Single Zone Runs Uncontrollably: Three Failure Modes

Zone Stuck On: Timer, Solenoid, or Valve Issue

One zone ran for hours. Only main water shutoff stopped it. Timer and valve box wiring appear normal.

Three Probable Causes

1. Timer "hot terminal": Zone output relay/triac failed closed, sending continuous 24VAC
2. Solenoid failure: Plunger stuck mechanically or coil shorted internally
3. Valve mechanical issue: Debris preventing diaphragm closure, worn seat, broken spring

Diagnostic Sequence

1. Disconnect zone wire at controller — if zone stops, timer is faulty
2. If zone continues, disconnect wire at valve solenoid — if stops, wiring short
3. If still runs, remove solenoid — if stops, solenoid defective
4. If still runs, valve mechanical failure — disassemble and clean/rebuild

Start at the controller — it's the easiest test and highest probability for "zone runs with timer off" symptoms.