Pressure loss when irrigation starts

Intermittent pressure event: pressure loss when irrigation starts

Quick Answer:

When your irrigation starts and water pressure falls instantly, first treat it as a system event, not a single fixture problem. Install a pressure gauge (ideally one with a max-needle), watch pressure while the irrigation cycles, and isolate whether the loss happens before or after your home shutoff or PRV. Use timed checks to correlate drops with the sprinkler start, storms, or PRV activity, then act on the specific cause.

Why This Happens

  • Irrigation systems draw a large, sudden flow. That increased demand can drop system pressure if supply, piping, or control devices are limited.
  • A pressure-reducing valve (PRV) that is worn or set near its limit can respond poorly to the surge, cycling or dropping pressure.
  • Upstream supply issues, a partially closed meter, or a leak on the supply side will show the same symptoms when irrigation runs.
  • Weather events and utility service changes can also cause temporary shifts in supply pressure that line up with irrigation starts.

Step-by-Step What to Do

1. Put a gauge on the system and record a baseline

  • Install a pressure gauge at the house main near the shutoff. Use one with a max-needle so it captures peak readings and recent extremes even if you miss the exact moment.
  • Record the resting pressure (no irrigation) and again while a zone runs. Note the lowest pressure reached and how quickly it recovers.

2. Correlate drops to specific triggers

  • Run the irrigation and watch the gauge. Note whether drops happen exactly when sprinklers start, during storms, or during certain PRV cycles.
  • Keep a short log: time of start, zone number, lowest pressure, and recovery time. This helps separate irrigation-caused drops from supply or weather issues.
  • If you see brief failures after power or service interruptions, compare your notes to patterns described in Intermittent pressure after outages.

3. Isolate upstream vs downstream causes

  • Install or move a second gauge upstream of the meter/PRV (if safe and allowed) or ask your water utility to check supply pressure. If the upstream gauge stays steady while the house side drops, the issue is downstream (inside your property).
  • If both sides drop together, the problem is upstream—contact the water utility.
  • Shut off the irrigation backflow or main irrigation valve and run a faucet. If pressure returns, the irrigation system or its controller/valves are causing the draw.

4. Check the PRV and irrigation equipment

  • Observe whether the PRV is cycling or dropping pressure when zones change. Use a gauge installed on the downstream side while monitoring the PRV. Patterns of repeated cuts suggest PRV interaction; see notes on PRVs and testing in Pressure cuts when PRV cycles.
  • Inspect irrigation valves, a stuck zone, or a leaking lateral line that could suddenly increase flow.

5. Narrow down and act

  • If the irrigation system is the cause, repair leaks, fix stuck valves, or adjust the controller so fewer zones run at once.
  • If the PRV is failing, a professional can test and replace or reset it to correct settings.
  • If the supply is weak, contact the utility with your gauge readings and log to request an investigation.

What Not to Do

  • Do not assume intermittent loss is a fixture issue—track system-wide behavior first.
  • Do not replace fixtures or appliances before confirming where the pressure drop is happening.
  • Do not bypass backflow devices or alter the irrigation controller wiring without knowing the consequences.
  • Do not dig near the meter or PRV without locating underground utilities and checking local rules for who can work on the meter.

When to Call a Professional

  • Call a plumber or irrigation technician if you cannot isolate the cause after the basic steps, if a PRV needs adjustment or replacement, or if you find a leak you cannot access.
  • Contact the water utility when upstream pressure is low or if neighbors are affected—supply problems are the utility’s responsibility.
  • Hire a licensed pro for any work on the meter or for complex irrigation repairs beyond simple valve changes.

Safety Notes

  • Turn off irrigation power before touching controllers, valves, or wiring to avoid electrical shock.
  • When installing gauges, use tools correctly and follow manufacturer instructions to avoid damaging the meter, PRV, or piping.
  • Call utility locate services and respect local rules before digging. Damaging underground lines is dangerous and costly.
  • If you smell gas or see exposed electrical hazards while inspecting, leave the area and call the appropriate emergency services.

Common Homeowner Questions

  • Why does pressure drop only when sprinklers run?
    Because irrigation draws a large flow; limited supply, leaks, or a restrictive device (PRV) will show a pressure drop under load.
  • Can I test this myself?
    Yes—install a gauge with a max-needle, log readings while irrigation runs, and isolate irrigation valves. Call pros if the cause is unclear or involves the meter/PRV.
  • How quickly should pressure recover after a zone stops?
    Pressure should rebound immediately or within a minute. Slow recovery suggests a continuing leak, supply restriction, or failing PRV.

Related Articles

If you’re troubleshooting a similar symptom, these guides may help:

For the full directory, see Intermittent Pressure Loss Events.

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