Troubleshooting Low Airflow in AHUs
Troubleshooting Low Airflow in AHUs
Introduction
An often-overlooked but critical issue in HVAC systems is low or inadequate airflow from the Air Handling Unit (AHU). When the AHU struggles to deliver the design air volume across the supply and return circuits, you’ll likely notice comfort problems (uneven temperatures, stuffy zones), increased energy bills and excess wear on system components. In this article we dive into why airflow drops in AHUs, how to diagnose it, and what you can do to rectify the problem.
Understanding Airflow & Why It Must Be Right
Airflow in an AHU is expressed in cubic-feet per minute (CFM) or cubic-metres per hour (m³/h). It ensures each zone receives the correct volume of conditioned air to meet thermal load, ventilation and comfort requirements. If airflow is too low:
- Heating or cooling capacity is under-utilised → zones don’t reach set-point.
- Duct pressures change, fan motors may run harder or stall.
- Filtration efficiency drops, indoor air quality may suffer.
- Therefore, maintaining designed airflow is foundational to both comfort and system longevity.
Common Causes of Low Airflow in AHUs
Clogged or Dirty Filters & Coils
Dirty filters restrict air entering the unit and increase pressure drop across the AHU. Dirty coils cause inefficient heat exchange and may force the fan to compensate.
Blocked or Undersized Return Air Path / Ducts
When the return side is restricted (blocked grilles, closed dampers, undersized ducts), the supply side airflow reduces accordingly.
Energy Design Systems
Fan/Blower Issues (Motor, Belts, Blades)
Worn belts, faulty motor or damaged/bladed fan wheel reduce delivered airflow.
Duct Leakage or Obstruction
Leaks in ducts or obstructions (nesting, collapsed duct, debris) lower effective flow into zones.
Poor AHU or System Design / Oversizing or Undersizing
AHU or ductwork incorrectly sized will adversely impact airflow. A too-small unit may be starved, and an oversized one may cycle and fail to deliver steady flow.
Changes in System Operation or Settings
Closed dampers, auto-changeover settings, fan speed reduced for energy savings—all can reduce flow if not calibrated properly.
Diagnostic Step-by-Step for Low Airflow
Identify Symptoms: Check for weak air from supply registers, significant temperature differences between zones, longer run times, higher static pressure.
Measure Static Pressure & CFM: Use a manometer to measure total external static pressure across the AHU and compare CFM to design values.
Inspect Filters and Coils: Remove filters, check pressure drop, inspect coils for dust. Replace/clean accordingly.
Check Fan/Blower Assembly: Inspect belts, motor current draw, blade cleanliness, bearing conditions and alignment.
Assess Return and Supply Ducts: Walk the duct path, check for obvious blockages, verify all grilles/registers are open, inspect duct size and integrity.
Perform Airflow Path Review: Confirm no dampers are closed inadvertently, check fan speed setting, ensure controls are properly configured.
Seal Duct Leaks & Repair Obstructions: Use appropriate duct sealant/mastic, repair collapsed or damaged ducts.
Compare to Design Documentation: Refer to original design/commissioning reports and ensure current performance aligns.
Practical Fixes & Preventive Measures
Replace filters regularly: Follow manufacturer guidelines—dirty filters are the simplest cause of low airflow.
Schedule coil cleaning: Annually for most systems; more often in dusty/industrial environments.
Maintain blower system: Keep belts tight, motors lubricated (if required), blades clean.
Ensure adequate return air: Avoid blocking return grilles; if necessary, install additional returns.
Inspect and repair ductwork: Fix leaks and damage; ensure duct sizing aligns with loads.
Use proper fan settings: Variable speed fans should be set correctly for zone loads; constant volume fans should run full speed when in operation.
Monitor system performance metrics: Log static pressure, CFM, motor current, energy use — trends will signal degradation early.
Include airflow checks in maintenance schedule: Periodic measurement and documentation will keep the system aligned with design intent.
Why Fixing Low Airflow Matters
Improved comfort: Adequate airflow ensures even temperature distribution, reduces hot/cold spots.
Better energy efficiency: A system working against restriction uses more power and wears faster.
Equipment longevity: Motors and fans that run harder or non-ideally will shorten their life.
Indoor air quality: Reduced airflow can lead to poor filtration, stagnant zones and higher pollutant concentrations.
System reliability: Faults are more easily identified early when baseline airflow is known and maintained.
Conclusion
Low airflow in AHUs is a critical issue that often signals underlying problems affecting system performance, comfort and operational cost. By systematically diagnosing and proactively maintaining filters, coils, fans, ducts and controls, facility managers and HVAC technicians can restore proper airflow, bolster system efficiency and extend equipment life. Don’t wait for the zones to get uncomfortable — airflow checks should be part of your standard maintenance regimen.
FAQs
Q1: How often should I check airflow in my AHU?
A1: At minimum annually, but high-demand systems (e.g., commercial buildings) benefit from bi-annual or quarterly checks of static pressure, CFM and duct integrity.
Q2: What is a typical pressure drop across filters that signals replacement?
A2: It depends on the filter and system, but when static pressure rise exceeds manufacturer’s recommendation (often ~0.5 in W.C. or ~125 Pa for many filters) you should replace filters.
Q3: Can low airflow cause compressor or coil damage?
A3: Yes — in cooling mode, low airflow over the coil can cause freeze-up, increased head pressure, compressor overload and eventual breakdown.
Q4: If I increase fan speed to compensate low airflow, is that okay?
A4: No — merely increasing fan speed without addressing the root cause can lead to higher static pressure, greater energy consumption and possible duct leakage or damage.
Q5: Should I call a professional technician for this?
A5: Yes, especially if measuring airflow/pressure, performing motor diagnostics or repairing ducts. Some tasks are suitable for in-house maintenance, but qualified HVAC professionals bring instrumentation and expertise to ensure accuracy and safety.
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