Quick Answer:
If the backflow device only fails when the sprinkler system starts, stop the irrigation and isolate the sprinkler supply. The most likely causes are a pressure surge or a leaking check that opens when zone valves start. Isolating the system and watching pressure changes will tell you whether the problem is with the irrigation side or the backflow assembly itself.
Why This Happens
Sprinkler systems change flow and pressure rapidly when valves open. That change can momentarily push pressure back through a backflow preventer or force a worn internal part to move. Typical triggers:
- Sudden pressure drops or spikes when multiple zones open.
- Worn seals or springs inside the backflow that hold under steady pressure but fail under surge.
- Leaks or pipe movement that only occur while irrigation is flowing.
In some cases a visible leak at the device or nearby pipe fittings is the clue; in others the device releases or fails only under the dynamic load. If you see steady leakage during irrigation, document it and consider the pattern similar to sprinkler system backflow leaking.
Step-by-Step What to Do
1. Stop the irrigation cycle
- Turn off the irrigation controller or manually stop the cycle immediately.
- Do not rerun the cycle until you’ve isolated the supply and checked pressure.
2. Isolate sprinkler supply
- Close the irrigation shutoff valve or isolation ball valve that feeds the sprinkler system. This keeps the house plumbing separate from the irrigation while you inspect.
- If you don’t have a clear irrigation isolation valve, shut off the controller and close the closest outdoor shutoff before the backflow device.
3. Inspect pressure surge
- If you have a pressure gauge on the irrigation or on the water main, watch it while briefly opening a zone to recreate the issue. Look for rapid spikes or drops.
- Repeat one zone at a time. If the backflow only fails when a specific zone opens, that zone may have a pipe break, stuck valve, or excessive flow demand causing the surge.
4. Check the backflow externally
- Look for obvious leaks, wet soil, or hissing noises at the device and nearby fittings.
- Do not attempt to disassemble the backflow device unless you are trained and permitted to do so.
5. Document what you observe
- Note which zones were running, how long before failure, and any pressure readings. If the device appears to open or release between cycles, record that pattern — it can match cases where the backflow device releases water randomly.
- Keep photos or video if possible; this helps a technician diagnose the intermittent issue.
What Not to Do
- Do not continue irrigation cycle — running it can worsen a leak, cause contamination, or lead to flooding.
- Do not remove covers or internal parts of the backflow device unless certified to work on it.
- Do not bypass or defeat the backflow preventer to get the sprinklers running.
When to Call a Professional
- If the backflow continues to open or leak after you isolate the sprinkler supply.
- If you detect large pressure surges or repeated failures when zones start.
- If you cannot find an isolation valve or the device requires repair or replacement.
- If local code requires a licensed tester to certify or repair the device.
Safety Notes
- Shut off controller power before working near electrically operated valves.
- Don’t use makeshift fixes that could allow backflow of irrigation water into your potable supply.
- Avoid working on pressurized piping; relieve pressure by isolating supply and opening a downstream faucet if needed.
Common Homeowner Questions
- Q: Why does the problem only happen when sprinklers run?
- A: Because valves opening change flow and pressure, which can force a weak seal or spring in the backflow to fail.
- Q: Can I keep using the sprinklers until a pro arrives?
- A: No. Stop the cycle and isolate the sprinkler supply to prevent contamination or water damage.
- Q: Will turning off the house water fix it?
- A: Turning off house water may stop the immediate problem but won’t repair a faulty backflow device or fix pressure-surge causes; call a technician for a proper diagnosis.