Lead/Lag Problems on Stulz Units
Lead–Lag Not Rotating — Quick Check (STULZ E²)
Goal: find why the Lead isn’t handing over to Standby/Assist and restore normal rotation/failover.
60-Second Screen Checks (on any unit)
Service → Group → Group Status
All expected units show Online (no “No Link”).
Current Lead unit is identified.
Active / Assist / Standby counts look right.
Service → Group → Group Rotation
Rotation is Enabled.
Rotate Every (hrs/days/weeks) > 0 and sensible.
Next Rotation time looks reasonable (clock not wrong).
Force Rotation is available (not greyed).
Service → Group → Group Config
Unit is Included in Group and Included in Rotation (not excluded).
Min Active / Min Assist aren’t blocking a changeover.
No unit is Inhibited / Manual / Locked Lead.
Service → Group → Group Alarm Setup
Alarms that should trigger changeover are enabled.
No active alarm is pinning the group in a protective state.
Date/Time (System)
Controller clock/time-zone is correct (rotation schedules depend on it).
Service → Group → Group Sensors / Averaging
Sensor source (Local/Lead/Average/Min/Max) matches your strategy.
Standby/Assist sensors included/excluded as intended (avoids “false no-demand”).
5-Minute Network/Setup Checks
pLAN addressing (E²)
Controllers addressed 1..8 with no gaps.
Terminal↔Controller pairs follow the sum-to-33 rule (32↔1, 31↔2 …).
Net/Bus status shows all controllers/terminals healthy.
Configuration consistency
All units share the same setpoint/deadband/modes (no “fighting” logic).
Same assist cut-in/out logic and humidity strategy.
No unit held Off by BMS/remote interlock.
Comms wiring
Correct polarity, good shields, proper termination at both ends only.
No stubs or crushed cables at doors/panels.
Safe Force-Rotation Test (proves the path)
Ensure both units are Online and no critical alarms.
Temporarily set Min Active = 1 (so a changeover is allowed).
Service → Group → Group Rotation → Force Rotation.
Confirm Lead flips to the next addressed unit and the prior lead drops to Standby/Assist.
Restore Min Active and any temporary changes.
If Force Rotation fails: almost always addressing/group membership or rotation disabled (re-check the two sections above).
Usual Culprits & Fixes
Rotation disabled / value = 0 → Enable and set a real interval.
Bad clock → Correct date/time; recheck “Next Rotation”.
Min Active too high → Lower so a handover is legal.
Unit excluded / inhibited / manual → Re-include and clear inhibit/manual.
Active alarm holding state → Clear alarm or change Group Alarm Setup logic.
Addresses skipped (e.g., 1 & 3 only) → Make them contiguous; rotation follows address order.
Terminal not paired to the right controller → Fix pairing (sum-to-33 rule).
BMS holding lead on (remote enable/global mode) → Release override; test again.
Mismatched setpoints/deadbands → Align; otherwise the group may never meet the condition to rotate.
Quick notes (older A-Tech-20 sites)
Level-3 password often 9998 (may be site-changed).
Communications → Unit Number must be 001, 002, 003… (no skips).
Match baud/parity on every unit/gateway.
Handy one-liner for the job sheet
Fix rule: If rotation is enabled, the clock is correct, addresses are contiguous, Min Active allows a swap, units are included & healthy, and Force Rotation works—lead-lag will rotate on schedule.
