Troubleshooting guide

Why Is My AHU Airflow Low Despite High Fan Speed?

Filters, dampers, duct restrictions, fan condition and measurement errors cause low AHU airflow at full speed. Includes a duct traverse example.

Mechanical, HVAC & Fluids · Last reviewed

A fan at full speed that delivers less air than designed is working against more resistance than the system was designed for, or it is not producing what it should, or the airflow is being measured wrongly. Static pressure readings help separate the three.

Air handlers have rotating fans, high-voltage drives and sharp metal edges. Lock out before opening access doors. Measurements with the fan running and any inside-unit work belong with qualified personnel following site procedures.

SymptomLikely causeWhat to check
Airflow low, filter pressure drop highLoaded filters or coilsPressure drop across the filter and coil against the clean value
Airflow low, static pressure higher than designA restriction in the duct: closed fire or balancing damper, collapsed duct, or a blocked outdoor air intakeDampers and duct inspection. Static pressure profile along the duct
Airflow low, static pressure lower than designFan problem: wrong rotation, belt slipping, wheel damage, or a drive limitFan rotation, speed measured at the shaft, belt tension, drive output frequency
Airflow low, static pressure low, leakage noiseLeaks in the ductwork or the unit casing, or open access doorsAccess doors, casing seams and duct joints
Airflow reading low but comfort is normalMeasurement error from a bad location or a drifting sensorMeasurement location (straight run), sensor calibration, and a duct traverse
Airflow was fine and dropped over timeGradual filter loading, a coil that has fouled, or belt wearTrend the pressure drops and the fan power

A duct traverse example

The airflow is velocity times area. In a duct 24 in. × 36 in. (6.0 ft²), a traverse of readings across the duct gives an average velocity of 1,200 ft/min:

Q = 1,200 ft/min × 6.0 ft² = 7,200 CFM

If the design airflow is 10,000 CFM, the delivery is 72% of design, even though the drive reads 100% speed. A single reading at the middle of the duct overstates the average, because the velocity is highest at the center and lowest at the walls. A traverse uses many points, and the measurement should be made in a long straight run.

Two quick checks

  • Compare the fan power with the fan curve. High power with low flow suggests a restriction. Low power with low flow suggests a fan or drive problem.
  • Check the fan laws: if the airflow is low and the speed is already at maximum, the system resistance is higher than design, and only removing the restriction will fix it.

Write a useful problem statement

  • Design and measured airflow and how it was measured. Fan type, speed and drive setting.
  • Static pressures at the fan inlet and outlet and across each filter and coil.
  • Filter type and last change, and any damper changes.
  • Fan current or power against the nameplate and the fan curve.

Related tools and labs

Calculators, labs and simulators are part of the All-Access Pass.

More troubleshooting

Questions

Why does a dirty filter reduce airflow at full speed?

It adds resistance, which moves the system curve up. The fan then operates at a point with higher pressure and lower flow, along its own curve.

Can I get more air by raising the fan speed?

Only to the limits of the motor, the drive and the fan. Power rises with the cube of the speed, and a restriction is still the more efficient thing to remove.

How accurate is a single point velocity reading?

It is a poor estimate of the average. Use a traverse, or a permanently installed airflow station that has been calibrated.

Educational material. Examples use simplified, hypothetical inputs and are not a design, a construction document or professional engineering advice. Confirm requirements with the codes and standards adopted where the work is built and with a licensed engineer.

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