi; you can also ask your water provider. For flow, time how long a 5-gallon bucket takes to fill from a fully open faucet with nothing else running, and divide 300 by the seconds to get gallons per minute. Rain Bird also says that for well or pump systems, pressure and flow capacity should come from the well and pump dealer or the pump's manual.

Rain Bird recommends testing both static pressure (nothing running) and dynamic pressure (with the zone running). A big drop when the zone opens points to a zone that asks for more than the supply can give, or to a leak on that zone.

Could a leak be stealing the pressure?

Yes. Rain Bird lists leaking pipes or fittings among the causes and suggests looking for leaks and wet spots. A broken lateral or a cracked fitting lets water escape before it reaches the heads, so the heads past the break are weak while the ground near it stays soggy. EPA's spruce-up advises checking between cycles for water pooling on the surface, which can indicate an underground leak. Our irrigation leak signs guide covers what to look for.

Could the heads or screens be clogged?

On one or two heads, very likely. Rain Bird lists dirty filters or screens and advises cleaning nozzle filters, and lists debris inside the stem among the reasons heads do not pop up. CSU notes that rotary nozzles are somewhat prone to plugging with water that is not filtered properly. After a mainline repair or a winter, grit gets pushed into the nearest heads; flushing the line and cleaning the screens usually brings them back. For drip zones, Rain Bird's guidance is that pressure should be between 20 and 30 psi, and CSU recommends flushing the mainline and cleaning the filter before the first spring watering.

Can elevation cause low pressure?

Yes, on sloped lots. Utah State University Extension notes that pressure changes by 0.433 psi per foot of elevation, rising as elevation drops and falling as it rises, and CSU's drip guide suggests adding 5 psi for every 10 feet of rise above the connection point. On a hillside lot above the valley floor, heads at the top of a zone can run noticeably weaker than heads at the bottom. CSU's sprinkler guide adds that a grade change of more than eight vertical feet on a single zone causes distribution problems. The answer is zones split by elevation and, where needed, pressure-regulated heads, as our landscaping slopes guide discusses.

What if the pressure regulator or the well is the problem?

Rain Bird lists a failing pressure regulator among the causes of low pressure, and EPA advises confirming the regulator is installed properly. A regulator that has failed partly closed starves everything after it. On well-fed systems, the pump and well set the ceiling: a zone sized beyond what the pump delivers at working pressure will always be weak, and running irrigation while the house uses water makes it worse. Our well water irrigation guide explains how to design inside that budget.

What simple checks can you do yourself first?

Work from the top of the system down. Confirm the main supply and the backflow assembly's valves are fully open. Run one zone at a time from the controller's manual mode and watch every head on it, noting which ones are weak and whether they are clustered at the far end, the high end or near one spot. Walk the zone while it runs and again an hour later, looking for wet ground, bubbling or sunken soil. Pull the nozzle on a weak spray head and rinse its screen. Lift a rotor that will not rise and look for grit in the stem, which Rain Bird lists as a common reason heads stick.

Write down what you find. A pattern (one zone, the uphill heads, every head after a certain point) usually points straight at the cause, and it shortens a service call considerably if you do end up calling one in.

When is low pressure a sign to call someone?

When the simple checks (valves open, screens clean, no obvious wet spots) do not fix it, or when the problem is a zone that has always been weak. Those usually need a pressure test at the zone, a leak search or a zone redesign. We diagnose before replacing parts, as described on our sprinkler repair page, and new layouts are covered on our irrigation systems page. Head choices that help on low-pressure supplies are in our sprinkler head types guide. This guide is part of our Montana Landscaping, Lawn & Irrigation Guide. Call HJ Property Care & Tree Service at (406) 215-7319 or request a free estimate. We work across Missoula and Ravalli counties, from Missoula and Lolo to Florence, Stevensville, Corvallis, Victor, Hamilton and Darby.

More resources