Quick Answer:
If you see water at the spigot only while the sprinkler runs, first isolate and shut off the irrigation zone feeding that spigot. Inspect the backflow assembly and its unions for loose nuts or failed seals, then run the sprinkler briefly while watching the feed connection and quick-connects for leaks. Replace damaged backflow parts or reseal the coupling rather than leaving the device pressurized.
Why This Happens
When the irrigation zone runs it raises pressure and flow in the supply line to the spigot and backflow preventer. That extra pressure can force water past a weak union gasket, a cracked body, or a failed quick-connect seal. Small leaks that only appear under flow are usually seal or coupling failures rather than a hole in the pipe. If you want related reading about similar issues, see Spigot leaks during irrigation.
Step-by-Step What to Do
1. Isolate and shut off the irrigation zone feeding the spigot
- Turn off the controller or the specific zone valve for the affected area.
- If there’s a manual shutoff at the irrigation manifold or a local shutoff valve near the backflow, close it.
- Open the spigot to relieve pressure before you touch unions or couplings.
2. Inspect the backflow assembly and its unions
- Look for loose union nuts at the feed connection and quick-connect fittings. Hand-tighten then give a small wrench turn if needed—don’t overtighten.
- Check for cracked plastic, corrosion, or visible deteriorated gaskets or O-rings at union faces.
- If seals look flattened, brittle, or missing, plan to replace them or the union.
3. Run the sprinkler briefly while observing connections
- Re-enable the zone for a short test while someone watches the backflow, union nuts, supply coupling, and any quick-connects at the spigot.
- Watch for drips that start only under flow; use a dry towel to detect small leaks.
- If the leak appears only when the zone runs, it confirms a pressure-related seal or coupling issue.
4. Replace damaged backflow components or reseal the coupling
- Shut the zone off and depressurize before disassembling unions.
- Remove the union nuts, inspect the gasket/O-ring and mating faces, and install a correct replacement seal. Light silicone lubricant can help seating if the manufacturer allows it.
- If the backflow body or union is cracked or the internal check is leaking, replace the whole backflow assembly or the damaged part per the manufacturer. Some backflow devices require certified service—see the next sections.
What Not to Do
- Avoid wrapping the backflow device with tape to hide a leak—this only delays the proper repair and may make the condition worse.
- Do not leave the system pressurized without addressing the device; ongoing pressure can damage seals and increase water loss.
- Don’t overtighten plastic union nuts; excessive force can crack fittings.
- Call a pro when the leak is coming from the backflow assembly or occurs only during irrigation cycles.
When to Call a Professional
- If the leak is coming from inside the backflow assembly, call a licensed plumber or an irrigation technician—some devices must be repaired or tested by certified personnel.
- If you can’t isolate the affected zone, if replacing seals doesn’t stop the leak, or if the backflow body is cracked, call a pro.
- Also call a professional if your local code requires certified backflow testing or replacement.
Safety Notes
- Always depressurize the line before loosening unions or removing hoses. Open a downstream spigot to relieve pressure.
- Wear gloves and eye protection when working near pressurized fittings.
- Don’t disassemble internal check valves or internal backflow components unless you are trained and the device is designed for homeowner service.
Common Homeowner Questions
- Why does the spigot leak only when the sprinkler runs?
Higher flow and pressure during irrigation can push past a weakened gasket or coupling, causing leaks that don’t show at low or static pressure. - Can I just tighten the union nuts myself?
Yes, you can snug loose union nuts, but don’t overtighten; replace deteriorated gaskets when present. - Will tape or pipe dope fix the leak temporarily?
No—taping the backflow device hides the issue and is not a safe or reliable fix; replace the seal or component instead.
If you’re unsure at any step, stop and call a qualified irrigation or plumbing professional to avoid damaging the backflow device or creating a safety/code issue.
More in this topic
For more related fixes and similar symptoms, see Outdoor Spigot Leaks Under Load.