Playbook: After-Hours Operation

Adapted from HVAC FDD Reference v1.0, Remediation Playbooks (pp. 157–159).

Step 1 — Verify the fault

1. Pull a 7-day trend of supply fan run status overlaid with the occupancy schedule; confirm the equipment runs during unoccupied periods.
2. Check for legitimate after-hours reasons: morning warmup / night cooldown (normal but should be time-limited to 1–2 hours) and tenant override requests (should be time-limited, never indefinite).
3. PNNL-27338 checks: unoccupied fan runtime > 30% of total unoccupied hours, or duct static pressure > 0.2 in. w.g. during unoccupied hours.
4. Check the BAS time zone setting — daylight saving mismatches are a common culprit.
5. Quantify the waste: unoccupied runtime hours × fan kW × (1 + thermal conditioning penalty). The thermal penalty is typically 1.5–3× the fan energy because the AHU is also conditioning outdoor air unnecessarily.

Step 2 — Remote fix

1. Correct the occupancy schedule if it’s wrong — the most common fix.
2. Remove stuck overrides: check the BACnet priority array on the fan command and release any manual override (a top retro-commissioning finding, PNNL-27338).
3. If morning warmup / evening cooldown runs too long: limit pre-conditioning to 1–2 hours maximum, ideally with an optimal start algorithm (PNNL EEM-27/28 starts the AHU just in time to reach setpoint by occupancy).
4. Set after-hours override time limits: each override event auto-expires after at most 2 hours; ensure an override can never run indefinitely.
5. Verify holiday schedules are current — missed holidays are a common source of after-hours waste.
6. Building-wide issues: fix the master schedule and verify all air handlers, exhaust fans, and lighting reference the same schedule.
7. Enable night setback temperatures: widen the unoccupied deadband to ~13 °C heating / ~29 °C cooling (55/85 °F, or site-appropriate values) — EEM-16 alone saves 3–16% of site energy depending on building type.

Step 3 — On-site service (rare)

1. If the fan runs despite a correct off command: check for a welded/stuck contactor or relay, a hardwired VFD run-signal bypass, or a hand-off-auto switch left in Hand.
2. If an occupancy sensor falsely detects occupancy: check location and sensitivity — PIR sensors trigger from HVAC air currents, rodents, or direct sunlight on the lens.

Step 4 — Confirm resolution

1. Monitor fan status during 3 consecutive unoccupied periods.
2. All after-hours faults should clear within 24–48 hours.
3. Verify lighting (SYS-0003) and exhaust fans (SYS-0004) also shut down on the corrected schedule — these often share the same root cause.