HVAC
Air distribution field guide: diffusers, grilles, and registers
Pick the supply device for the throw, set the pattern, balance in the duct and not at the face, and put the return where the air actually needs to go.
Direct answer
Diffusers, grilles, and registers are the terminal devices that deliver supply air into a room and pull return air back. A grille is a faced opening with no damper, a register is a grille with a damper, and a diffuser spreads and mixes supply air, usually at the ceiling. Manufacturer performance data governs throw and noise.
Key takeaways
- A grille is a faced opening with no damper, a register is a grille plus a damper, and a diffuser is a supply device that spreads and mixes air.
- Throw is the distance a supply jet travels before decaying to terminal velocity, commonly 50 fpm, listed in catalogs as the T50 number.
- Balance in the branch duct several feet upstream, not at the face damper, since a closed opposed-blade damper makes noise inches from the room.
- ADPI scores occupied-zone comfort roughly 0 to 100, ties throw to room length per ASHRAE, and selections commonly aim for about 80 or better.
- Select diffusers by four inputs together: airflow in cfm, required throw, the NC noise limit, and mounting, at the actual operating airflow.
The terminal devices, where the air meets the person
A diffuser, a grille, and a register are the last things the air touches before a person feels it. They sit at the end of the run, after the air handler has conditioned the air and the ductwork has carried it across the building, and they set how the air enters the room. How far it throws. How well it mixes. Whether it lands on someone's neck as a draft or pools as a cold puddle on the floor.
The air handler and the duct system have their own guides. This one is about the terminal devices, the point where the engineering on the drawings finally reaches the occupant. Everything upstream can be right and the room can still be wrong, because comfort is decided at the face of the device, not at the coil.
These devices carry two jobs. On the supply side they deliver conditioned air and spread it so it mixes with room air before it reaches anyone. On the return side they collect spent air and send it back to the system at low velocity. Most comfort complaints on an otherwise healthy system trace back to one of those two jobs being done by the wrong device in the wrong spot.
What is the difference between a diffuser, a grille, and a register?
A grille is a faced opening with fixed or adjustable vanes and no damper. It is a covered hole that lets air pass, used most often on the return side. A register is a grille with a damper behind it, so you can throttle or close the air right at the opening. A diffuser is a supply device built to spread and mix the air, usually mounted in the ceiling, with a face geometry of cones, louvers, slots, or a perforated panel that fans the air into a pattern instead of shooting it in one stream.
The shorthand that holds up: grille means no damper, register means a grille plus a damper, diffuser means the spreader. That is the distinction worth carrying.
The words drift by region and by catalog. Some manufacturers call a louvered supply device a register and a perforated one a diffuser. Some sidewall supplies have a damper and still get called a grille on the plans. So read the submittal and the part number, not the word someone used in the field. The performance table tells you what the device actually does.
Supply devices throw, return devices collect
Supply and return devices do opposite work, and confusing the two is a common sizing error. A supply diffuser or register has to push air out with enough velocity to throw it across the room and induce room air into the stream so the conditioned air arrives mixed and tempered. That takes a face designed to fan and entrain, and a neck velocity high enough to project the air.
A return grille does the opposite and far less of it. Air does not throw on the way back in. It drifts toward the opening at low velocity from every direction at once, so a return grille is sized for low face velocity to stay quiet, with a large free area for the airflow it has to pass. You cannot aim a return and you cannot make it reach. It only pulls from the air right in front of it.
That is why a supply device and a return device of the same nominal size are not interchangeable. The supply is tuned to project and mix. The return is tuned to pass air quietly. Swap them and you get a noisy return or a supply that dumps.
Ceiling diffusers: square, round, and multi-pattern
The square or round ceiling diffuser is the workhorse of commercial overhead distribution. The classic version uses stacked cones or curved louvers to throw the air horizontally along the ceiling in a one-, two-, three-, or four-way pattern. Hugging the ceiling matters: the air rides the surface, slows by induction as it pulls room air in, and is mixed and tempered by the time it drops into the occupied zone. That ceiling-attachment effect is what keeps cold supply air from falling straight down on people.
Pattern controls where the air goes. A four-way pattern in the middle of a space, a one-way or two-way along a wall or a perimeter so the throw does not blow into an obstruction or across into another diffuser. Many catalog diffusers ship with adjustable or field-changeable pattern controllers for exactly that reason.
Square and round ceiling diffusers fit standard ceiling heights and ordinary office and retail loads well. Push them into very high spaces or very high airflow and the throw and induction stop keeping up, which is where the swirl and linear types earn their place.
| Diffuser type | Typical use | What it is good at |
|---|---|---|
| Square or round ceiling | Offices, retail, standard ceilings | Horizontal throw along the ceiling, adjustable pattern |
| Linear slot | Perimeter, glass lines, architectural ceilings | Long narrow throw, clean look, continuous runs |
| Round or swirl, high-induction | High ceilings, high airflow, atriums | Fast mixing, short drop, tolerates cold supply |
| Perforated face | Lay-in tile ceilings, low-draft spaces | Even low-velocity supply, blends with ceiling |
| Displacement | Floor or low sidewall, tall or high-load spaces | Low-velocity cool air to the occupied zone |
Linear slot diffusers
A linear slot diffuser delivers air through one or more narrow slots in a long, thin face. Architects reach for them because they read as a clean line in the ceiling instead of a grid of square tiles, and because you can run them continuously along a glass wall or down a corridor. Inside each slot sits a small adjustable deflector that aims the throw left, right, or straight down.
The performance reason to use them is the perimeter. A slot running along a window line can wash the glass with a sheet of air, which fights the heat gain in summer and the cold downdraft off the glass in winter. Aimed correctly, the slot holds a curtain of conditioned air at the surface where the load actually is.
The catch is the plenum behind them. A linear slot is only as even as the plenum feeding it, and a long run fed from one end without an equalizing plenum or proper inlet collars throws hard near the inlet and dribbles at the far end. The pretty line on the ceiling hides a balancing problem if the plenum is an afterthought.
Round, swirl, and high-induction diffusers
Swirl and high-induction diffusers spin the air as it leaves the face, sending it out in a rotating pattern that drags in a large volume of room air fast. High induction means the supply air mixes and loses its temperature difference quickly, over a short distance, so the air is neutral by the time it reaches anyone.
That property is what makes them the choice for high ceilings and high airflow. In a tall space, a standard ceiling diffuser has to throw a long way down and the cold air can fall before it mixes. A swirl diffuser tempers the air in a short drop, so it can deliver a lot of cold air from a high ceiling without dumping it on the floor.
They also tolerate a wider supply temperature range, which is why they show up on systems pushing colder-than-usual supply air to cut fan energy. The trade-off is that the strong mixing pattern is built into the device. You select the diffuser for the space, because unlike a four-way cone there is far less to adjust in the field once it is in.
Perforated-face diffusers
A perforated-face diffuser is a panel of small holes, usually a lay-in unit that drops into a standard ceiling tile grid and blends with the perforated or flat ceiling around it. Behind the face sits a backpan and often a deflector that sets the pattern, so the visible perforated panel is the look while the guts do the throwing.
They suit spaces that want low face velocity and minimal visible hardware. The many small openings spread the airflow over a large face area, which keeps the discharge gentle and the device quiet, and the look disappears into the ceiling. That is why you see them in open offices and clean, modern interiors.
Two things to watch. A perforated face with no internal deflector just dumps straight down, so the backpan and pattern device are not optional if you want a horizontal throw. And the small holes load up with dust and lint over time, which the ceiling staining around an old perforated diffuser will show you plainly. The face you can wipe. The plenum behind it is where the airflow problem usually hides.
Displacement diffusers
Displacement ventilation flips the usual overhead approach. Instead of throwing cool air from the ceiling and mixing the whole room, a displacement diffuser introduces cool air at low velocity right at floor level or low on a sidewall, and lets it spread across the floor in a shallow layer. Heat from people, equipment, and lights makes that air rise, carrying the warm, stale air up and out a high return.
Because the supply enters slow and cool and stays low, the occupied zone gets fresh, cool air with very little draft, and the contaminated warm air collects above head height where it is exhausted. Done right it can use less fan energy and deliver better air quality in the breathing zone than a mixing system.
It only works where the physics cooperate. You need the ceiling height for the warm layer to form above the people, a cooling-dominated load, and a supply temperature that is not so cold it chills ankles. Displacement is a poor fit for low ceilings and for heating, since warm supply air at the floor wants to rise immediately and never reaches across the space.
Grille and register types: sidewall, floor, return, transfer, bar
The grille and register family covers the devices that are not ceiling diffusers, and each has a spot it fits. A sidewall or floor register is a supply device with a damper, common in residential and light commercial work, with adjustable vanes that set a single-direction throw. The damper lets you throttle the room, though that is not where you should be balancing the system.
A return grille is the plain faced opening that pulls air back, sized large and quiet, often with a removable core for cleaning. A linear bar grille is the architectural version, a run of closely spaced bars used for supply or return where the long, clean line matters, frequently at floor level along a glass wall.
A door or transfer grille is the one people forget. When a room has supply but the door blocks the return path, you need a transfer grille in the door or the wall, or an undercut door, so the air has a way back to the return. Skip it and you pressurize the room, the door whistles, and the supply air cannot get in because the air already there has nowhere to go.
What is throw in HVAC?
Throw is how far a supply device drives the air before the stream slows to a defined terminal velocity. The reference is commonly 50 fpm, the speed at which the air has effectively stopped projecting and merged with the room, so a manufacturer's throw figure is the distance from the device to where the jet decays to that velocity. Throw to 50 fpm is often written as the T50 number in catalog tables.
Two companion terms go with it. Spread is how wide the air pattern fans out from the device. Drop is how far a cold supply stream falls below the device as it travels, which matters because cold air is heavier and wants to sink. A diffuser with too little throw drops its air short. A diffuser with too much throw blasts past the zone and bounces off the far wall.
Throw, spread, and drop are device properties measured by the manufacturer on a test setup, and they shift with airflow, pattern setting, and supply temperature. Use the catalog values for the specific device and the specific cfm you are running. The numbers in a generic chart are a starting point, not the device on your ceiling.
Terminal velocity and ADPI, the comfort metric
Terminal velocity is the cutoff speed that defines the end of the throw, commonly 50 fpm for ceiling diffusers and sidewall grilles. It is a convention, not a law of physics, but it lets everyone compare devices on the same basis: the throw is the distance to that speed.
ADPI, the air diffusion performance index, is the comfort score that ties throw to how the room actually feels. It is a single number, expressed roughly 0 to 100, that captures how uniform the air speed and temperature are across the occupied zone. A high ADPI means most of the space is well mixed and most occupants are comfortable. ASHRAE guidance ties ADPI to the ratio of throw to a characteristic room length, and selections are commonly aimed at an ADPI of about 80 or better.
The practical version: there is a sweet spot for throw relative to room size. Too short and the air dumps and the room stratifies. Too long and you get drafts and the air short-circuits. The ADPI tables in the ASHRAE Handbook give the throw-to-length ratios that land in the comfortable middle for a given device and load. Treat them as the design basis and verify against the manufacturer's data for the actual device.
Sizing the throw to the room
The job in layout is to get the supply air into the occupied zone mixed and tempered, without dumping it short or blowing it across the space. The occupied zone is the part of the room people actually use, generally floor level up to around 6 ft and a foot or so off the walls. That is the volume the device has to condition, and the volume that has to stay out of a draft.
Match the throw to the distance the air has to cover, which is usually the span to the wall or half the distance to the next diffuser. The ASHRAE ADPI approach puts that ratio in a comfortable band, often around half to one and a half times the characteristic length depending on load and device. Land in that band and the air arrives mixed at the edge of the zone instead of dumping in the middle or piling into a corner.
Spacing and obstructions decide as much as the device. Beams, light fixtures, partitions, and tall shelving all interrupt a throw that looked fine on the reflected ceiling plan. Walk the actual ceiling, not just the drawing, before you commit the layout.
Why is cold air dumping from my diffuser?
Cold-air dumping is when the supply air falls straight down out of the diffuser onto the people below instead of throwing across the ceiling and mixing first. You feel it as a cold column under the diffuser and a warm, stale room everywhere else. The cause is almost always too little throw for the air to stay attached to the ceiling, the air too cold for the velocity it is leaving at, or the airflow turned down so far that the velocity can no longer hold the pattern.
Variable-air-volume systems make this common. At part load the box throttles the airflow, the diffuser velocity drops, the ceiling-attachment effect breaks, and the cold air sheets down. A diffuser sized for design cfm can dump badly at 40 percent flow. That is why diffusers on VAV systems are selected to hold their pattern across the turndown range, not just at peak.
The draft is the opposite failure: too much velocity in the occupied zone. Both come back to the same lever, the velocity and throw at the face relative to the room and the load. Fix dumping by raising the throw, warming the supply, or selecting a higher-induction device. Fix the draft by cutting velocity or redirecting the pattern, not by closing the face down until it whistles.
Short-circuiting: supply straight to return
Short-circuiting is when supply air heads straight for the return grille without doing its work in the room. The conditioned air leaves the diffuser, takes the short path to a nearby return, and goes home barely used, while the far side of the room never gets mixed. The thermostat near the return reads satisfied, the system cycles off, and the occupants in the dead zone are uncomfortable.
It usually comes from a supply and a return placed too close together, or a diffuser pattern aimed at the return, or a return that is oversized and dominant enough to pull the supply jet straight into it. Ceiling returns set right beside ceiling supplies are the classic offender.
Layout fixes it. Separate the supply and return so the air has to cross the occupied zone to get back. Aim the diffuser pattern away from the return, not toward it. In a perimeter-load space, supply at the perimeter and return at the interior, so the path of the air runs through the room rather than across the ceiling between two nearby openings. You cannot balance your way out of a short-circuit. The geometry is wrong, and the geometry is what you change.
Neck velocity, face velocity, and the NC limit
Two velocities describe a device and they are not the same. Neck velocity is the speed of the air in the duct collar feeding the device, the inlet side, and it is the main source of the device's own noise. Face velocity is the speed across the visible face after the air has spread out over the larger face area. Push the neck velocity up to get more throw and you get more noise along with it.
Noise is rated as an NC, or noise criterion, number. It is a single figure that describes how loud the device is across the sound spectrum, and manufacturer catalogs list the NC for each device at a given airflow and neck size. The faster you drive the neck, the higher the NC, and a device that throws beautifully at high velocity can be too loud for the space it serves.
The selection discipline is to hold the device under the NC limit for the room. An open office tolerates more noise than a conference room or a recording space, and the spec or the design will name the target. When the airflow you need pushes the device over its NC, the answer is a larger device or another device, not cranking the velocity and hoping nobody notices. Confirm the NC against the manufacturer's table for the actual selection.
How do you pick the right diffuser?
Pick the device by four inputs together: the airflow it has to deliver in cfm, the throw it needs to reach the zone, the NC limit for the space, and the mounting and look the ceiling allows. Those four are what the manufacturer's performance table is built around, and the right device is the one that satisfies all four at the airflow you are actually running, not three of the four.
Work it in order. Start from the cfm the zone needs. Find the throw required from the room geometry and the ADPI target. Then go to the catalog and find the device and size that delivers that throw at that cfm while staying under the NC limit. If nothing fits, the airflow per device is too high, so add devices and split the air rather than overdriving one.
Two cautions from the field. Select for the operating range, not just design cfm, because a VAV zone spends most of its life at part load where the throw collapses. And select the device for the season that is harder, which in an overhead cooling system is cooling, since cold air dumps and warm air is more forgiving. The catalog has the numbers. The skill is feeding it the real airflow and the real room.
Balance in the duct, not at the face
The single most abused part on these devices is the damper, because crews balance the system by closing it. That is the wrong move. An opposed-blade damper sitting right behind the diffuser face is a noise machine: close it down to cut airflow and the air screams through the partly shut blades inches from the room. You traded an airflow problem for an acoustic one.
Balance belongs in the duct. Put the balancing damper in the branch duct, set well back from the outlet, commonly on the order of several feet or more upstream so the turbulence it makes has room to settle before the air reaches the device. Throttle there and the device sees clean, even airflow and stays quiet. The opposed-blade damper at the face is for fine trim and for shutting off a device, not for hauling the airflow down a large amount.
This is a core part of test and balance, which has its own discipline and standards. The takeaway for the install: get the volume dampers in the branches where they belong, and leave the face damper near open. The branch duct and its takeoffs are covered in the ductwork guide. Balance there.
The diffuser plenum and the flex connection
How the device is connected decides whether its catalog performance ever shows up in the room. Most ceiling diffusers sit on a plenum box, a sheet metal box that turns the round duct or flex into the square or round neck of the diffuser and gives the air a settling chamber before it hits the face. A good plenum has the inlet sized and positioned so the air arrives even, often with the inlet on the side and a deflector or an inlet straight up rather than blasting directly at the face.
Flex duct is where the performance dies. A kinked or sharply bent flex connection to the diffuser chokes the airflow and ruins the pattern, and it is the most common field defect on the supply side. The flex sags, gets crushed above the tile, or makes a hard turn right at the collar, and the diffuser that was selected for clean air now starves and dumps.
Connect the flex straight and supported into the plenum, with a gentle radius and the inner liner pulled taut, not bunched. A short, straight, hard-pipe connection at the diffuser beats a long, lazy run of flex every time. The flex duct and its install rules are in the ductwork guide.
Heating versus cooling, and the adjustable pattern
The hard truth of overhead air distribution is that heating and cooling want opposite things. Cold air is heavy and falls. Warm air is light and rises. A ceiling diffuser throwing cold supply across the ceiling, mixing it down gently, is doing exactly the right thing in cooling. The same diffuser in heating throws warm air across the ceiling where it wants to stay, stratifies the room, and leaves the floor cold while the ceiling bakes.
That is why heating from the ceiling is harder than cooling from the ceiling, and why perimeter heat, floor registers, or a vertical throw-down pattern often handle the heating load that an overhead cooling diffuser cannot. In spaces that swing between the two, the device and the layout have to serve the harder case.
Some diffusers offer an adjustable or temperature-responsive pattern, a one-, two-, three-, or four-way controller you set in the field, or a thermal element that shifts the pattern between a horizontal cooling throw and a downward heating throw. They help, within limits. They do not repeal the physics. If a space has a real heating load, plan the heating distribution for it instead of expecting an overhead cooling diffuser to do a job the warm air will not cooperate with.
The return side, plus louvers and filter grilles
The return is sized for low velocity and placed to pull the room's air across the occupied zone, not to short-circuit the supply. Because you cannot aim a return, location does the work: a return that draws spent air through the room earns its keep, one parked next to the supply does not. Conventional placement puts a high return where you want to pull off the warm layer in cooling, and a low return where a heating-dominated space needs to clear cold air off the floor. The right answer depends on the system and the load, so follow the design intent.
A filter grille, or eggcrate return with a filter behind it, is the return that carries the system filter at the grille face instead of at the unit, common in residential and light commercial. It works only if someone changes the filter, and a clogged return filter starves the whole system quietly.
On the outside-air side, an exterior louver is the device that lets outdoor air in while keeping rain and birds out. What matters there is free area, the actual open portion after the blades, and the rain rejection at the design wind, both of which come from the louver manufacturer's tested ratings under the AMCA program. Size the louver to the free area, not the wall opening.
Commissioning: setting and measuring the device
A device is not done when it is screwed to the ceiling. It is done when the airflow is measured, the pattern is set, and the numbers are recorded. The airflow at each diffuser and grille is measured with a flow hood, the capture hood the balancer holds over the face that reads the cfm passing through it. That reading, compared to the design cfm, is what tells you whether the device and the branch damper are set right.
The balancer sets the branch dampers to bring each device to its design airflow, trims with the face damper only as needed, and sets the diffuser pattern to suit the space. Then the readings get logged on a balance report, device by device, so there is a record of what the system was delivering on the day it was accepted.
This is the test and balance discipline, governed by the TAB standards from bodies such as NEBB and AABC, and it is where the design intent either becomes real or quietly fails. A system that was never properly balanced runs wrong for years without anyone able to point to the device that started it. Get the flow-hood numbers on paper at acceptance. They are the baseline every future complaint gets checked against.
Matching the device to the space
Different spaces ask for different devices, and a pro picks by the room, not by habit. A standard office or retail floor with an ordinary ceiling height takes square or round ceiling diffusers or perforated lay-ins, selected for the cfm and held under a moderate NC. An open-plan space rewards even coverage and low draft, so the device spacing and the part-load throw matter as much as the peak.
High-ceiling spaces, atriums, lobbies, and big-box retail are where swirl and high-induction diffusers earn their cost, because they mix cold air in a short drop instead of letting it fall the whole height. Perimeter and glass lines call for linear slots or bar grilles that wash the surface and fight the solar and downdraft loads.
Data centers and clean spaces are their own world. A data hall pushes very high airflow and cares about containment, delivering cold air to the equipment inlets and keeping it separated from the hot return, often through perforated floor tiles or overhead supply tied to hot- or cold-aisle containment. A cleanroom or operating room uses laminar, low-turbulence perforated supply to sweep particles out rather than mix them around. Those spaces follow their own design standards, and the device selection serves the airflow and contamination control, not just comfort.
What to document
The record on the air-device side is what lets the next person diagnose a complaint without re-engineering the whole system. For each device, capture what it is, where it is, what it was selected and balanced to, and the pattern it was set to. When a room runs cold or noisy a year out, that record is the difference between a five-minute check and a guessing game.
Log the device type and model, the location and the zone it serves, the design cfm and the measured cfm at balance, the neck size, the throw and NC from the selection, the pattern setting, and the branch damper position if it is recorded. Tie it to the balance report so the as-built airflow is on paper, device by device.
| Device | Use | Key spec to record |
|---|---|---|
| Ceiling diffuser (square/round) | Overhead supply, standard ceilings | Model, design cfm, measured cfm, throw, NC, pattern |
| Linear slot diffuser | Perimeter, glass line, architectural | Length, slot count, plenum/inlet detail, throw, NC |
| Swirl / high-induction | High ceilings, high airflow | Model, design cfm, drop, supply temperature range |
| Perforated face | Lay-in ceilings, low draft | Model, backpan/deflector, design vs measured cfm |
| Return / filter grille | Return and exhaust | Free area, face velocity, filter size if applicable |
| Register (supply/floor) | Single-direction supply with damper | Neck size, measured cfm, vane and damper setting |
| Exterior louver | Outside-air intake | Free area, rain rejection, AMCA-rated data |
Common mistakes
- Selecting for design cfm only, so the diffuser dumps cold air when the VAV box throttles to part load.
- Balancing the system by closing the opposed-blade damper at the face, which makes noise instead of balancing in the branch duct.
- Placing the supply and return too close together so the supply short-circuits to the return and the room never mixes.
- Kinked, crushed, or sharply bent flex into the diffuser plenum, which chokes the airflow and ruins the pattern.
- Ignoring the NC limit and overdriving the neck velocity to get throw, leaving an occupied space too loud.
- Using an overhead cooling diffuser to carry a real heating load and getting a stratified, cold-floor room.
- Undersizing the return so it runs at high face velocity, gets noisy, and starves the system.
- Forgetting the transfer grille or door undercut, so a closed room pressurizes and the supply cannot get in.
Field checklist
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Standards and references
Comfort is where ASHRAE governs the air-device side. ASHRAE Standard 55 sets the thermal comfort conditions, including the air-speed and draft limits in the occupied zone that a supply device has to respect, and the air diffusion performance index and the throw-to-length selection tables live in the ASHRAE Handbook, in the air distribution chapter of the Applications volume. Ventilation airflow comes from ASHRAE Standard 62.1. Treat the ADPI targets and throw ratios as design guidance and verify them against the device manufacturer's tested performance.
Device performance itself, throw, spread, drop, neck and face velocity, and NC, comes from the manufacturer's catalog, with the test methods and ratings standardized through AMCA and the related ASHRAE and ANSI test standards for air outlets and louvers. The numbers are device-specific. Pull them for the actual model and the actual airflow, not from a generic chart.
Setting and measuring the devices in the field is the test and balance discipline, with procedures and tolerances from the TAB standards published by NEBB and AABC, and duct construction and leakage from SMACNA. The exact figures for throw, NC, and ADPI depend on the device and the design, so hedge to the manufacturer's data and the ASHRAE selection method, and confirm the project specification, which can be tighter than any rule of thumb.
Units, terms, and conversions
The air-device world has its own vocabulary, and the same device shows up under different names across a drawing set, a submittal, and a catalog. GRD is the catchall acronym for grilles, registers, and diffusers, and air terminal or outlet is the engineering term for the same family.
Airflow is in cubic feet per minute (cfm), or liters per second or cubic meters per hour in metric sets. Velocity is in feet per minute (fpm), with the 50 fpm terminal velocity as the throw reference, or meters per second in metric. Throw is a distance in feet, often the T50 figure. Noise is the NC, the noise criterion number. Comfort mixing is the ADPI, the air diffusion performance index. Louver capacity is its free area, the actual open portion of the face.
- GRD
- Grilles, registers, and diffusers, the family of air terminal devices at the ends of the duct system
- Throw (T50)
- Distance from the device to where the supply jet decays to a terminal velocity, commonly 50 fpm
- Spread and drop
- How wide the supply pattern fans out (spread) and how far a cold stream falls as it travels (drop)
- ADPI
- Air diffusion performance index, a 0 to 100 measure of how uniform and comfortable the occupied zone is
- NC
- Noise criterion, a single-number rating of a device's sound level used to hold it under a room limit
- Neck vs face velocity
- Air speed in the inlet collar (neck, the noise source) versus across the visible face after spreading
- OBD
- Opposed-blade damper at the device, for trim and shutoff, not for balancing the system
- Free area
- The actual open portion of a grille or louver face that air passes through, less than the nominal size
FAQ
What is the difference between a diffuser, a grille, and a register?
A grille is a faced opening with no damper, used mostly for return air. A register is a grille with a damper behind it, so you can throttle or shut the air at the opening. A diffuser is a supply device, usually in the ceiling, built to spread and mix the air into a pattern rather than throw one stream.
What is throw in HVAC?
Throw is how far a supply device drives the air before the stream slows to a defined terminal velocity, commonly 50 fpm. Manufacturers list it as the T50 distance for a given device and airflow. Too little throw and the air dumps short. Too much and it overshoots the zone and bounces off the far wall.
Why is cold air dumping from my diffuser?
Cold air dumps when the supply has too little throw to stay attached to the ceiling, the air is too cold for its velocity, or the airflow is turned down so far the pattern collapses. It is common on VAV systems at part load. Fix it by raising the throw, warming the supply, or using a higher-induction device.
How do you pick the right diffuser?
Pick by four inputs together: the airflow in cfm, the throw needed to reach the occupied zone, the NC noise limit for the space, and the mounting. Start from the cfm, find the throw from the room geometry and the ADPI target, then find the catalog device that meets the throw at that cfm while staying under the NC.
Should I balance the system by closing the damper at the diffuser?
No. An opposed-blade damper closed at the diffuser face makes noise inches from the room. Balance with the volume damper in the branch duct, set several feet back from the outlet, so the turbulence settles before the air reaches the device. The face damper is for fine trim and shutoff, not for hauling the airflow down.
Where should the return grille go, high or low?
It depends on the load and the system. A high return pulls off the warm layer in a cooling-dominated space, while a low return helps clear cold air off the floor where heating dominates. Whatever the height, place the return so it draws air across the room, not right beside the supply, or the air short-circuits.
What is ADPI in air distribution?
ADPI, the air diffusion performance index, is a 0 to 100 score of how uniform the air speed and temperature are across the occupied zone. A higher number means better mixing and more comfortable occupants. ASHRAE ties it to the ratio of throw to room length, and selections commonly aim for an ADPI of about 80 or better.
Why is my supply diffuser noisy?
Device noise comes mostly from neck velocity, the air speed in the inlet collar. Driving the neck hard for more throw raises the NC rating. A closed face damper makes it worse. Cut the noise with a larger device or another device to split the airflow, and balance in the branch duct instead of throttling at the face.
Can one diffuser handle both heating and cooling from the ceiling?
Within limits. Cold air falls and warm air rises, so a ceiling diffuser that mixes cold air well in cooling will stratify and leave the floor cold in heating. Adjustable or temperature-responsive patterns help, but a real heating load is usually better served by perimeter heat, floor registers, or a downward throw.
What does a flow hood measure on a diffuser?
A flow hood, or capture hood, measures the airflow in cfm passing through a diffuser or grille face. The balancer compares that reading to the design cfm and adjusts the branch dampers to match. Logging the measured cfm device by device at acceptance gives the baseline every future comfort complaint gets checked against.
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Codes cited in this guide
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