Quick Answer:
If opening an irrigation zone causes the house water pressure to fall sharply, it’s usually a supply-demand issue rather than a sudden fixture failure. Run a few quick checks: Pressure fine until second fixture opens, test pressure with fixtures isolated, confirm the PRV setting, and observe the water meter for flow patterns to tell whether the irrigation is overwhelming the supply or there’s a leak or equipment problem.
Why This Happens
Most homes share the same main supply for irrigation and indoor plumbing. When an irrigation zone calls for water, it can draw enough flow to lower pressure for the rest of the system. Common causes:
- High combined demand — a large sprinkler zone plus indoor use can exceed supply or pump capacity.
- A pressure regulator (PRV) that’s set low or failing, so downstream pressure drops when flow increases.
- Well pump or municipal supply limits — the source can’t maintain pressure under sudden high flow.
- Hidden irrigation leaks that only show when a zone opens and create continuous flow.
Step-by-Step What to Do
Step 1 — Test pressure with fixtures isolated
- Turn off all indoor and outdoor fixtures and appliances that use water (dishwasher, washing machine, showers, hose spigots).
- Attach a pressure gauge to an outdoor spigot or a hose bib near the main line and note the static pressure.
- Start one irrigation zone and watch the gauge. If pressure drops drastically only when the zone runs, the irrigation demand is the likely cause.
Step 2 — Observe meter flow patterns
- Go to the water meter while the irrigation zone runs. Look for a constant sweep or continuous flow indicator; that shows sustained demand or a leak.
- Compare patterns: a steady high flow when the zone opens suggests expected usage, while unusual spikes or continued flow after the zone turns off suggests a leak or valve problem.
Step 3 — Confirm PRV setting
- Locate the PRV (usually near the main shutoff). Use a pressure gauge downstream of the PRV to check the set point — many systems are set 50–60 psi but municipal limits vary.
- If the downstream pressure collapses under flow but the upstream pressure stays high, the PRV may be undersized, failing, or set too low. Do not force adjustments beyond recommended values.
Step 4 — Check source limits (well or supply)
- If you’re on a well, watch pump cycling and pressure tank behavior when the irrigation runs. Rapid cycling or inability to maintain pressure points to pump or tank issues.
- If on municipal water, confirm with your provider if there are known pressure limits or supply restrictions at peak times.
Step 5 — Try practical fixes
- Stagger irrigation start times so not all zones draw at once.
- Reduce the number of heads in a zone or shorten run times to lower peak flow.
- If testing suggests the PRV or pump can’t cope, plan for professional adjustment, PRV replacement, or a pressure booster for irrigation only.
What Not to Do
- Do not assume fixture failure when pattern is demand-related. A drop tied to irrigation opening is often normal supply behavior, not a household plumbing fault.
- Do not crank a PRV above recommended settings to “force” pressure; that can damage fixtures and may violate local rules.
- Do not try to repair pressurized irrigation components without shutting off the zone and relieving pressure first.
When to Call a Professional
- Visible continuous flow at the meter when irrigation should be off (possible leak or stuck valve).
- PRV problems that require replacement or adjustment beyond simple verification.
- Well pump issues: frequent cycling, inability to maintain pressure, or loss of prime.
- If you’re unsure how to safely isolate systems or read the meter and gauges.
Safety Notes
- Turn off the irrigation zone and relieve line pressure before working on valves or PVC pipe.
- If you suspect electrical problems with a well pump, cut power at the breaker before inspecting equipment and call an electrician or well contractor.
- When in doubt, hire a licensed plumber or irrigation contractor rather than attempting risky repairs yourself.
Common Homeowner Questions
- Why does pressure only drop when irrigation runs? — The irrigation adds large flow demand; if supply or device limits are reached, pressure to the house falls.
- Can I adjust my PRV to fix it? — You can check the setting, but replacing or adjusting a PRV is best left to a pro if flow collapse is significant.
- Is this the same problem as two showers affecting water temperature? — It’s a similar demand issue: Two showers running causes cold water loss is an example of supply limits causing symptom changes.
For more related articles, see the Pressure Drops When Multiple Fixtures Run hub.