Air Terminal Design in HVAC

Air Terminal in HVAC: Types, Design and Diffuser Sizing (with CFM Chart)

An air terminal in HVAC is the point where conditioned air enters or leaves a room. The term covers two different things, and drawings use both.

Controls how much

Air terminal unit (ATU)

A box in the ductwork that sets the airflow to a zone. The VAV box is the common example.

Controls where it goes

Air terminal device (ATD)

The diffuser, grille or register at the ceiling or wall that spreads air into the room.

Sizing diffusers from CFM is a job HVAC draftsmen do every day, yet most pages on the air terminal in HVAC stop at definitions. This guide covers both meanings, then works through the sizing: formula, catalogue, a full office example, and a diffuser size chart taken from published manufacturer data.

TL;DR

An HVAC air terminal is where supply air enters a room or return air leaves it.
Air terminal units (VAV, fan-powered, dual duct boxes) control airflow volume; air terminal devices (diffusers, grilles, registers) distribute it.
A VAV box is the most common type of air terminal unit, not a separate thing.
Diffusers are sized with Q = A × V, using a neck velocity limit, then checked against catalogue throw and NC data.
Throw is quoted at terminal velocities of 150, 100 and 50 fpm (about 0.76, 0.51 and 0.25 m/s).
ASHRAE design guidance puts private offices at about NC 30, and every extra diffuser in a room raises the sound level.

Definition Terminal units Diffusers and grilles Throw and NC Design methods Worked example CFM chart Installation FAQ

What Is an Air Terminal in HVAC?

An air terminal in HVAC is any component at the end of the air distribution system where conditioned air is delivered to, or taken from, an occupied space. Everything upstream (the AHU, the ducts, the dampers) exists to get the right amount of air at the right temperature to these points.

Air Terminal Unit vs Air Terminal Device

An air terminal unit controls the volume of air sent to a zone; an air terminal device controls how that air enters the room. A VAV box above an office ceiling is a unit: it opens or closes a damper to deliver, say, 560 CFM when the room is hot. The four square diffusers in the ceiling below it are devices: they spread that 560 CFM across the room without draughts or noise. ASHRAE's definition of a terminal unit includes the functions of controlling airflow rate, temperature or pressure and mixing air streams, which is why boxes and diffusers sit in different chapters of the handbook.

Is an Air Terminal Unit a VAV Box?

A VAV box is the most common type of air terminal unit, but not the only one. Constant volume boxes, fan-powered boxes and dual duct boxes are also air terminal units. Pages that treat "ATU" and "VAV box" as identical are describing the usual case, not the definition.

Air terminal unit above the ceiling and air terminal devices at the ceiling: a VAV unit controls airflow to downstream diffusers through distribution ducts, flexible connections and plenum boxes.

Figure 1: Air terminal unit above the ceiling and air terminal devices at the ceiling: a VAV unit controls airflow to downstream diffusers through distribution ducts, flexible connections and plenum boxes.

Types of Air Terminal Units in HVAC

Every air terminal unit has the same core parts: a damper that throttles airflow, an airflow sensor at the inlet that measures it, and a controller that compares the reading with what the room thermostat is asking for. Some units add a fan, a reheat coil or a second inlet.

Single duct

One inlet, one damper. Varies cold primary air to the zone.

Cooling only: interior offices
With reheat: perimeter zones needing heat

Fan powered

Adds a fan that draws warm air from the ceiling plenum.

Series: fan always runs, constant airflow to the room
Parallel: fan runs only at low load or for heating

Dual duct

Two inlets, one hot and one cold, blended at the box.

Mixing: both streams mixed to one outlet
Non-mixing: streams kept separate downstream

On a parallel fan-powered unit, part of the air leaving the box is recirculated plenum air, so the zone's outdoor-air requirement under ASHRAE 62.1 must be met by the primary air alone.

Types of Air Terminal Devices: Diffusers, Grilles and Registers

Table 1: Air terminal devices compared
Device	Supply or return	Typical use
Diffuser	Supply	Ceiling outlets that spread air in several directions and mix it with room air
Grille	Mostly return or exhaust	Fixed-blade openings in walls or ceilings; also sidewall supply
Register	Supply or return	A grille with a built-in volume damper behind the face
Linear slot diffuser	Supply or return	Narrow continuous slots along perimeter glazing or feature ceilings
Jet nozzle	Supply	Long throws in tall, open spaces

Square diffuser: the standard office outlet, sized to drop into a ceiling grid tile, with a four-way spread.
Round diffuser: concentric cones giving an even 360-degree pattern, often used with exposed ducts.
Linear slot diffuser (LSD): one or more slots that can wash glazing or follow architectural lines.
Swirl diffuser: angled vanes give a rotating jet that mixes quickly, suited to high ceilings.
Jet nozzle: a directional nozzle that throws air tens of metres in atria, halls and auditoria.

Common air terminal devices: square ceiling diffuser, round ceiling diffuser, linear slot diffuser, swirl diffuser and jet nozzle. Products are shown for identification, not to a common scale.

Figure 2: Common air terminal devices: square ceiling diffuser, round ceiling diffuser, linear slot diffuser, swirl diffuser and jet nozzle. Products are shown for identification, not to a common scale.

Role of the Plenum Box

A plenum box is a sheet-metal box mounted on the back of a diffuser that spreads incoming air evenly across the diffuser face. Without it, air from a side-entry duct tends to favour one side of the diffuser, distorting the throw pattern. Insulated plenum boxes also absorb some duct-borne noise before it reaches the room.

Key Air Terminal Design Terms: Flow Pattern, Throw and Noise Criteria

Flow pattern is the direction the air leaves the device: one-way, two-way, three-way, four-way or radial. Ceiling diffusers depend on the Coanda effect, in which the jet clings to the ceiling surface and travels along it before dropping into the room.

Throw is the distance from the outlet to the point where the jet has slowed to a stated terminal velocity. Catalogues tested to ANSI/ASHRAE Standard 70 publish three values:

T150

150 fpm / 0.76 m/s

Jet still strong; keep occupants out of this zone

T100

100 fpm / 0.51 m/s

Middle of the jet's reach

T50

50 fpm / 0.25 m/s

The value used for room air diffusion checks

Noise criteria (NC) is a single-number rating of background sound. Diffuser catalogues quote NC for one outlet with an assumed 10 dB room absorption. ASHRAE's design guidance for HVAC background sound, as reproduced in acoustic industry references, gives about NC 30 for private offices and conference rooms and NC 40 for open-plan offices.

Throw measured to terminal velocities of 150, 100 and 50 fpm (approximately 0.76, 0.51 and 0.25 m/s). Distances are illustrative; use the selected manufacturer’s catalogue values.

Figure 3: Throw measured to terminal velocities of 150, 100 and 50 fpm (approximately 0.76, 0.51 and 0.25 m/s). Distances are illustrative; use the selected manufacturer’s catalogue values.

How to Design Air Terminals in HVAC

There are two ways to size a diffuser, and good practice uses both: the formula to get the size, the catalogue to confirm throw and noise.

Method 1: Formula Approach (Q = A × V)

Airflow equals neck area multiplied by neck velocity. Choose a velocity limit for the space, then solve for area.

Diffuser neck sizing

IMPERIALCFM = Area (ft²) × Velocity (fpm)

SIL/s = Area (m²) × Velocity (m/s) × 1000

CONVERT1 CFM = 0.472 L/s | 500 fpm = 2.54 m/s

A 12 in round neck has an area of 0.785 ft², so at 500 fpm it passes 393 CFM (185 L/s). That matches the published catalogue figure for that neck exactly, because catalogues are built on the same relationship.

Method 2: Manufacturer's Catalogue

The catalogue adds what the formula cannot: throw, total pressure and NC for each neck size and velocity. The values in this article come from the Titus performance data sheets for the TMSA square-face, round-neck diffuser and the TDC square-neck louvered diffuser. Any manufacturer's equivalent data, tested to ANSI/ASHRAE Standard 70, works the same way; use the catalogue for the product actually specified.

Worked Example: Sizing Diffusers for an Office Room

BriefPrivate office, 7.2 m × 5.0 m, ceiling height 2.7 m. The heat load calculation gives a room sensible load of 3.5 kW. Room 24°C, supply air 13°C. Target NC 30.

1. Airflow3500 ÷ (1.2 × 1.006 × 11)264 L/s
559 CFM

2. LayoutTwo diffusers on the long axis, 3.6 m apart and 1.8 m from each end wall, so each serves half the room.280 CFM each

3. VelocityQuiet office: hold neck velocity near 500 fpm (2.54 m/s).≤ 500 fpm

4. Area280 ÷ 500 = 0.56 ft². A 10 in round neck is 0.545 ft².10 in neck

5. Check V280 ÷ 0.545, slightly above the target, which is acceptable.514 fpm

6. NoiseCatalogue NC for a 10 in TMSA is below 10 at 500 fpm and 15 at 600 fpm. Two outlets add 3 points, so the room stays well below NC 30.< NC 20

7. ThrowCatalogue T50 at 500 fpm is 8 ft (2.4 m). Check T50 ÷ L, where L is the distance to the wall, or half the spacing plus 0.9 m (ceiling to the 1.8 m occupied zone) between diffusers: side wall 2.4 ÷ 2.5 = 0.96; end wall 2.4 ÷ 1.8 = 1.33; between units 2.4 ÷ 2.7 = 0.89.All 0.5 to 1.5

ASHRAE's room air diffusion guidance gives a T50/L range of 0.5 to 1.5 for an air diffusion performance index (ADPI) of 80 or better with round-pattern ceiling diffusers, with the best result near 0.8. All three ratios fall inside it, so two 10 in diffusers work for this room.

Diffuser Size vs CFM Chart

Table 2: Round neck diffuser size chart, CFM by neck velocity
Neck	400 fpm	500 fpm	600 fpm	700 fpm	800 fpm	L/s at 500	NC at 500 / 700	T50 at 500
6 in	78	98	118	137	157	46	<10 / 14	6 ft (1.8 m)
8 in	140	175	209	244	279	83	<10 / 17	6 ft (1.8 m)
10 in	218	273	327	382	436	129	<10 / 20	8 ft (2.4 m)
12 in	314	393	471	550	628	185	12 / 23	10 ft (3.0 m)
14 in	428	535	641	748	855	252	<10 / 18	19 ft (5.8 m)
Source: Titus TMSA performance data (24 x 24 in module, horizontal discharge). NC assumes 10 dB room absorption; throw is isothermal per ANSI/ASHRAE Standard 70. L/s converted at 0.472.

Table 3: Square neck diffuser size chart, CFM by neck velocity
Neck	400 fpm	500 fpm	600 fpm	700 fpm	800 fpm	L/s at 500
6 x 6 in	100	125	150	175	200	59
9 x 9 in	225	281	338	394	450	133
12 x 12 in	400	500	600	700	800	236
15 x 15 in	625	781	938	1094	1250	369
18 x 18 in	900	1125	1350	1575	1800	531
Source: Titus TDC square neck performance data. Airflow per neck size does not depend on blow pattern; NC and throw do, so read those from the catalogue for the pattern specified.

Neck size finder

Total airflow (CFM) Number of diffusers Max neck velocity

Enter values to size each neck.

The finder uses neck area alone (Q = A × V), which matches the catalogue airflow in Tables 2 and 3. Confirm NC and throw in the catalogue before issuing.

Design and Installation Considerations

A fixed rule such as "keep diffusers at least 15 feet apart" does not hold up. Spacing follows from throw and room geometry: place outlets so that each one's T50 divided by its characteristic length L stays within the recommended range, as in the worked example.

Zone by load, not by ceiling grid. Perimeter rooms with glazing need different airflow from interior rooms, often on separate terminal units.
Account for ceiling height. High ceilings may need swirl or jet outlets; low ceilings shorten the distance before the jet drops into the occupied zone.
Keep the T150 zone above heads. Air still moving at 0.76 m/s on someone's neck reads as a draught, whatever the room temperature.
Make a clean duct connection. Catalogue data assumes ideal entry; a kinked flexible duct or an elbow right at the neck raises noise and distorts throw.
Add the outlets together for sound. Price Industries' selection data adds 3 NC for two outlets, 6 for four and 9 for eight in one room.
Derate throw on exposed ducts. Without a ceiling for the Coanda effect, catalogue throws drop to about 70% of the listed values.

Frequently Asked Questions

What are air terminals in HVAC?

They are the components where conditioned air is delivered to or removed from a space. Air terminal units, such as VAV boxes, control how much air a zone gets; air terminal devices, such as diffusers, grilles and registers, distribute it into the room.

Is an air terminal unit the same as a VAV box?

A VAV box is the most common air terminal unit, but constant volume, fan-powered and dual duct boxes are air terminal units too.

What is the difference between a diffuser, a grille and a register?

A diffuser is a supply outlet that spreads air in several directions and mixes it with room air. A grille is a fixed-blade opening, usually for return or exhaust. A register is a grille with a volume damper behind it.

How do you size a diffuser from CFM?

Divide the airflow by your neck velocity limit to get the required neck area (CFM ÷ fpm = ft²), choose the next standard neck size, then check throw and NC in the manufacturer's catalogue. For example, 280 CFM at 500 fpm needs about 0.56 ft², which a 10 in round neck provides at 514 fpm.

What is throw in an air diffuser?

Throw is the distance from the diffuser to the point where the air jet slows to a stated terminal velocity: 150, 100 or 50 fpm (about 0.76, 0.51 and 0.25 m/s). T50 is the value used to check room air distribution against the characteristic length L.

What noise criteria (NC) level is used for offices?

ASHRAE's design guidance for HVAC background sound gives about NC 30 for private offices and conference rooms and about NC 40 for open-plan offices. Select diffusers quieter than the target, because multiple outlets and real duct connections raise the sound level above the catalogue figure.

Sources

ASHRAE Handbook: Fundamentals, room air distribution chapter, for throw, characteristic length and ADPI; T50/L guidance as summarised in HPAC Engineering (round ceiling diffusers: 0.5 to 1.5 for ADPI 80, best near 0.8; test method ANSI/ASHRAE Standard 113).
ASHRAE Handbook: HVAC Applications (2019), Noise and Vibration Control, design guidelines for HVAC-related background sound, as reproduced in the Rockfon ceiling and HVAC acoustics white paper.
Titus TMSA performance data, titus-hvac.com, for round neck airflow, NC and throw (Table 2).
Titus TDC performance data, titus-hvac.com, for square neck airflow (Table 3).
Price Industries air outlet selection charts, priceindustries.com, for NC additions with multiple outlets and throw reductions on exposed ducts.
ANSI/ASHRAE Standard 70, method of testing air terminal devices, and ANSI/ASHRAE Standard 62.1, ventilation.

This article was last reviewed on 15 October 2026.

Air Terminal in HVAC: Types, Design and Diffuser Sizing (with CFM Chart)

TL;DR