8 Reasons for Pressure Fluctuations – and What to Do About Them
Pressure fluctuations in a comfort-cooling variable air volume (VAV) HVAC system arise from various design, operational and external factors. These issues must be addressed to ensure consistent airflow, energy efficiency and occupant comfort.
Here are eight reasons you may experience pressure fluctuations in your VAV system.
1. Improper Ductwork Design
Suboptimally designed ductwork can sometimes lead to static pressure imbalances, particularly at branch points or near VAV boxes. Sharp bends, abrupt transitions and incorrect duct sizing disrupt airflow, creating turbulence and pressure losses. The static regain duct-design method is effective for minimizing pressure drops in complex systems, while the equal friction method suits smaller systems, ensuring balanced airflow and consistent pressure.
2. Inadequate Pressure Control
Pressure sensors are critical for monitoring and maintaining system stability, particularly during fluctuating demands, such as when occupancy in a space changes suddenly. Misplaced sensors, failed calibration and poorly commissioned pressure controls can cause fans and dampers to over- or underreact, leading to control oscillations, inaccurate readings and pressure instability.
3. Improper System Balancing
Balancing ensures that each zone receives the designed airflow. Incorrect damper positions – either too open or too closed – force the system’s fans to compensate, leading to pressure fluctuations, reduced efficiency and occupant discomfort. Proper balancing aligns airflow supply with demand, preventing instability.
4. Equipment Issues
Variable-speed drive (VSD) fans that fail to modulate properly can destabilize system pressure. Worn fan blades, belts and bearings disrupt airflow, while VAV box dampers that don’t open or close smoothly cause sudden pressure changes. Calibration errors and faulty programming further exacerbate inconsistencies.
5. System Leakage
Air leaks in ducts lower static pressure as the system compensates to meet demand. Leakage often results from improper duct sealing, incorrect installation or duct leakage specifications falling short of application requirements. Proper sealing ensures pressure consistency throughout the system.
6. Start-Up and Shutdown Sequences
Uncoordinated start-up and shutdown processes can cause pressure surges or drops. Fans must activate and begin pressurizing the system before dampers and other components engage to ensure proper system operation.
7. Inadequate Maintenance
Neglected maintenance can cause significant disruptions. Clogged air filters increase resistance, affecting pressure balance, while obstructed terminals, diffusers and grilles create localized buildups. Regular maintenance keeps system components functioning efficiently.
8. Overcomplex Systems
Systems with too many zones often struggle to maintain stable pressure as individual zones are continuously adjusting the airflow. Simplifying designs by grouping spaces with similar load profiles into single zones improves control and reduces variability.
There are several solutions that engineers can turn to for addressing these problems, either to correct an existing issue or to prevent it from happening in the first place:
- Optimize duct design using ASHRAE-approved methods
- Calibrate and position sensors correctly to maintain stable control
- Balance the system to align air supply with zone demands
- Inspect and maintain fans, dampers and components regularly
- Seal ducts effectively to prevent air leaks
- Synchronize start-up and shutdown processes to avoid surges
- Simplify zoning for more efficient and stable system operation
Price’s project and application engineers are ready to assist design engineers, building owners, architects and mechanical contractors with product choices that can optimize air distribution system efficiency.
Click here to find a sales representative or technical expert near you, or reach out to our team at airmovement@priceindustries.com.
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Mark Mahon is Business Development Manager for Fan Coils and Terminals for Price's Air Moving team. He is based out of Price's Progress facility in Lawrenceville, GA. Click here to connect with him on LinkedIn. |
