Quick Answer:
When an irrigation zone starts, the sudden demand can make indoor pressure surge or dip briefly. Check house fixtures and the water meter to tell whether the change is caused by the sprinkler demand itself, a pressure regulator, or a plumbing fault before assuming a broken part.
Why This Happens
Sprinkler valves open large-diameter outlets and create a high flow event. That flow can reduce static pressure at other fixtures or cause short-term pressure swings as the system rebalances. If your house has a pressure-reducing valve (PRV) or a well pressure system, those devices can respond to the surge and produce unstable indoor pressure until the zone finishes its cycle.
Similar demand-related patterns show up with other appliances, for example **Pressure weakens when water heater refills** or **Pressure drops when dishwasher fills** — those are useful comparisons when you try to figure out whether the issue is the irrigation event or a separate plumbing failure.
Step-by-Step What to Do
Step 1 — Observe the meter while a zone starts
Have someone start a sprinkler zone while you stand at the water meter (mechanical or electronic). Watch for a clear spike in flow or a steady flow reading during the irrigation cycle. A clean, repeatable spike that matches the zone start usually means the irrigation demand is the cause.
Step 2 — Test pressure with fixtures isolated
Isolate indoor fixtures by turning them all off and closing any valves that feed appliances (dishwasher, hose valves). With fixtures isolated, start a sprinkler zone again and note indoor pressure at a cold-water tap or a pressure gauge. If pressure still swings, the issue is system-wide; if pressure stabilizes when fixtures are off, the problem is demand-related.
Step 3 — Confirm PRV setting and behavior
Locate your PRV and visually check its setting (many have an adjustment screw and a pressure range stamped on the body). If the PRV looks set correctly but house pressure bounces or doesn’t return to normal after irrigation, the PRV may be slow to react or failing. Do not force the adjustment if you are unsure — see “When to Call a Professional.”
Step 4 — Observe meter flow patterns over several cycles
Repeat zone starts for a few cycles and note whether the meter pattern is consistent. Consistent, repeatable patterns point to normal demand behavior. Irregular or continuous flow on the meter between cycles suggests a leak or valve that isn’t closing fully.
Step 5 — Check for visible leaks and valve operation
While a zone is running, walk the irrigation lines and nearby house plumbing for wet spots, hissing sounds, or valves that don’t fully shut. A physical leak or a stuck zone valve can cause a pressure change that looks similar to normal demand but is actually a fault.
What Not to Do
- Do not assume fixture failure when pattern is demand-related.
- Do not rapidly crank the PRV adjustment to “fix” pressure without confirming the cause.
- Do not open or force irrigation solenoids or controllers while the controller is powered unless you know the sequence; you can damage the system or create uncontrolled flow.
When to Call a Professional
- If pressure is unstable even with all fixtures isolated and the meter shows unexplained continuous flow.
- If you see water pooling, persistent leaks inside walls, or evidence of pipe rupture when zones run.
- If the PRV appears to be malfunctioning and you’re not comfortable replacing or testing it; PRV work can require a permit or specialized tools.
Safety Notes
- Turn off the main shutoff before doing any invasive inspection inside the home.
- Avoid working on pressurized irrigation lines without isolating and draining them; unexpected pressure can cause injury or property damage.
- If you must climb near sprinkler heads or valves, make sure controllers are off and power to solenoids is disconnected to prevent unexpected starts.
Common Homeowner Questions
- Why does my house pressure drop only when sprinklers run? Short answer: high irrigation flow reduces available pressure; check meter patterns to confirm.
- Can a PRV be adjusted to stop the swings? Sometimes—if the PRV is responsive and correctly sized. If it’s slow or failing, replacement is the safer option.
- Could a leak cause the same symptoms? Yes. Persistent or irregular meter flow and unexplained water loss indicate a leak rather than normal demand.
For more related articles, see the Pressure Drops When Multiple Fixtures Run hub.