HVAC & TAB Glossary

Clear definitions of common terms used in air balancing, TAB, ventilation, and HVAC performance — organized alphabetically.

ACH (Air Changes per Hour) (ACH): The number of times per hour that the total air volume of a space is supplied by the HVAC system. Calculated as (CFM × 60) ÷ room volume in cubic feet.
Air Handling Unit (AHU): The central mechanical unit that conditions and distributes air — typically containing a fan, heating/cooling coils, a filter section, and dampers.
Balometer (Flow Hood): An instrument that captures airflow at a supply, return, or exhaust outlet and directly measures the volumetric flow rate in CFM.
CFM (Cubic Feet per Minute) (CFM): The standard US unit of volumetric airflow. Design CFM values are assigned to each supply and return outlet by the mechanical engineer.
Damper: An adjustable plate inside a duct that restricts or allows airflow. Balancing dampers are used by TAB technicians to proportion airflow; control dampers open and close based on zone demand.
Design Intent: The mechanical engineer's documented objectives for an HVAC system — what conditions (temperature, humidity, airflow, pressure) each space should maintain.
DCV (Demand-Controlled Ventilation) (DCV): An HVAC control strategy that modulates outdoor air intake based on measured occupancy (typically using CO₂ sensors), saving energy when spaces are not fully occupied.
EER / IEER: Energy Efficiency Ratio and Integrated EER — measures of cooling equipment efficiency. Higher values indicate greater efficiency. IEER accounts for part-load operation.
External Static Pressure (ESP): The total resistance to airflow from all ductwork, coils, filters, dampers, and terminal devices — everything outside the fan housing. Measured in inches of water gauge (in. w.g.).
Fan Curve: A graph showing a fan's performance (CFM vs. static pressure) across its operating range. The intersection of the fan curve and the system resistance curve determines the actual operating point.
HEPA Filter: High-Efficiency Particulate Air filter — removes ≥99.97% of particles ≥0.3 µm. Required in cleanrooms, operating rooms, and isolation rooms. MERV 17 equivalent.
Hydronic System: An HVAC system that uses water (chilled water, hot water, or condenser water) to transfer heating or cooling energy. TAB includes balancing these water flows alongside air flows.
in. w.g. (Inches of Water Gauge) (in. w.g.): The standard unit for measuring HVAC static pressures. 1 in. w.g. = 249 Pa. Typical residential systems have 0.3–0.8 in. w.g. external static pressure.
ISO 14644: The international standard defining cleanroom classifications by airborne particle count (ISO classes 1–9) and specifying test methods for cleanroom performance verification.
Manometer: An instrument that measures pressure or pressure differential. Used in TAB to measure duct static pressure, filter pressure drop, fan performance, and room-to-room pressure relationships.
MERV (Minimum Efficiency Reporting Value) (MERV): A filter efficiency rating per ASHRAE 52.2. Ranges from MERV 1 (coarse) to MERV 16 (hospital-grade). HEPA filters are rated MERV 17–20.
NEBB: National Environmental Balancing Bureau — a certification organization for TAB firms and technicians. Publishes procedural standards for TAB, sound testing, and commissioning.
AABC: Associated Air Balance Council — a certification organization for independent TAB agencies. Publishes National Standards for TAB and emphasizes third-party independence.
Pitot Tube: A probe that measures velocity pressure (the difference between total pressure and static pressure) in a duct. Combined with a manometer, used to calculate air velocity and perform duct traverses.
Proportional Balancing: A systematic TAB method where outlets are adjusted proportionally toward the outlet with the lowest measured/design ratio, ensuring balanced distribution without starving any branch.
RH (Relative Humidity) (RH): The amount of moisture in the air as a percentage of the maximum moisture air can hold at that temperature. ASHRAE recommends maintaining 30–60% RH in occupied spaces.
Static Pressure: The pressure of air acting perpendicular to flow — the resistance force in a duct system. Higher static pressure means the fan must work harder to move the same airflow.
TAB (TAB): Testing, Adjusting, and Balancing — the process of measuring, adjusting, and documenting HVAC system performance to verify compliance with design specifications.
Tachometer: An instrument that measures rotational speed (RPM) of fans and pumps. Used in TAB to verify fan speed and calculate sheave adjustments for proper airflow.
Total Static Pressure: The sum of static pressure and velocity pressure at a point in a duct. Used in fan performance calculations.
VAV Box (VAV): Variable Air Volume terminal device that modulates airflow to a zone based on thermostat demand. Contains an airflow sensor and damper actuator. TAB sets minimum and maximum airflow setpoints.
VFD (Variable Frequency Drive) (VFD): An electronic controller that varies fan or pump motor speed by adjusting the electrical frequency supplied to the motor. Allows precise airflow control and significant energy savings at part load.
Ventilation Rate Procedure (VRP): The primary calculation method in ASHRAE 62.1 for determining outdoor air requirements — combining a per-person rate and a per-floor-area rate for each space type.

Missing a term? These definitions cover the concepts most relevant to air balancing and TAB work. For manufacturer-specific terminology or code-specific definitions, refer to the relevant standard.