The diffuser is the only part of the air system most occupants ever notice. When it is selected well, people do not think about it at all. When it is selected poorly, they complain about drafts, stuffiness, hot and cold spots, or a constant hiss from the ceiling. Many comfort complaints that get blamed on the air handler or the controls actually start at the outlet.
This guide covers how to select supply diffusers, return grilles and registers so that the air mixes properly, the space stays comfortable across the full range of airflow, and noise stays within the target for the room.
What a supply outlet has to do
A mixing-type supply outlet has three jobs:
- Deliver the design airflow at an acceptable pressure drop.
- Mix supply air with room air before it reaches the occupied zone, so that people feel neither cold drafts nor stagnant warm pockets.
- Stay quiet at design airflow, within the room's noise criterion.
The occupied zone is usually taken as the volume from the floor to about 1.8 m (6 ft) high and about 0.6 m (2 ft) in from walls, consistent with ASHRAE Standard 55. Comfort depends mainly on air speed and temperature in that zone.
Key selection terms
| Term | Meaning | Why it matters |
|---|---|---|
| Throw | Distance from the outlet to where the air jet slows to a stated terminal velocity, usually 0.75, 0.5 or 0.25 m/s (150, 100 or 50 fpm) | Determines how far the air travels and whether it reaches walls or collides with other jets |
| Characteristic length (L) | Distance from the outlet to the nearest wall or to the midpoint between outlets | Used to match throw to the space |
| ADPI | Air Diffusion Performance Index: percentage of points in the occupied zone meeting draft and temperature criteria | A measure of mixing quality; higher is better |
| NC rating | Noise criterion of the outlet at a given airflow, per manufacturer data | Must be below the room target after adding other sources |
| Pressure drop | Total pressure required at the outlet | Affects fan energy and balance |
| Coanda effect | Tendency of a jet to attach to an adjacent surface such as a ceiling | Keeps cool air at the ceiling until it has mixed |
Matching throw to the room
The ASHRAE Handbook, Fundamentals provides ADPI tables relating throw-to-length ratio (T/L) with ADPI for each outlet type at different cooling loads. The approach is:
- Determine the characteristic length for each outlet from the layout.
- Look up the recommended T/L range for the outlet type.
- Select a size and pattern that gives the target throw at the design airflow.
- Check the selection at minimum airflow, especially in VAV systems.
As a general pattern, ceiling diffusers achieve good ADPI when the throw to 0.25 m/s (50 fpm) is roughly in the range of 0.8 to 1.5 times the characteristic length, but the exact range depends on outlet type and load, so use the tables or the manufacturer's ADPI data. An ADPI of 80 or higher is commonly targeted for offices.
Throw too short
Air does not reach the far side of the room. Warm spots form near exterior walls, and in cooling, cold air may drop out of the jet near the diffuser and dump onto occupants.
Throw too long
Jets hit walls or collide with each other and turn down into the occupied zone at high velocity, producing drafts. This is common when a large diffuser is placed in a small office.
The VAV turndown problem
In a VAV system, airflow through each outlet may drop to 30% to 50% of design, or lower. Throw shortens and jet velocity falls. Some outlets lose the Coanda effect and cold air drops straight down, known as dumping.
Ways to manage this:
- Select outlets that maintain ceiling attachment at low flow, such as slot or plaque-type diffusers rated for VAV.
- Avoid oversizing outlets for the peak airflow.
- Set VAV minimums with comfort and ventilation in mind, not just energy.
- Consider supply air temperature reset at part load; see the controls library for reset strategies.
Heating from overhead
In Canadian perimeter zones, overhead heating with warm air is a known weak point. Warm air is buoyant and tends to stay at the ceiling, stratifying and short-circuiting to ceiling returns. ASHRAE Standard 62.1 accounts for this with a lower zone air distribution effectiveness for ceiling supply of warm air with ceiling return when the supply air is more than about 8 K (15 F) above room temperature.
Practical guidance:
- Keep the heating supply air temperature low, typically no more than about 8 to 11 K (15 to 20 F) above space temperature.
- Use linear slot diffusers along the perimeter that throw down the glass.
- Consider perimeter radiation, radiant panels or fan-powered boxes where heat loss through the envelope is high.
Noise: getting NC right
Room noise targets come from the ASHRAE Handbook, HVAC Applications. Typical design ranges are roughly NC 30 to 40 for private offices, NC 25 to 35 for classrooms and conference rooms, and lower for theatres and recording spaces. Owners and designers should agree on targets early.
Manufacturers publish outlet NC at given airflow, typically based on a standardized room absorption. A few cautions:
- Multiple outlets in one room add noise. Two equal sources add about 3 dB.
- Published ratings assume smooth, straight inlet conditions. A diffuser fed by a sharp elbow or a partly closed damper right at the neck can be much louder than its rating.
- Balancing dampers should be placed at the branch takeoff, well upstream, not at the diffuser neck.
- Flexible duct runouts should be short and straight. Kinked flex adds noise and pressure drop.
A common rule is to select outlets at least 5 NC points below the room target, leaving room for the terminal unit, duct breakout and other sources. For more on sound paths, see HVAC noise and vibration control.
Return and exhaust grilles
Return grilles have little effect on room air patterns because air approaching an inlet does so from all directions at low velocity. Their placement matters less for mixing, but they still need attention:
- Velocity and noise: size for face velocities that keep noise below the target. Returns in quiet spaces should be generously sized.
- Short-circuiting: avoid placing returns right beside supply outlets.
- Transfer paths: in plenum return systems, each enclosed room needs a path back, such as a ceiling return grille, transfer duct or door undercut. Rooms without one will be pressurized and get less supply air.
- Acoustic privacy: transfer grilles between offices can carry speech. Lined transfer ducts with an elbow help.
Outlet types at a glance
| Outlet type | Typical use | Notes |
|---|---|---|
| Square or round ceiling diffuser | General offices, retail | Four-way or radial pattern; easy to coordinate with ceiling grid |
| Plaque or louvered face | Offices, architectural ceilings | Good horizontal pattern and appearance |
| Linear slot diffuser | Perimeter zones, open offices, VAV | Long throw, good Coanda attachment, adjustable pattern |
| Sidewall register | Hotel rooms, residential, retrofits | Throw depends on deflection setting |
| Swirl diffuser | High ceilings, atria, large rooms | High induction, rapid mixing |
| Jet nozzle | Large volumes, arenas, halls | Long throw, adjustable for heating and cooling |
| Displacement outlet | Classrooms, auditoriums | Low velocity, low level, see displacement guide |
For low-level, low-velocity supply, see displacement ventilation in commercial spaces.
Key takeaways
- Select diffusers by throw, ADPI and NC, using the room's characteristic length and ASHRAE data.
- Check every selection at minimum airflow in VAV systems to avoid dumping.
- Keep overhead heating supply temperatures low and throw warm air down perimeter glass.
- Leave a noise margin below the room target and give outlets clean inlet conditions.
- Provide a proper return path from every enclosed room. Proper air balancing then confirms outlet airflows and pattern settings.
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