Air distribution glossary

A ceiling unit in which primary air from nozzles induces room air through a water coil, which cools (or heats) it before it is discharged into the room.

The percentage of points in the occupied zone where the effective draft temperature is between −3 and +2 °F (−1.7 and +1.1 K) and the air speed is below 70 fpm (0.35 m/s). Measured according to ASHRAE 113, for cooling; 80% or more is a usual design target.

Go deeper: Thermal comfort

The time since the air at a point entered the room. Where the local mean age is high, the air has been breathed and loaded with pollutants longer; good mixing keeps it even across the occupied zone.

Go deeper: Indoor air quality

The airflow supplied to a room in one hour divided by its volume. A convenient way to compare rooms, but it says nothing about where the air goes once it is inside.

The equipment that filters, heats or cools, and moves the air through the duct system, mixing outdoor air with return air.

The effective area of a diffuser, used in the field to calculate the airflow from a measured velocity: airflow = Ak × velocity. It is specific to each model and size, and to where and with which instrument the velocity is measured.

55: Thermal Environmental Conditions for Human Occupancy (comfort). 62.1: Ventilation and Acceptable Indoor Air Quality (outdoor air, Ez). 70: Method of Testing the Performance of Air Outlets and Air Inlets (the catalogue data). 113: Method of Testing for Room Air Diffusion (ADPI).

Measuring and adjusting the airflow of every outlet to its design value after installation, so each room gets the air it was designed for.

A digital model of the building in which each product is an object with its geometry and data. A Revit family is a product’s object for Autodesk Revit.

In ASHRAE 62.1, the region between 3 and 72 in (75 and 1800 mm) above the floor and more than 2 ft (600 mm) from walls. The outdoor air requirement is defined for this zone.

Go deeper: Air distribution effectiveness

The force that makes warm air rise and cold air sink. It is why warm air supplied from a high ceiling tends to stay up there, and why a cold jet drops once it loses speed.

Go deeper: Heating from high ceilings

A damper adjusted from the room through a cable, without opening the ceiling: useful in drywall or finished ceilings.

An instrument placed over a diffuser to read its airflow directly. Its reading can be less accurate on diffusers whose jets swirl or leave at a shallow angle, so the measurement method matters.

The space between the suspended ceiling and the structure above, where ducts, plenums and services run. Architects want the highest ceiling, so this space is often tight.

Numerical simulation of airflow and heat transfer in a room. A full study maps velocity, temperature, comfort (PMV/PPD), age of air, CO₂ and Ez before construction.

Go deeper: CFD validation

Airflow in cubic feet per minute (1 cfm = 0.472 L/s), velocity in feet per minute (100 fpm = 0.508 m/s), pressure in inches of water (1 in. w.g. = 249 Pa).

For a ceiling diffuser, the distance from the diffuser to the wall in the direction of the jet, or to the midplane between two diffusers: the room each jet has to fill.

A jet discharged along a surface clings to it, because the pressure between the jet and the surface drops. It keeps cold supply air along the ceiling while it mixes. Beams, light fixtures or a slow, cold jet can break the attachment.

Go deeper: Thermal comfort

Two jets that meet head-on, for example between two diffusers, turn downward together. Where they meet, and how fast they still are, decides whether people below feel a draft.

A self-adjusting damper that holds a preset airflow across a range of duct pressures, with no power or controls.

Blades in the duct or plenum that adjust the airflow for balancing. In an opposed-blade damper, neighbouring blades turn in opposite directions, which keeps the airflow more even downstream.

A system that treats and supplies the outdoor air separately from the system that heats and cools the space (radiant panels, chilled beams, fan coils). Its diffusers often carry small airflows that still have to reach and mix with the whole room.

Outdoor air adjusted to the actual occupancy, often from CO₂ sensors. The airflow at the diffusers then varies, so they must perform well at low airflow too.

A diffuser discharges air in several directions, usually from the ceiling, to mix it with the room. A grille is a louvred face that supplies or returns air in one direction; a register is a grille with a damper.

The speed of the air as it leaves the diffuser face. It sets how strongly the jet pulls in room air: a fast jet induces and mixes, a slow one carries the supply air into the room almost unchanged.

Go deeper: High induction

Air distribution that supplies cool air slowly near the floor. It spreads across the floor and rises with the heat from people and equipment, leaving warm, used air near the ceiling. It suits cooling more than heating.

Unwanted local cooling of the body by air movement. Cold, fast air on the neck, the head or the ankles is the most common comfort complaint in mechanically ventilated buildings.

Go deeper: Thermal comfort

How far a horizontal jet falls by the end of its throw, measured from the diffuser to the lower edge of the jet. It matters most for sidewall grilles and cold jets, which sink as they slow down.

An exposed duct that distributes air along its whole length through perforations or slots, instead of feeding separate diffusers. Suited to open ceilings, where there is no ceiling for a jet to cling to.

Go deeper: Architectural duct diffusers

Cold supply air that falls into the occupied zone before it has mixed, usually because the jet is too slow to stay on the ceiling, for example at a VAV minimum airflow. Occupants feel it as a cold draft under the diffuser.

Go deeper: Thermal comfort

Combines the temperature difference from the room setpoint and the local air speed into one number: EDT = (tx − tc) − 0.07 (Vx − 30), in °F and fpm; in SI, (tx − tc) − 8 (Vx − 0.15), in K and m/s. It is the basis of ADPI.

Go deeper: Thermal comfort

How well the outdoor air supplied to a zone actually reaches its breathing zone (ASHRAE 62.1). The required zone outdoor airflow is Voz = Vbz / Ez: an Ez below 1 means more outdoor air to supply. Table 6-4 of the standard gives default values by configuration: 1.0 for cool air supplied at the ceiling; 0.8 for warm air supplied and returned at the ceiling, unless the supply is less than 15 °F (8 °C) above the room and the jet still reaches 150 fpm (0.8 m/s) within 4.5 ft (1.4 m) of the floor, in which case 1.0. The standard also allows Ez to be calculated (Normative Appendix C), which can give higher values.

Go deeper: Air distribution effectiveness

The visible part of a diffuser, in the ceiling plane. Its pattern of openings shapes the jets; its finish is what the architect sees.

A jet that travels through the room without a surface to cling to, such as a nozzle jet in a hall or air supplied under an open ceiling. It spreads and slows faster than an attached jet and responds directly to buoyancy.

Go deeper: Long throws

Ready-to-edit specification text for a product, which the engineer adapts and includes in the project specifications.

A filter that captures at least 99.97% of particles of 0.3 µm, the size hardest to capture.

Go deeper: Indoor air quality

A diffuser designed to discharge air at high velocity from a pressurising plenum, so that each jet pulls in a large amount of room air and mixes at the ceiling. EffectiV’s NEX, OTO, PLAY and AXO families work on this principle.

Go deeper: High induction

Room air drawn into a moving jet. The induced air mixes with the supply air, so the jet warms up (in cooling) and slows down before it reaches people: the basis of draft-free mixing.

Go deeper: High induction

The total air moving in the jet (supply plus induced room air) divided by the supply air, at a stated point of the jet. The higher the ratio, the closer the jet is to room temperature when it slows down.

Go deeper: High induction

The manufacturer’s instructions to install, adjust and maintain a product.

A jet supplied at room temperature. Catalogue throws are isothermal; a cold jet behaves differently (it sinks once it leaves the ceiling) and a warm jet tends to rise.

A flexible duct bent sharply just before the diffuser, often because the ceiling void is shallow. The air arrives skewed, so most of it leaves one side of the diffuser: drafts on one side, a warm zone on the other, and more noise.

Go deeper: High induction

A lay-in (T-bar) ceiling holds tiles, flat or tegular (with a stepped edge), in a suspended grid; a drywall ceiling is continuous; an open ceiling leaves the structure and ducts exposed. Each calls for its own diffuser mounting.

Go deeper: Architectural diffusers

A long, narrow diffuser with one or more slots, often run continuously along a window or a wall. It can wash a glazed façade or throw along the ceiling, and blends into the architecture.

The average temperature of the surfaces that exchange heat by radiation with a person. A cold window or a sunny curtain wall changes it, and with it comfort, even at the same air temperature.

The ASHRAE 52.2 rating of an air filter, from 1 to 16, by how well it captures particles from 0.3 to 10 µm. A rating with the suffix “A” (MERV-9A, for example) is obtained after the optional conditioning step of Appendix J, which exposes the filter to fine particles to show the efficiency it can lose in service, for example when an electrostatic charge fades.

Go deeper: Indoor air quality

Air distribution that supplies air at a velocity high enough to mix it with the whole room, so temperature and air quality are nearly uniform. Ceiling diffusers work this way.

Go deeper: Room First

A single number rating a sound spectrum against the NC curves. Diffuser catalogues give the NC of the diffuser alone, usually with 10 dB of room attenuation; the ASHRAE Handbook recommends design values by type of space.

Go deeper: Airflow performance

The round or rectangular inlet that connects the diffuser or its plenum to the duct. Its nominal size is the one used to read the performance tables.

The airflow divided by the area of the inlet collar. Performance tables are organised by it; throw, pressure drop and sound all rise with it.

Go deeper: Airflow performance

A diffuser with one or more nozzles that throw air over long distances, for halls, atriums, auditoriums and high spaces. The nozzles can be aimed to set the throw and the spread.

Go deeper: Long throws

The part of the room where people are, where comfort is assessed. ASHRAE 55 generally takes it from the floor up to 6 ft (1.8 m), more than 3.3 ft (1 m) from exterior walls, windows and fixed HVAC equipment and 1 ft (0.3 m) from interior walls.

Go deeper: Thermal comfort

The temperature people actually feel, combining air temperature and the mean radiant temperature of the surfaces around them. At low air speed it is close to their average.

Air brought in from outside to dilute the pollutants produced indoors. ASHRAE 62.1 sets the minimum quantity per person and per unit of floor area.

Go deeper: Air distribution effectiveness

The manufacturer’s tested tables for each size: airflow, throw, pressure drop and NC by neck velocity, measured according to ASHRAE 70. They describe the diffuser alone in a standard test room, not your room.

Go deeper: Airflow performance

The box above a diffuser face that receives the duct and distributes the air to the face. A plenum fed from the side and tall enough lets the air pressurise and leave evenly on every side, whatever the duct connection.

Go deeper: High induction

PMV predicts the average thermal sensation of a large group on a scale from −3 (cold) to +3 (hot); PPD is the share of people expected to be dissatisfied at that PMV. A CFD study can map both across a room.

Go deeper: CFD validation

The pressure the fan must supply to push the selected airflow through the diffuser and its plenum, in in. w.g. or Pa. It adds to the losses of the duct system and changes with the square of the airflow.

Go deeper: Airflow performance

A product offered in place of the one specified. The engineer has to check that it really matches the specified performance (throw, induction, NC, behaviour at minimum airflow), not just the size.

Room air drawn back to the air handler through return grilles, part of it to be exhausted and the rest mixed with outdoor air. Where the return sits decides which air leaves the room: the warmest, or mixed air.

Go deeper: Energy efficiency

The difference between the sound power a diffuser emits and the sound pressure heard in the room, which depends on the room’s volume and finishes. Catalogue NC values usually assume 10 dB; a small, hard room attenuates less.

EffectiV’s design approach: define what the occupied room needs first, then the air distribution that delivers it, then the airflow, the ductwork and, last, the equipment sized to fit.

Go deeper: Room First

The temperature the control system tries to hold at the thermostat. If the air at the thermostat is not the air people feel, the system holds the wrong room at the right temperature.

Go deeper: Energy efficiency

Supply air that reaches the return without passing through the occupied zone. Air that short-circuits is conditioned and ventilated air that never serves anyone.

Go deeper: Indoor air quality

The width the jet covers at the end of its throw, or the angle at which it diverges. A wide spread covers more ceiling with a shorter throw; a narrow one reaches farther.

A part of the room where the air barely moves. It tends to be too warm or too cold, and the air there is older than in the rest of the room.

Go deeper: Thermal comfort

Static pressure pushes on the duct walls; velocity pressure is the pressure of the moving air; total pressure is their sum. Diffuser pressure losses are usually stated in total pressure.

Layers of air at different temperatures, warm above and cool below. Wanted in displacement ventilation, it is a problem when heating from the ceiling: the warm air stays up and leaves through the return while the occupied zone stays cold.

Go deeper: Energy efficiency

The drawings and data the contractor submits for approval before ordering: dimensions, performance, finish, accessories.

The difference between the supply air temperature and the room temperature. The larger it is in cooling, the more the jet needs induction to avoid drafts; in heating, warm air with a large ΔT supplied from the ceiling tends to stay up.

Go deeper: Heating from high ceilings

A radial jet that also rotates around the diffuser. The rotation adds contact between the jet and the room air, so it mixes quickly over a short distance.

Go deeper: High induction

A ceiling diffuser whose face gives the jet a rotation around the diffuser, for fast mixing and short throws, well suited to high airflow per square foot.

Go deeper: High induction

The 50 fpm throw divided by the characteristic length of the room. For each type of diffuser, the ASHRAE Handbook gives the range of T50/L that gives the highest ADPI; it is the classic way to check a selection for comfort.

Go deeper: Thermal comfort

The jet velocity at which a throw is measured. A lower terminal velocity gives a longer throw: the 50 fpm throw always reaches farther than the 150 fpm throw of the same diffuser.

The condition of mind that expresses satisfaction with the thermal environment (ASHRAE 55). It depends on air temperature, radiant temperature, air speed and humidity, and on people’s clothing and activity. ASHRAE 55 aims for at least 80% of occupants satisfied.

Go deeper: Thermal comfort

A diffuser that changes its discharge pattern with the supply air temperature: along the ceiling in cooling, down toward the occupied zone in heating. EffectiV’s ACTIF models do it with a thermal element (wax piston), with no power or controls.

Go deeper: Heating from high ceilings

The distance from the diffuser to the point where the highest velocity in the jet has dropped to a stated terminal velocity. Catalogues give several throws for one selection, one per terminal velocity (commonly 150, 100 and 50 fpm), measured with isothermal air.

Go deeper: Airflow performance

The reduction of airflow in a VAV system, down to the box’s minimum airflow. At low airflow, a diffuser whose discharge velocity drops too far lets cold air fall into the occupied zone.

Go deeper: Thermal comfort

Ultraviolet light in the UV-C band (below 280 nm), which inactivates bacteria and viruses by damaging their genetic material. In a UV diffuser, the air is exposed in a lit chamber before it enters the room, out of sight of occupants.

Go deeper: Indoor air quality

A VAV system varies the airflow to each zone to follow the load at a constant supply temperature; a CAV system supplies a constant airflow and varies the temperature. With VAV, the diffuser must perform from maximum down to minimum airflow.

Go deeper: Energy efficiency

The terminal unit that sets the airflow to a zone from its thermostat, between a minimum and a maximum airflow, sometimes with a reheat coil.

Go deeper: Energy efficiency

Vbz is the outdoor air the breathing zone needs: Rp × people + Ra × floor area (ASHRAE 62.1). Voz is what must be supplied to the zone to deliver it: Vbz / Ez.

Go deeper: Air distribution effectiveness

The difference in air temperature between head and ankles. ASHRAE 55 limits it to 5.4 °F (3 K) for seated occupants (ankles at 4 in / 0.1 m, head at 43 in / 1.1 m) and 7.2 °F (4 K) for standing occupants (head at 67 in / 1.7 m).

Go deeper: Heating from high ceilings