Sprinkler System Won't Run but Backflow Preventer Leaks

A 4-zone Toro controller indicates zones are running, but no water reaches the heads. The backflow preventer (Watts 800M4QT) leaks from its top vent when any zone is activated. Recent changes: removal of a wooden cover exposing the backflow to sun, recent mowing, and drip conversion work on one zone (currently off).

Diagnosis: Restricted Supply to Backflow Preventer

The backflow preventer vents water when inlet pressure is too low to seat its check valves. This indicates a supply restriction upstream of the backflow.

Troubleshooting Steps

1. Ensure all ball valves before and after the backflow are fully open.
2. Locate and operate the main irrigation supply valve (often near the water meter, in the basement, or buried). Cycle it open/closed several times to dislodge any stuck gate.
3. Remove the backflow bonnet and poppet assembly; turn on the water. If flow is still poor, the restriction is further upstream.
4. Check for kinked or damaged supply pipe from recent yard work.
5. If in a freezing climate, the supply valve may be buried below frost line – locate via the water meter.

Once the supply restriction is cleared, the backflow should seal and zones will operate normally.