Quick Answer:
If the leak only happens while the irrigation runs, the backflow assembly or a vacuum breaker is likely letting water past a seal when the system is pressurized. Stop the irrigation, relieve pressure, then check zone valves and the vacuum breaker seating to narrow the source quickly.
Why This Happens
- Backflow devices and vacuum breakers are designed to hold back pressure and prevent contamination. When internal seals, springs or seats wear, they may leak only under the higher pressure created while a zone runs.
- Valve problems or a failed diaphragm in a zone valve can pressurize a section and allow water to find a leak path it won’t use at rest.
- Damage from winter freeze or rough startup behavior can weaken seals. If you recently restarted the system after a shutdown, think of the situation as similar to backflow failure after sprinkler startup.
- Corrosion, debris, or a displaced seal after colder months can also cause a pressurized leak — for examples like this see issues similar to Backflow device leaking after winter.
Step-by-Step What to Do
Stop irrigation and relieve pressure
- Turn the controller off or set to rain mode so no zones run.
- Shut the irrigation supply valve (where the water enters the irrigation system) and open a downstream test port or a hose bib to relieve pressure before touching anything.
Inspect zone valves
- With the system off, visually inspect each zone valve assembly for obvious leaks, damaged wiring, or rodents.
- Manually operate one zone at a time from the controller (or using the manual lever) and watch the backflow and piping while that zone runs. This helps identify which zone pressurizes the leaking path.
- Check solenoids and diaphragms on the zone valves — a failed diaphragm can allow water to backflow into a spot that then leaks when pressurized.
Confirm vacuum breaker seating
- Locate the atmospheric vacuum breaker (AVB) or pressure vacuum breaker (PVB) — usually mounted on top or near the backflow assembly.
- With system pressure on briefly, observe the vacuum breaker for drips or weeping at the seat. Small drips under pressure point to a seating issue.
- Do this carefully: relieve pressure first, then apply pressure only briefly to test. If the vacuum breaker fails to seal, it often needs a new seat or spring.
Check the backflow assembly
- Open the test cocks one at a time (only after relieving pressure) to see if water escapes past the internal check valves. Leakage from test cocks under pressure suggests internal check failure.
- Tightening test cocks or replacing small seals can help, but internal check replacement or full assembly replacement may be required for a reliable fix.
Isolate and confirm the source
- Run single zones and watch the assembly and piping to confirm the leak only appears when certain zones run.
- If the leak follows a single zone, focus repair or replacement on that zone’s valves and piping. If it happens regardless of zone, the backflow or vacuum breaker is the likely culprit.
Temporary steps and repair decisions
- As a temporary measure, keep the irrigation off and use the regular potable supply if you need water.
- If confident and competent, replace small seals, packing, or test cocks. If the internal checks are bad, plan for professional replacement of the backflow assembly.
What Not to Do
- Do not bypass irrigation protection — never remove or bypass the backflow device to stop a leak. That risks contaminating your household water.
- Do not use tape, glue or temporary plugs as a long-term fix on pressurized parts.
- Do not run the system continuously to “see where it leaks” without someone watching the assembly; uncontrolled pressure can worsen damage.
When to Call a Professional
- Call a licensed irrigation or plumbing pro if the leak continues after basic checks, if internal backflow checks are leaking, or if you need backflow assembly replacement. Many jurisdictions require certified technicians to work on backflow devices.
- If you’re unsure how to safely relieve pressure or operate valves, get professional help rather than risk injury or further damage.
- When the leak risks contaminating your potable supply or when the assembly is older or corroded, professional replacement is the safer long-term choice.
Safety Notes
- Always relieve system pressure before loosening any fittings or test cocks.
- Turn off electrical power to controllers if you’ll be working near wiring or solenoids.
- Backflow devices prevent contamination; do not tamper with certification tags or try to avoid required testing in your area.
Common Homeowner Questions
- Why does it only leak when irrigation runs? Short answer: the leak path requires system pressure — worn seals or seats let water pass only when a zone pressurizes the assembly.
- Can I keep using the system with a small leak? Short answer: not recommended — it can waste water, worsen damage and risk contamination if the backflow protection is compromised.
- Will winterization fix this? Short answer: only if damage was caused by freeze and is visible; most internal check or seat failures require repair or replacement.
For more related articles, see the Backflow Preventer Malfunctions hub.