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Playbook: Exhaust Fan Schedule Misalignment
Applies toSYS-0004 (exhaust running unoccupied), SYS-0008 (exhaust/AHU misalignment), CLU-08
Fix complexityRemote fix (90%) · On-site (10%)
Typical time15–30 min remote
Typical cost$0 remote
Energy impactEEM-07: 0.5–3% site energy. ~35% prevalence per PNNL 151-building study. The waste is twofold: direct fan energy plus the conditioning penalty from exhausting conditioned air and drawing in unconditioned outdoor air. An exhaust fan running during unoccupied hours in winter pulls heated air out of the building while drawing freezing air in through cracks, dramatically increasing heating energy.

Adapted from HVAC FDD Reference v1.0, Remediation Playbooks (pp. 173–175).

Step 1 — Verify the fault

  1. For exhaust running unoccupied (SYS-0004):
    • Pull a 7-day trend of exhaust fan status overlaid with the building occupancy schedule.
    • Confirm the exhaust fan is running during unoccupied periods for more than 15 minutes.
    • Check whether the run is a legitimate demand override (e.g., lab exhaust, parking garage, kitchen hood).
  2. For exhaust/AHU misalignment (SYS-0008):
    • Compare exhaust fan status to supply fan status over 7 days.
    • Condition 1: exhaust ON but supply OFF — this creates negative building pressure, pulling in unconditioned air through the envelope.
    • Condition 2: supply ON but exhaust OFF during occupied hours — this creates positive pressure and can cause IAQ issues.
  3. Quantify the waste: exhaust fan energy = fan_rated_kW × (speed/100)³. Add the conditioning penalty: for each CFM of exhausted air, the building must condition the same volume of makeup air.

Step 2 — Remote fix

  1. Synchronize the exhaust fan schedule with the AHU schedule:
    • Both should reference the same master occupancy schedule.
    • If the exhaust fan is on a separate controller or timer, reprogram it to match.
  2. Release any manual overrides on the exhaust fan command.
  3. For exhaust fans controlled by standalone timers or switches (not integrated into the BAS):
    • Reprogram the timer to match the building schedule.
    • Better: connect the exhaust fan to the BAS via an interlock relay so it follows the supply fan status automatically.
  4. Verify that holiday schedules are synchronized between the AHU and exhaust systems.
  5. For demand-controlled exhaust (e.g., garage CO sensors, kitchen hoods): verify the demand sensor is working correctly and not falsely triggering the fan.

Step 3 — On-site service (rare)

  1. If the exhaust fan runs despite receiving the correct off command:
    • Check the contactor or relay for welding (stuck closed).
    • Check for a hand-off-auto switch left in hand (manual on) position.
    • Check for a hardwired interlock with another system (e.g., fire alarm override) keeping the fan running.
  2. If the exhaust fan VFD has a fault keeping the fan running at a fixed speed, clear the VFD fault and restore normal BAS control.

Step 4 — Confirm resolution

  1. Monitor exhaust fan status vs. supply fan status for 3 consecutive occupied/unoccupied cycles.
  2. Exhaust fans should start within 5 minutes of the supply fan and stop within 5 minutes of it shutting down.
  3. During unoccupied hours, exhaust fans should remain off unless a legitimate demand override is active.
  4. The fault should clear within 24–48 hours.