Why Is AHU Airflow Low? Causes and Testing Tools Malaysia
Meta Description: Discover why AHU airflow becomes low and how anemometers, differential-pressure meters, tachometers and clamp meters support factory HVAC troubleshooting.
Low airflow from an air handling unit can cause poor cooling, high humidity, uneven room temperature, condensation and unstable production conditions.
The cause may involve a dirty filter, blocked cooling coil, incorrect fan speed, slipping belt, closed damper or restricted duct.
Maintenance teams should measure the system before replacing parts or increasing fan speed.
Common Causes of Low AHU Airflow
The most common causes include:
Dirty or blocked filters
Blocked cooling coil
Fan running too slowly
Slipping or worn belt
Incorrect fan rotation
Closed or damaged damper
Blocked duct
Air leakage
Dirty fan blades
Motor or VFD problem
1. Dirty Air Filters
As filters collect dust, resistance to airflow increases.
Possible signs include:
Reduced supply airflow
Increasing filter pressure drop
Higher fan-motor load
Uneven room conditions
Whistling or abnormal air noise
Use a differential-pressure meter to measure the pressure drop across the filter.
Filter replacement should follow the filter and AHU manufacturer’s recommended limit rather than appearance alone.
2. Blocked Cooling Coil
Cooling coils can collect:
Dust
Dirt
Oil
Biological growth
Moisture-related contamination
A blocked coil restricts airflow even when the filter is clean.
Measure pressure before and after the coil, and inspect its surface condition according to the maintenance procedure.
3. Incorrect Fan Speed
The fan may run too slowly because of:
Incorrect VFD setting
Belt slippage
Wrong pulley size
Motor problem
Control-system error
Gearbox issue
Use a tachometer to verify actual motor and fan RPM.
The UNI-T UT371 is one non-contact tachometer option for routine speed checks.
4. Slipping or Worn Belt
For belt-driven AHUs, inspect for:
Insufficient tension
Excessive tension
Cracks
Glazing
Belt dust
Pulley wear
Misalignment
A slipping belt may allow the motor to run at normal speed while the fan rotates more slowly.
Measure both motor and fan RPM.
5. Incorrect Fan Rotation
After electrical work, the fan may rotate in the wrong direction.
Possible effects include:
Low airflow
Higher noise
Poor cooling
Unexpected motor current
Weak air delivery
Use a phase sequence meter and confirm actual fan direction according to the equipment markings.
6. Dirty Fan Blades
Dust buildup can reduce fan performance and create imbalance.
Possible signs include:
Reduced airflow
Increasing vibration
Uneven fan rotation
Higher motor current
Dust inside the fan housing
The AHU must be safely isolated before internal inspection or cleaning.
7. Closed or Faulty Damper
A damper may be:
Partially closed
Stuck
Disconnected from its actuator
Incorrectly controlled
Mechanically damaged
Obstructed
Check the commanded position and actual blade position.
8. Duct Obstruction
Airflow may be restricted by:
Collapsed flexible duct
Foreign material
Closed fire damper
Product buildup
Damaged insulation
Poor duct modification
Blocked grille
Incorrectly installed component
Measure air velocity at several points to identify where the reduction begins.
9. Duct Leakage
A major duct leak can reduce airflow reaching the production area.
Possible signs include:
Normal airflow near the AHU
Low airflow at distant outlets
Air noise above ceilings
Damaged duct joints
Loose flexible connections
Pressure loss along the route
Inspect accessible ducts and compare measurements at upstream and downstream locations.
10. Motor or VFD Problem
Motor speed or torque may be affected by:
Incorrect VFD frequency
Current limitation
Voltage problem
Motor overloading
Bearing condition
Control signal
Drive alarm
Use a clamp meter and tachometer to compare electrical current with actual RPM.
How to Measure AHU Air Velocity
Use an anemometer at:
Supply grilles
Return grilles
Outdoor-air intake
Exhaust outlet
Selected duct locations
For large grilles, take several readings across the area.
A single centre-point reading may not represent the average airflow.
Air Velocity vs Airflow Volume
Air velocity describes how quickly air moves.
Airflow volume describes the quantity of air delivered over time.
Airflow Volume = Average Air Velocity × Opening Area
Correct area measurement and representative velocity readings are required.
How to Measure Filter Pressure Drop
Step 1: Identify the Filter
Record the filter type and AHU.
Step 2: Confirm Fan Operation
Note fan speed and operating condition.
Step 3: Connect the Upstream Pressure Point
Connect one port before the filter.
Step 4: Connect the Downstream Pressure Point
Connect the second port after the filter.
Step 5: Zero the Meter
Follow the instrument procedure.
Step 6: Record the Reading
Document the pressure unit, fan speed and filter condition.
Useful AHU Troubleshooting Instruments
What to CheckRecommended InstrumentSupply and return air velocityAnemometerFilter and coil pressure dropDifferential-pressure meterMotor and fan RPMTachometerFan and motor vibrationVibration meterMotor currentClamp meterMotor and bearing temperatureThermal imaging cameraSpot temperatureInfrared thermometerTemperature and humidityTemperature-humidity meterLong-term room conditionsData logger
Using a Vibration Meter
Increasing fan vibration may indicate:
Imbalance
Bearing wear
Loose mounting
Misalignment
Belt problems
Dirty fan blades
Measure the fan and motor bearings at consistent locations.
Using Thermal Imaging
A thermal camera can help inspect:
Fan motor
Bearings
Belt and pulley area
Electrical connections
Cooling-coil temperature pattern
Condensation
Pipe insulation
Thermal imaging does not directly measure airflow.
Basic Low-Airflow Investigation
Step 1: Confirm the Complaint
Measure airflow rather than relying only on hand sensation.
Step 2: Record AHU Operating Conditions
Note fan speed, damper position and filter condition.
Step 3: Measure Filter Pressure Drop
Determine whether the filter is restricting airflow.
Step 4: Verify Motor and Fan RPM
Check for belt slippage or incorrect VFD settings.
Step 5: Inspect the Cooling Coil
Look for blockage and contamination.
Step 6: Check Dampers
Confirm actual blade position.
Step 7: Measure Airflow Along the System
Compare the AHU outlet, main duct, branch ducts and grilles.
Step 8: Check Motor Current and Vibration
Investigate fan, motor and bearing condition.
Low Airflow at Every Grille
Possible causes include:
Dirty filter
Blocked coil
Low fan speed
Incorrect fan rotation
Major fan problem
Main damper restriction
Low Airflow at One Grille
Possible causes include:
Closed branch damper
Blocked branch duct
Damaged flexible duct
Dirty grille
Local duct leak
Poor system balancing
This distinction helps narrow the investigation.
Common AHU Troubleshooting Mistakes
Avoid:
Replacing filters without measuring pressure drop
Increasing fan speed before identifying the restriction
Comparing airflow at different damper positions
Measuring only one point on a large grille
Ignoring belt slippage
Assuming the motor RPM equals fan RPM
Ignoring incorrect fan rotation
Checking the AHU but not the ductwork
Ignoring cooling-coil blockage
Recording airflow without its unit
Entering the AHU without correct isolation
AHU Testing Tools Supplier in Malaysia
MTM Precision supplies instruments for investigating low AHU airflow, blocked filters, incorrect fan speed, vibration, motor loading and unstable temperature or humidity.
Our range includes anemometers, differential-pressure meters, UNI-T UT371 tachometers, vibration meters, clamp meters, thermal imaging cameras, infrared thermometers and temperature-humidity data loggers.
Contact MTM Precision
MTM Precision Sdn. Bhd.
Showroom & Service Centre
No. 29-1 & 29-2, Jalan Bandar 18,
Pusat Bandar Puchong,
47160 Puchong, Selangor, Malaysia
🌐 Website: www.mtmpre.com.my
📧 Email: mtmpre@yahoo.com
📱 WhatsApp: +6016-660 7346
31 Aug 2026
