VAV Rooftop Short on Airflow During TAB

Measured total static: 1.55 + 0.95 = 2.50 in. w.c. against 2.0 design, a 0.5 in. w.c. excess - with airflow 17% below design. High static plus low airflow means extra resistance, not a lack of fan speed.

Static budget:

• Filter: 0.85 measured vs 0.35 clean = +0.50 in. w.c. - that alone accounts for the entire excess.
• Coil: 0.45 vs 0.30 = +0.15 in. w.c. - the coil is dirty or wet beyond its schedule, so the true excess is more than 0.5; the ducts must be slightly better than the design assumption.
• Outdoor-air damper at 15% minimum is adding its scheduled resistance and should be verified at the scheduled setpoint.

Conclusion: the first move is not fan speed. It is a filter change and a coil cleaning/inspection, plus a check that all boxes, fire dampers, and manual balancing dampers are actually open (the boxes are commanded open - verify the damper actuators actually responded).

Fan-curve reasoning: at fixed rpm, removing 0.5 in. w.c. of system resistance moves the operating point up the fan curve. For a centrifugal fan of this type, a drop from 2.5 to 2.0 in. w.c. typically buys roughly 8–12% more airflow - about 9,000 to 9,300 CFM. Most of the 1,700 CFM deficit disappears without touching the drive.

Required speed ratio: 10,000 ÷ 9,000 = 1.11, so about 1,050 → 1,165 rpm.

Fan laws applied:

• Static pressure: 2.0 × 1.11² ≈ 2.46 in. w.c. - check that the casing, coil, and filter rack are rated for it.
• Power: 1.11³ ≈ 1.37×.
• Motor current: 21 A × 1.37 ≈ 29 A, which exceeds the 27 A nameplate FLA. The motor would be operating in the service-factor region with the overloads lifting. That is not acceptable.

So the honest answer to the owner's rep is no - not as proposed. Speed changes here require a drive change plus a motor and starter/overload review, and even then the operating point has moved toward the unstable part of the fan curve where small duct changes swing the airflow.

Find the rest of the resistance first. At 9,000 CFM with a clean filter the remaining shortfall is 10%, which is beyond balancing tolerance and points to something real: a kinked or crushed flex run, an unopened fire damper or manual volume damper, an undersized section versus the design drawing, a coil that is still fouled on the entering face, or a scheduled component installed in the wrong size. Fixing one of those is cheaper and safer than an electrical upgrade.

Also verify: belt tension and sheave setting versus the schedule, blower rotation, and that the measured rpm is genuinely 1,050 - a mis-set variable-pitch sheave is a common root cause of 'the fan won't make design.'

Acceptance criteria:

• Each VAV box within ±10% of its design airflow at maximum, and each zone within its minimum airflow tolerance; total airflow within ±10% of 10,000 CFM.
• Outdoor air at the minimum setpoint measured at the intake hood/louver (about 1,500 CFM nominal for 15%), and verified that the economizer returns to minimum and that the unit holds slight building positive pressure.
• Static pressure profile recorded at every point of measurement: filter in/out, coil in/out, supply trunk, return, and at the last box - with the filter's loaded-vs-clean condition noted.
• Fan rpm, motor voltage and amps, sheave setting, and belt condition.

Conditions to record:

• Date, time, outdoor and indoor dry/wet bulb, and whether the economizer was at minimum or full open.
• Balancing performed with the system at or near design airflow conditions, or corrected using an approved method; a winter balance at 45°F outdoor is not the same test as a summer balance.
• A mandatory re-check at 100% outdoor air and at the economizer changeover point, since a system that balances at minimum OA can be starved or over-pressurized at full economizer.

Controls verification with the balance: static pressure reset setpoints, box minimum/maximum, damper actuator stroke versus command, and a documented safe position on any failure. Sign the report only when the measured numbers and the corrected fan/drive configuration match what is stamped.