Fan Curves

The payoff

Turning the curve into dollars: a VAV system spends most of its life at part load. How you get from design flow down to part flow — throttle the system, or slow the fan — is the single biggest energy decision on the airside. Same 10,000 CFM fan as every other demo; the meter runs below.

Input power vs. airflow — four ways to get to part load

Where the hours actually live

Annual $ = the power chart weighted by these hours. The bins around 50–70% flow dominate — exactly where the cube law pays best.

Ride the curve + throttle (dampers)
—
— kWh/yr
VFD, fixed duct-static setpoint
—
— kWh/yr
VFD + static-pressure reset (G36)
—
— kWh/yr
Saved per year — VFD + reset vs. dampers, this one fan
—
× every AHU, RTU, and pump in the building
Reading the gap
  • Dampers: the fan keeps making pressure you then destroy across a blade. Power barely falls.
  • Fixed setpoint: the VFD slows the fan, but the loop holds 1.5″ at the sensor even when nobody needs it — you ride above the ideal parabola.
  • Setpoint reset: trim the setpoint until one damper is nearly wide open; the system curve relaxes toward the origin and you hug the cube law.
This is baked into the code (ASHRAE 90.1-2022)
  • Fan power limits — Table 6.5.3.1-1 caps hp per CFM (VAV: bhp ≤ CFM×0.0013 + A). You can’t damper your way past it.
  • Fan Energy Index — §6.5.3.1.3: FEI ≥ 1.00 at design (≥ 0.95 for VAV). FEI is “how far from a good spot on a good curve did you select?” — codified.
  • Heat-rejection fans ≥ 5 hp — §6.5.5.2.1: ≤ 30% of design watts at 50% airflow. That number is the cube law (0.5³ ≈ 12.5%, plus margin).
  • Static-pressure reset — §6.5.3.2.3: DDC systems must reset the setpoint by zone demand — the aqua line is a code requirement, not a nicety.
Pumps: the same movie
  • Identical affinity laws — flow ∝ N, head ∝ N², power ∝ N³ (ASHRAE S&E ch. 44). A pump curve reads exactly like a fan curve.
  • One twist: static head. Lifting to a cooling tower or holding a ΔP setpoint means the system curve doesn’t pass through the origin — so real savings land between the orange and aqua lines, never on the ideal. Same story you just watched.
  • “Don’t go there” zone: for fans it’s stall/surge on the left of the curve; for pumps it’s cavitation/NPSH and dead-heading. Different physics, same discipline: stay in the happy part of the curve.