Zone-Level Airflow Schedule Disagrees With System Inspector Before Issue
A 12-zone office floor is being issued for construction. Your supply air schedule, built from space-level airflow parameters, totals 14,200 L/s. The mechanical engineer's load calculation totals 13,600 L/s. The system inspector on the main AHU shows 13,900 L/s. You have three different numbers and a deadline. Investigation shows: (a) two spaces are unoccupied corridors that were included in the schedule but excluded from the load calc; (b) one VAV box serves two spaces but the schedule lists each space separately; (c) the AHU's system flow is computed from the connected terminals, not from the spaces. The schedule is filtered by a 'Zone' shared parameter and grouped by level. The permit set requires a schedule that reconciles to the load calculation within 5%.
1. Reconcile the three numbers and explain what each figure actually represents.
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The three numbers measure different things and should not be expected to match without adjustment:
- 14,200 L/s (schedule) is the sum of space-level design airflow parameters. It includes the two corridors (which the load calc excludes) and double-counts the space served by the shared VAV box if that box's airflow was entered on both spaces. It is a space-level demand figure, not a system figure.
- 13,600 L/s (load calculation) is the engineer's calculated cooling/heating demand for occupied spaces only, before any diversity or system-level allowance. Corridors are excluded by the engineer's scope convention.
- 13,900 L/s (system inspector) is the AHU system flow computed from the connected terminals - the VAV boxes - not from spaces. It reflects what the terminals are set to deliver, including any box whose airflow was set independently of the space parameters.
The gap between schedule and load calc is largely explained by the corridors plus the double-counted space. The gap between load calc and system inspector is the terminal-vs-space distinction: boxes may carry a design airflow that differs from the sum of their spaces, and transfer air or diversity handling may differ.
Reconciliation approach: remove corridors from the schedule (filter them out or exclude unoccupied spaces), correct the double-counted space so its airflow appears once, then compare schedule total to load calc. Then compare terminal airflow totals to the load calc to find boxes set inconsistently.
2. The permit set requires the schedule to reconcile to the load calculation within 5%. Describe the concrete schedule and model changes, and the checks you run before issuing.
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Model and schedule changes:
- Fix the double count. For the VAV box serving two spaces, decide the convention: either assign the box's full airflow to one space and zero to the other, or split it proportionally. Document the convention. The schedule must not sum the same box twice.
- Exclude unoccupied spaces. Add a filter to the schedule on the space's occupancy or a dedicated 'Include in Schedule' shared parameter set to No for corridors. Do not delete the corridors - they still need to exist for geometry and egress coordination.
- Align space parameters to the load calc. Update each space's design airflow to the engineer's values so the schedule is derived from one source of truth, not two.
- Reconcile terminals. Compare each VAV box's design airflow to its served space(s). Where a box differs, determine which is correct and correct the other. The system inspector total should then track the schedule total.
Pre-issue checks:
- Recompute the schedule total and confirm it is within 5% of 13,600 L/s.
- Confirm the schedule filter on the 'Zone' shared parameter still returns every intended zone - a filter change can silently drop rows.
- Confirm grouping by level is intact and no zone appears on the wrong level.
- Verify the system inspector total now matches the terminal schedule total.
- Add a note or a calculated total row to the schedule so reviewers can see the reconciliation.
If after correction the gap exceeds 5%, do not force the numbers - issue with a written variance explanation, because a schedule massaged to hit a target is worse than a documented discrepancy.