Low Water Pressure in a Sprinkler System: Causes and Fixes

Low pressure is one of the most common sprinkler complaints. Heads barely rise out of the ground, spray falls short, rotors stall, and dry patches spread between heads. The cause can be as simple as a partly closed valve or as involved as a hidden leak. This article explains how to find out which one you are dealing with.

Start by Deciding: One Zone or All Zones?

Run each zone by itself from the controller and watch. This one step cuts the list of possible causes in half.

• If every zone is weak, the problem is in a shared part of the system: the water supply, the isolation valve, the backflow preventer or the mainline.
• If only one zone is weak, the problem is in that zone: its valve, its lateral pipe or its heads.

Causes When Every Zone Is Weak

A partly closed shutoff or backflow valve

The irrigation isolation valve and the two shutoff valves on the backflow preventer should be fully open during normal use. After winterizing or a repair, one of them is sometimes left partly closed. Check each handle. On most backflow assemblies, the handles are open when they run parallel to the pipe.

A problem at the backflow preventer

Debris lodged in a check valve, or internal damage from a freeze, can restrict flow through the whole system. If the assembly is also leaking or dripping, read our guide to a leaking backflow preventer. Repairs to backflow assemblies in Fort Worth must be followed by a test from a registered tester.

A mainline leak

A break in the mainline lets water escape before it reaches the valves, so every zone loses pressure. You may see a wet spot that never dries or a meter that keeps moving when everything is off. Our guide to using your water meter to find leaks shows how to confirm it.

City supply pressure

Water pressure at the street can vary by location and time of day. Homes at higher elevation or at the end of a long line may see lower pressure. If your pressure has always been on the low side, the system may need fewer heads per zone or more efficient nozzles, not a repair.

Causes When Only One Zone Is Weak

Too many heads on the zone

Over the years, heads get added to fill dry spots. Each one takes a share of the flow. Eventually the zone asks for more water than the pipe and valve can deliver, and every head on it runs weak. Splitting the zone or switching to lower-flow nozzles usually solves it.

A lateral line leak

A crack or broken fitting on the zone’s pipe lets water escape before it reaches the heads. Look for a soggy patch, a small fountain or bubbling in the grass while the zone runs. Our article on broken sprinkler pipe repair explains how lateral repairs work.

A broken or missing head

One broken head can drain pressure from the rest of the zone. When a riser snaps off, water pours out of the open fitting and the other heads on the zone may barely rise.

The valve’s flow control is turned down

Many zone valves have a flow control knob or handle on top. If it has been turned down, that zone will run weak. Open the valve box and turn the flow control counterclockwise to open it.

A failing valve diaphragm

A torn diaphragm or debris in the valve can keep it from opening fully. Our guide to sprinkler valve repair covers how to inspect and clean a valve.

Causes at the Head Level

Sometimes only a few heads are weak. That usually points to clogged nozzles or filter screens, sand and grit in the line after a repair, or a head that is buried too deep and cannot pop up fully. Pull the nozzle and screen, rinse them, and try again. If the head sits below the grass, raise it. Our guide to sprinkler head repair covers these fixes.

Mixing Heads and Nozzles

Spray heads and rotors put out water at very different rates. When they share a zone, the spray heads take more of the available flow, and the rotors can stall or cover poorly. Mixing nozzle brands can cause a similar problem. Keeping each zone to one type of head, with matched nozzles, makes pressure more even.

Check Pressure Over Time

Pressure problems sometimes come and go. If your zones run fine some mornings and weak on others, note the day and time. Heavy water use in the neighborhood early on watering days can lower pressure for everyone on the line. If your weak zones line up with busy times, try moving your start time a little earlier or later within the allowed hours. If the zones are weak every time, the cause is inside your system.

What Not to Do

It is tempting to fix a weak zone by adding more heads, but that usually makes it worse. Opening the flow control on a zone that is already fully open does nothing. Running the zone longer to make up for poor coverage wastes water and still leaves dry spots. Find the cause first.

When to Call a Technician

If you have checked the valves, cleaned the nozzles and still have low pressure, the cause is likely a hidden leak, a failing valve, an undersized zone or a backflow problem. Our sprinkler repair service can test pressure at different points in the system, find the restriction and fix it.

If low pressure is only one of several problems you are seeing, our guide to the most common sprinkler problems can help you sort them out.