HVAC Air Distribution key terms

Conditioned air delivered from an air handling unit through ducts into a space to meet its heating, cooling, and ventilation loads. Its volume, temperature, and throw determine how the occupied zone is served.

Air drawn back from a conditioned space to the air handling unit for reconditioning, exhausting, or mixing with outdoor air. Return path design affects pressure balance and contaminant control.

Fresh air introduced from outside to dilute indoor contaminants and satisfy ventilation requirements. It carries a heating, cooling, and dehumidification load that must be accounted for in coil sizing.

The number of times the volume of air in a space is replaced per hour, calculated as supply airflow divided by room volume. Used for ventilation checks, dilution of contaminants, and rough load estimates.

The portion of a conditioned space normally occupied by people, typically from the floor to about 1.8 m (6 ft) and bounded away from walls and ceilings. Comfort criteria such as temperature and air speed apply here.

The horizontal or vertical distance a supply air jet travels from the outlet before its velocity decays to a specified terminal value, often 0.25 m/s (50 fpm). It indicates how far the jet reaches into the room.

The vertical distance a supply air jet falls below the outlet centerline as it travels, caused by buoyancy and entrainment. Excessive drop can create drafts in the occupied zone during cooling.

The process by which a moving air jet draws surrounding room air into itself, growing in size and slowing as it travels. Entrainment governs mixing, throw, and temperature equalization in the space.

The tendency of an air jet to stay attached to an adjacent surface, such as a ceiling, rather than separating. Designers use it to extend throw along ceilings and keep cold supply air out of the occupied zone.

Localized air movement that causes unwanted cooling of occupants, usually from air speeds above about 0.25 m/s (50 fpm) at normal room temperatures. Draft risk rises with low supply temperatures and poor outlet selection.

ADPI. The percentage of occupied-zone measurement points meeting combined criteria for effective draft temperature and air speed. It is used to evaluate and select air outlets for cooling conditions.

An index combining local air temperature and air velocity into a single comfort measure, computed relative to room average temperature. It is the temperature criterion used within ADPI evaluations.

A distribution strategy that supplies air at high velocity to entrain and mix with room air, producing relatively uniform temperature and contaminant levels. It is the most common approach in commercial spaces.

A strategy that supplies cool air at low velocity near the floor, letting it spread across the floor and rise as it warms, carrying contaminants upward to high-level returns. It relies on stratification rather than mixing.

The formation of horizontal layers of air with different temperatures, typically warmer near the ceiling and cooler near the floor. It affects ventilation effectiveness and return location in tall spaces.

A buoyant upward flow of air generated by a warm object such as a person, light, or equipment. Plumes drive contaminant transport and are central to displacement ventilation design.

A measure of how well supply air removes contaminants from the occupied zone compared with perfect mixing. Values above one indicate better-than-mixing performance, as in displacement systems.

A supply outlet mounted in or near the ceiling that discharges air radially or in multiple directions, often along the ceiling using the Coanda effect. Selection depends on throw, pressure drop, and noise.

A linear supply outlet with one or more narrow slots that produces a long, flat jet, often directed along a ceiling or wall. It suits perimeter zones and spaces needing uniform, draft-free coverage.

Variable air volume terminal unit that modulates the airflow to a zone in response to a thermostat, often with a reheat coil. It allows zone-by-zone control while a central fan varies total airflow.

The pressure exerted by air against duct walls, independent of velocity, expressed in pascals or inches of water. Fan control and duct sizing rely on it, and VAV systems often reset it to save energy.

The pressure associated with the speed of moving air, equal to one-half the density times velocity squared. Total pressure is the sum of static and velocity pressure.

The average speed of airflow through a duct cross-section, calculated from airflow divided by area. It influences friction loss, noise, and the balance between duct size and fan energy.

Pressure lost to resistance as air moves along ductwork, depending on velocity, duct size, roughness, and length. It is a primary input to fan total pressure and duct sizing calculations.

The straight-duct length that would produce the same pressure loss as a fitting such as an elbow or transition. Adding fitting equivalent lengths to straight runs gives total effective duct length.

A dimensionless factor, often denoted C, that quantifies pressure loss through a duct fitting relative to its velocity pressure. It lets designers compare elbows, tees, and transitions consistently.

A movable plate or blade assembly in ductwork or an outlet used to regulate or shut off airflow. Dampers serve balancing, volume control, fire protection, and pressure relief functions.

The process of adjusting dampers and fan settings so each zone receives its design airflow within acceptable tolerance. It is verified by measuring airflow at terminals and comparing with design values.

A device installed in ductwork that measures airflow using pressure differentials across a calibrated array. It supports balancing, control, and verification of outdoor air quantities.

A field method of measuring duct velocity by averaging velocity pressure readings at multiple points across the duct section. The result, multiplied by area, gives total airflow.

A device that closes automatically upon detection of heat or fire to prevent flame and smoke spread through duct penetrations of rated assemblies. Its location and access for inspection are code-regulated.

A damper designed to resist smoke movement through ductwork and often controlled by a smoke detection system. It is used where ducts cross smoke barriers and must be accessible for testing.

The pressure of a space relative to adjacent spaces, controlled by balancing supply and return or exhaust air. Positive pressure limits infiltration; negative pressure contains contaminants.

A control strategy that uses cool outdoor air for free cooling when conditions allow, reducing mechanical cooling. It involves modulating outdoor and return dampers and is common in air handling units.

A graph of the pressure a fan can develop across a range of airflows at a given speed. The operating point is where the fan curve intersects the system resistance curve.

A plot of the pressure required to move air through a duct system versus airflow, rising with the square of flow. Its intersection with the fan curve defines the actual operating point.

Relationships showing how fan airflow, pressure, and power change with speed and diameter. Airflow varies with speed, pressure with speed squared, and power with speed cubed.

Air lost from or drawn into ductwork through joints, seams, and penetrations, reducing delivered airflow and wasting fan energy. It is controlled by sealing and verified by leakage testing.