9 Common Causes of Poor Airflow in AAON HVAC Systems
AAON HVAC units are engineered for high-performance environmental control, often prized for their durability and precise modulation capabilities. However, even the most robust rooftop units (RTUs) and air handlers can suffer from airflow degradation. In a commercial setting, poor airflow isn’t just a comfort issue—it leads to soaring energy costs, premature component failure, and compromised indoor air quality (IAQ).
Identifying the root cause of airflow restriction in AAON systems requires a blend of mechanical intuition and an understanding of their specific design features, such as backward-curved impellers and sophisticated control logic.
Here are the nine most common culprits behind poor airflow in AAON parts systems and how to address them.
1. The Silent Performance Killer: Loaded Air Filters
It seems elementary, but the most frequent cause of airflow issues is neglected filtration. AAON units often handle high volumes of outside air. If the pleated filters are clogged with particulate matter, the static pressure increases significantly.
The AAON Edge: Many AAON units utilize differential pressure sensors. If your system is reporting a "High Static" alarm or if the Variable Frequency Drive (VFD) is ramping up to 100% without meeting the airflow setpoint, the filters are your first stop.
Solution: Implement a strict quarterly replacement schedule and consider upgrading to high-capacity MERV-rated filters that offer lower initial resistance.
2. VFD Programming and Communication Errors
Most modern AAON systems utilize Variable Frequency Drives (VFDs) to manage fan speeds. If the VFD is not receiving the correct 0-10V or 4-20mA signal from the building automation system (BAS) or the onboard controller, the fan may "hunt" or stay at a minimum speed.
The Issue: Parameters within the VFD may be incorrectly set, limiting the maximum Hertz (Hz) output, or a communication "timeout" may have forced the drive into a default low-speed bypass mode.
Solution: Verify that the VFD frequency matches the demand signal and ensure that the "Auto" mode is engaged rather than a manual "Hand" speed setting.
3. Slipping or Misaligned Belt Drive Assemblies
While many newer AAON units utilize direct-drive plenum fans, thousands of legacy units in the field rely on belt-driven blowers. A belt that has stretched over time or a pulley that has fallen out of alignment will result in a significant drop in Revolutions Per Minute (RPM).
Warning Signs: Squealing on startup or black dust (belt "fines") inside the blower cabinet.
The Impact: A 5% drop in fan RPM can lead to a much larger percentage drop in delivered CFM (Cubic Feet per Minute) due to the laws of fan physics.
Solution: Tighten belts to the manufacturer's tension specifications and use a laser alignment tool to ensure sheaves are perfectly parallel.
4. Obstructed or Malfunctioning Economizer Dampers
AAON units are famous for their economizer sections, which allow for "free cooling" using outside air. However, if the damper linkages become disconnected or the actuator fails, the dampers may get stuck in a closed or near-closed position.
The Restriction: If the return air dampers and outside air dampers are both closed—or fighting each other—the fan is essentially trying to pull air through a brick wall. This creates high vacuum pressure in the cabinet but zero airflow at the registers.
Solution: Periodically stroke the dampers through their full range of motion to ensure the seals aren't sticking and the linkages are secure.
5. Fouled Evaporator or Reheat Coils
Airflow doesn't just happen in the ducts; it has to pass through the dense fins of the cooling and heating coils. AAON units often feature thick, multi-row evaporator coils and optional dehumidification (hot gas reheat) coils.
The Build-up: Dust, microbial growth, or "white rust" on aluminum fins can bridge the gaps between fins.
The Result: This increases the pressure drop across the coil. In humid environments, a dirty coil can also hold onto water droplets (carryover), further blocking the air path.
Solution: Annual coil cleaning using a non-acidic, self-rinsing foam cleaner is essential for maintaining the design CFM.
6. Incorrect Fan Rotation (The "Reverse" Trap)
This is surprisingly common following a motor replacement or a change in electrical service. Because AAON often uses high-efficiency centrifugal or plenum fans, they will still move some air even when spinning backward—but only about 20% to 50% of their rated capacity.
How to Spot It: The fan will be noisier than usual, and the amp draw on the motor will be significantly lower than the nameplate rating because the fan isn't "catching" the air properly.
Solution: Check the rotation arrow on the fan housing. If it's reversed, swap any two leads on the load side of the motor starter or VFD.
7. Ductwork Leakage and Internal Liner Delamination
Sometimes the airflow leaves the AAON unit perfectly but never reaches the occupied space. Internal insulation (liner) inside the unit or the initial discharge ductwork can sometimes pull away from the metal walls.
The "Flap" Effect: A loose piece of insulation can act like a check valve, flapping upward under high pressure and choking off the main supply trunk.
Solution: During routine maintenance, inspect the "high-velocity" zones of the unit cabinet for any signs of peeling insulation or adhesive failure.
8. Closed Fire or Smoke Dampers
In large commercial facilities, the ductwork is equipped with fire and smoke dampers (FSDs). These are designed to spring shut in the event of a fire signal or a blown thermal link.
The Hidden Barrier: If a damper trips due to a false alarm or a localized heat spike, the AAON unit will continue to run, but the air will be dead-headed against the closed damper.
Solution: If airflow is zero in one wing of a building but fine in others, check the status of the FSDs in the main trunk line.
9. Improper Static Pressure Setpoints
AAON systems are frequently used in Variable Air Volume (VAV) applications. The system relies on a static pressure sensor located about two-thirds of the way down the ductwork to tell the fan how fast to spin.
The Error: If the static pressure transducer is out of calibration, or if the "Pitot tube" in the duct is clogged with dust, the controller may "think" the duct is pressurized when it isn't. This causes the fan to throttle back prematurely.
Solution: Calibrate the pressure transducer using a handheld manometer to ensure the BAS reading matches reality.
Conclusion: The Path to Peak Airflow
Maintaining an AAON HVAC system is an investment in the longevity of your facility. Airflow issues are rarely isolated; they are usually a symptom of either mechanical wear or a breakdown in the control logic. By systematically checking these nine areas starting with the filters and moving toward the VFD and dampers you can ensure your system delivers the precise climate control it was designed for.
Looking to maintain optimal airflow in your HVAC system? PartsHnC offers high-quality HVAC parts and supplies, including air filters, belts, fan motors, dampers, coils, and pressure sensors from trusted brands. Get the right parts fast to ensure your system delivers proper CFM, improves efficiency, and keeps your building comfortable all year!
FAQs
What causes poor airflow in AAON HVAC systems?Clogged filters, VFD errors, worn belts, stuck dampers, or dirty coils can all reduce airflow. Multiple issues often combine, so a full inspection is recommended.
How do dirty air filters affect airflow?
Clogged filters increase resistance, forcing fans to work harder and deliver less air. This causes uneven temperatures and higher energy use.
wLoose, stretched, or misaligned belts lower fan RPM and decrease CFM. Regular inspection and proper tension keep airflow consistent.
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