Why Is My Irrigation System Losing Pressure?
Low pressure can mean a leak, undersized pipe, a failing PRV, or something as simple as a clogged filter. Here's how to diagnose each cause — systematically, zone by zone.
Recognizing a Pressure Problem
Low pressure doesn't always announce itself with puddles or obviously broken heads. The symptoms are often subtle: a rotor that stops mid-arc and just jets in one direction, spray heads that only pop up halfway, drip zones that wet a two-foot circle around the emitter instead of the plant root zone, or a section of lawn that's always a shade more yellow than the rest.
Before you start digging, confirm you actually have a pressure problem — not a coverage problem caused by improper head spacing or a programming issue. The fastest way to confirm low pressure is with a pressure gauge on the hose bib. If your static reading has dropped more than 5–10 PSI from the last time you checked, or if it drops more than 20 PSI when a zone runs, something has changed.
The Six Most Common Causes
Most residential pressure loss traces to one of six causes. They look similar on the surface but have very different fixes. Work through them in order — each check eliminates one possible cause before you move on.
Underground leak
A cracked pipe or failed fitting bleeds pressure away before it reaches the heads. Leaks under pressure show up as wet spots or soft ground over the pipe run, even when no zones are scheduled to run. Turn on a zone manually and walk the entire pipe run looking for bubbling, pooling, or unusually saturated ground. A zone that shows 10+ PSI less than adjacent zones almost always has a leak.
Partially closed or failing zone valve
Zone valves are solenoid-operated and have a manual bleed screw. If the solenoid is sticking, the valve isn't opening fully — it restricts flow just like a half-open faucet. To test: manually open each valve's bleed screw and check if the zone's pressure (gauged at the end head) improves. If it does, the solenoid is the problem, not the pipe. You can also check solenoid resistance with a multimeter — most solenoids read 20–60 ohms; anything outside that range means replacement.
Clogged inline filter or backflow preventer screen
Every properly installed system has a filter upstream of the zone valves, and backflow preventers have built-in screens. These catch debris from the main supply line — sediment, pipe scale, small grit. Over time they restrict flow. If your pressure has declined gradually over one or two seasons with no leaks or valve issues, a clogged filter is the likely culprit. Locate the filter housing (typically near the backflow preventer), disassemble it, and inspect the mesh screen.
Undersized mainline or lateral pipe
Friction loss in undersized pipe is proportional to velocity — as GPM increases, pressure drop increases steeply. If you or a previous owner added zones or heads to an existing system without upsizing the pipe, you may have too much flow for the pipe diameter. This shows up as pressure that's fine when running one zone but drops significantly when high-GPM zones run. Confirm by running your highest-demand zone and measuring pressure at the farthest head vs. the valve.
Pressure Reducing Valve (PRV) set too low or failing
If your home has a PRV (usually a bell-shaped fitting on the main supply line near the meter), it limits pressure to the house. PRVs are set at the factory to 50–60 PSI but can be adjusted, and they wear out after 7–12 years. A failing PRV diaphragm lets the set pressure drop well below spec. Check by reading static pressure at the hose bib. If it's consistently under 40 PSI and your supply from the street is higher (ask your utility), the PRV is restricting flow.
Clogged or worn sprinkler heads
A single clogged rotor or spray nozzle won't lower system pressure — but it will look like a pressure problem at that head. Debris (sand, grass clippings, insects) can pack into the nozzle and reduce throw distance or stop rotation entirely. Pull the head out of the ground, unscrew the nozzle, and rinse it under running water. If the filter screen at the base of the head is compacted with sediment, soak it in a cup of water for 5 minutes.
Quick Diagnosis by Symptom
| What You See | Most Likely Cause | First Test |
|---|---|---|
| All zones weak; static pressure low | PRV set wrong, utility supply issue | Check static PSI at meter vs. hose bib |
| One zone weak; others normal | Leak, valve not opening, undersized lateral | Walk the zone's pipe run for wet spots |
| Pressure drops when zone activates | Leak, too many heads, undersized pipe | Gauge at hose bib during zone run |
| Pressure declined gradually over seasons | Clogged filter, failing PRV | Remove and inspect inline filter screen |
| One head has no throw; others fine | Clogged nozzle, broken head | Pull head, remove nozzle, inspect and rinse |
| Wet ground when system is off | Underground leak or valve not closing | Check valve manual bleed; walk pipe run |
Running a Zone-by-Zone Pressure Test
The most reliable diagnostic method is a zone-by-zone pressure comparison. Screw your pressure gauge onto the hose bib nearest to your system's backflow preventer. With all zones off, record static pressure. Then activate each zone one at a time and record the pressure drop on the gauge.
A healthy system typically drops 5–12 PSI from static when a zone runs. If one zone drops 20+ PSI while others drop 8–10 PSI, that zone has a restriction — most likely a leak, a partially closed valve, or undersized lateral. If all zones cause a large pressure drop, the problem is upstream of the zone valves: the mainline, the backflow preventer, or the PRV.
Keep a log of these readings over seasons. A slow, year-over-year decline in dynamic pressure usually points to a maturing clog in the filter or PRV. A sudden drop that appeared this season points to a new leak or valve failure.
The Elevation Factor
If your property slopes significantly, heads at higher elevations will always see less pressure than heads at lower elevations — by 0.43 PSI per foot of vertical rise. This isn't a system problem; it's physics. A zone that climbs 20 feet up a slope loses nearly 9 PSI at the top heads compared to the bottom heads.
The fix is not to increase overall system pressure — that over-pressurizes the low heads. Instead, zone uphill and downhill areas separately, or use pressure-compensating heads that maintain consistent output across a range of inlet pressures. Hunter and Rain Bird both make pressure-regulating rotor stems that adjust the head's internal pressure, evening out coverage on slopes without affecting the rest of the zone.
- Do a full pressure test at the start of every irrigation season — before you need the system. Leaks that developed over winter are much cheaper to fix before the first scheduled run.
- If you're on a municipal water supply, call your utility and ask for the pressure zone data for your address. Municipal pressure varies by neighborhood and time of day, and low city pressure is not something you can fix on your side of the meter.
- A water hammer (bang when zones shut off) indicates pressure spikes that stress fittings and valves over time. Install a water hammer arrester near the control valves if you hear this.
- When testing for leaks, turn off the system and watch your water meter for 30 minutes. If the meter keeps moving with everything shut off, there's a leak somewhere — not necessarily in the irrigation system.
- Pressure-compensating drip emitters mask zone pressure problems by maintaining rated flow across 10–50 PSI. If you're troubleshooting a drip zone, swap to non-compensating emitters temporarily to see if pressure is actually low.
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