HVAC System Performance
Understanding how an HVAC system performs — not just whether it's running, but whether it's running correctly — requires knowing a few key concepts: static pressure, fan curves, total airflow, and what affects efficiency.
Static Pressure
Static pressure is the resistance to airflow in an HVAC duct system, measured in inches of water gauge (in. w.g.). Every component that air flows through — ductwork, elbows, filters, coils, diffusers — contributes pressure drop (resistance). The sum of all these resistances is the total external static pressure (ESP) the fan must overcome.
Key relationships:
- Higher static pressure → lower airflow (for a given fan speed)
- Longer duct runs, more elbows, and finer filters all increase static pressure
- Dirty filters and coils significantly raise static pressure over time
- Closed dampers increase static pressure by restricting airflow paths
Design documents specify the expected static pressure at design airflow conditions. TAB technicians measure actual static pressure and compare to design to diagnose system problems.
ℹ Static pressure units
Fan Curves
A fan curve is a graph that shows how a specific fan performs across a range of airflows and static pressures. The fan curve has:
- X-axis: airflow (CFM)
- Y-axis: static pressure (in. w.g.)
- Fan curve line: the pressure the fan can develop at each airflow rate
- System curve: the resistance of the duct system at each flow rate (increases as the square of flow)
- Operating point: where the fan curve and system curve intersect — actual operating conditions
If the actual system curve is higher than the design system curve (higher resistance than expected), the operating point shifts left — lower airflow. This is exactly what happens when filters are dirty, dampers are closed, or ductwork is undersized. TAB technicians use fan curves and field measurements to diagnose whether the fan is sized and set correctly.
Total System Airflow
Total supply airflow (total CFM) is the volume of air delivered by the air handling unit per minute. This number must match the design to ensure each zone receives its proportional share when the distribution system is balanced.
Total airflow can be measured several ways:
- Summing all terminal devices — add up measured CFM at every supply register. This is the most common approach in TAB.
- Duct traverse — measure velocity at multiple points across a duct cross-section using pitot tubes and calculate total flow from average velocity × duct area.
- Fan curve method — measure static pressure and RPM, then read CFM from the fan curve. Less accurate due to field installation variations.
System Efficiency
HVAC system efficiency has several dimensions:
- Fan efficiency — how effectively the fan converts motor power to airflow. Expressed as total efficiency (%) or specific fan power (W/CFM).
- Cooling equipment efficiency — COP (coefficient of performance), EER (energy efficiency ratio), or IEER for part-load conditions.
- Distribution efficiency — whether the air delivered actually reaches occupied zones or is wasted through duct leakage and poor mixing.
- Control efficiency — whether the system modulates to match actual loads rather than running at full capacity regardless of conditions.
ASHRAE Standard 90.1 sets minimum efficiency requirements for commercial HVAC equipment and maximum fan power limits. A TAB report often includes fan power calculations to verify compliance.
Performance Diagnostics
Common performance problems and their typical causes:
| Symptom | Likely Cause |
|---|---|
| Total airflow below design | Dirty filter, dirty coil, closed damper, duct obstruction, wrong fan speed |
| Total airflow above design | Fan too fast, ductwork larger than design, external static lower than expected |
| High static pressure | Dirty components, closed dampers, undersized ductwork |
| Low static pressure | Duct leakage, missing sections of ductwork |
| Motor amperage above nameplate | Fan operating at higher than design speed; system resistance lower than design |
| Zone temperature not reaching setpoint | Insufficient supply CFM to zone, controls sequence issue, coil capacity problem |