Airflow Measurement & Balancing for HVAC Engineers

Testing, Adjusting, and Balancing (TAB) verifies HVAC system delivers design airflows to all spaces. Proper measurement techniques and documentation ensure comfort, energy efficiency, and code compliance.

Duct Traverse Methods

Pitot Tube Traverse

Velocity pressure measurement:

$$v = 4005 \times \sqrt{\frac{P_v}{\rho}}$$

For standard air ($\rho = 0.075$ lb/ft³):

$$v = 4005 \times \sqrt{P_v}$$

Where:

$v$ = velocity (FPM)
$P_v$ = velocity pressure (“w.g.)

Traverse procedure:

Drill test holes per ASHRAE/SMACNA (equal area method)
Measure velocity pressure at each point
Calculate average velocity
Multiply by duct area for CFM

Number of measurement points:

Round duct: 20+ points (5 × 4 grid minimum)
Rectangular duct: Based on aspect ratio (6-64 points)

Accuracy: ±5% with proper technique

Flow Hoods (Capture Hoods)

Principle: Captures all air from diffuser/grille

Advantages:

Fast (30 seconds per measurement)
No duct access required
Easy for existing buildings

Disadvantages:

±15% accuracy (lower than pitot)
Sensitive to mounting position
Expensive ($2,000-$5,000)

Applications: VAV terminal verification, diffuser balancing

Balancing Procedure

Preliminary checks:
- Verify all dampers open
- Check belt tensions, sheave settings
- Measure motor amps (should be < nameplate)
Proportional balancing:
- Measure all terminal flows
- Calculate ratios to design
- Adjust furthest/highest pressure drop branch first
- Re-measure and iterate
Final measurements:
- Document actual CFM, static pressure
- Record damper positions
- Photograph balancing damper settings

Hydronic Balancing

Flow measurement methods:

Calibrated balancing valves: Read flow directly
Pressure drop method: $\Delta P$ across coil/valve correlates to flow
Temperature method: $\Delta T$ and heat transfer rate infer flow

Balance tolerance: ±10% of design flow

Documentation Requirements

TAB report must include:

Design vs. actual CFM for all terminals
Fan motor amps, voltage, speed
System static pressures
Outdoor air measurements
Damper positions
Deficiency list

Standards:

ASHRAE Standard 111: Practices for Measurement, Testing, Adjusting, and Balancing
AABC National Standards
NEBB Procedural Standards

Common TAB Issues

Low airflow: Dirty filters, closed dampers, incorrect sheaves
Noisy terminals: Excessive velocity (>2,000 FPM)
Unbalanced zones: Duct leakage, undersized branch dampers
Low system airflow: Undersized ductwork, excessive fittings

Practical Applications

New construction: Full TAB per contract documents
Renovation: Verify affected zones only
Troubleshooting: Selective measurements to diagnose complaints
Commissioning: TAB is critical component of Cx process

Related Technical Guides:

References:

ASHRAE Standard 111: Practices for Measurement, Testing, Adjusting, and Balancing of Building HVAC Systems
SMACNA HVAC Systems Testing, Adjusting and Balancing, 3rd Edition