Air balancing procedures and instruments

Air balancing sounds simple: measure the air at each outlet and adjust dampers until the numbers match the drawings. In practice it requires the right instruments, a systematic procedure and an understanding of how changes at one outlet affect every other outlet on the same system. A disciplined approach gets the job done in fewer passes, with less throttling and lower fan energy.

This guide describes the instruments used for commercial air balancing and the procedure typically followed, based on ASHRAE Standard 111 and the procedural standards of the TAB certification bodies.

Instruments

All instruments should have current calibration records, and the instrument list with calibration dates belongs in the TAB report. Capture hoods in particular must be used correctly: the hood should fully cover the outlet, and the reading may need a correction factor for diffuser type and back pressure as described by the manufacturer.

Measuring airflow in ducts

The most accurate way to measure total airflow in a duct is a pitot tube traverse. The technician drills test holes in a straight section of duct and measures velocity pressure at a grid of points across the cross section.

• Traverses are most accurate in straight duct runs, ideally several duct diameters downstream and a shorter distance upstream of elbows, transitions and dampers. Poor locations produce uneven velocity profiles and inaccurate results.
• Point locations follow either the equal-area method or the log-Tchebycheff method described in ASHRAE Standard 111. Log-Tchebycheff placement generally gives better accuracy for a given number of points.
• Each velocity pressure reading is converted to velocity. At standard air density, velocity in m/s is approximately 1.29 times the square root of the velocity pressure in pascals, and in imperial units, velocity in fpm is about 4005 times the square root of velocity pressure in inches of water. Velocities are averaged, not velocity pressures.
• Airflow equals average velocity times duct area.

Density corrections matter at altitude. In Calgary, at roughly 1,000 m above sea level, air density is around 10% lower than at sea level, which affects velocity calculations and fan performance. See air density, altitude and HVAC design in Calgary and the Rockies.

The balancing procedure

1. Preparation

1. Review the drawings, specifications, equipment submittals and sequences of operation.
2. Prepare report forms listing every fan, terminal unit and outlet with its design airflow.
3. Confirm the system is ready: ductwork complete and sealed, filters installed, coils clean, all volume and fire dampers open, access doors closed, controls operating and fan rotation correct.
4. Set VAV systems to a full-flow condition, usually by commanding all terminal units to maximum airflow, or to the design diversity condition if specified.

2. Fan total airflow

1. Measure fan speed, motor current and the static pressure profile: entering and leaving the fan and across filters, coils and other components.
2. Traverse the main supply duct, and the return and outdoor air ducts where possible.
3. Compare total airflow with design. If it is significantly low or high, adjust before balancing branches. Use the fan laws to estimate the required speed change. See fan laws and how to apply them.

3. Branch and outlet balancing

The proportional (ratio) method is widely used because it reduces the number of repeat measurements.

1. Measure all outlets on a branch and calculate each one's percentage of design.
2. Identify the outlet with the lowest percentage. Leave its damper fully open; it becomes the reference outlet.
3. Adjust the other outlets so that each has the same percentage of design as the reference outlet. Because adjusting one outlet affects others proportionally, they stay in ratio.
4. Repeat for each branch, then balance the branches against one another at the branch dampers using the same principle.
5. When all outlets and branches are in proportion, adjust the fan speed so that every outlet reaches 100% of design together.

This approach leaves at least one path with dampers fully open, so the fan delivers design flow at the lowest possible static pressure.

4. VAV terminal units

For each VAV box:

1. Verify that the box airflow sensor reading matches the measured airflow at maximum and minimum setpoints. If not, adjust the controller's flow calibration factor so that the controller reads correctly.
2. Confirm that the box can achieve maximum airflow with the available duct static pressure.
3. Balance the outlets downstream of the box proportionally at maximum airflow.
4. Record the box static pressure, damper position and calibration factors.

The duct static pressure setpoint is then set so that the most demanding box can reach its maximum airflow. Static pressure reset strategies should then lower it when demand drops. See VAV box types and selection.

5. Outdoor air and pressurization

Measure outdoor air quantity at minimum and economizer positions, and confirm it meets the design minimum, which is often set to satisfy ASHRAE 62.1 or the applicable provincial code. Where room or building pressure relationships are specified, measure differential pressures and adjust return or exhaust airflow accordingly.

6. Final checks and marking

Record final fan speed, motor current, static pressures and VFD settings. Mark the final position of every balancing damper. Report any outlets that could not reach design, with the likely cause.

Common problems found during air balancing

• Missing or inaccessible balancing dampers.
• Closed fire or volume dampers left from construction.
• Duct leakage that prevents outlets from reaching design flow.
• Fans running backwards or with the wrong drive installed.
• Poorly placed VAV box flow sensors that give unstable readings.
• Excessive static pressure due to dirty filters, undersized ductwork or system effect at the fan.

Finding these during balancing is common, and they should be reported promptly rather than masked by increasing fan speed.

Frequently asked questions

Can balancing be done with a capture hood only?

Capture hoods are convenient for outlets but are less accurate for total system airflow. A duct traverse remains the preferred method for fan totals, with hood readings used for individual outlets.

Why balance at maximum airflow on VAV systems?

Balancing at maximum ensures each outlet receives its share of air when the zone needs it most. At lower flows, the VAV box controls total zone airflow and the outlets stay roughly in proportion.

Key takeaways

• Calibrated micromanometers, pitot tubes and capture hoods are the core air balancing instruments.
• Duct traverses using log-Tchebycheff or equal-area points give the most reliable fan totals.
• Balance proportionally, leaving one path wide open, then trim fan speed to meet design.
• VAV box flow sensors must be calibrated against measured airflow before static pressure setpoints are finalized.
• Report installation problems rather than compensating with higher fan speed. Our air balancing team follows these procedures on commercial projects.