Aircon Diffuser: What the Fitting in the Ceiling Decides

A diffuser gives the air a shape on the way out, and that is its whole job. Price Industries defines a grille as an outlet with a square or rectangular face and neck carrying louvres that deflect the air. A register is a grille with a damper for volume control.

The shaping goes past deflection: concentric cones, a plaque or pattern controllers turn the discharge into a form the maker has measured and published. Two fittings can pass identical air and put it in different places.

None of that gets worked out on site. Titus states its performance data comes from tests run to ANSI/ASHRAE Standard 70. Price puts the pressure side the same way: an outlet's static pressure loss depends on its geometry and free area, and must be derived by test.

Most Singapore flats never carry one. On a wall unit the machine and the outlet are the same object, since air leaves through the body's own face. These fittings appear once that body moves into a ceiling, where a ducted system ends every branch at one.

Ceiling fittings fall into a handful of families, and the family settles the discharge shape. Round and square cone diffusers stack concentric cones over a round neck. Price describes the square version as holding a uniform 360 degree horizontal pattern even at very low flows, suited to variable airflow.

Louvre face and perforated face diffusers work from a modular core allowing a one, two, three or four way pattern, Price says. On a perforated face, aiming is done by deflection vanes at the face or in the neck; the perforations aim nothing.

Plaque diffusers hang a flat disc in front of a round inlet. Price lists three plaque positions that can be set on site, taking one fitting from fully horizontal throw through to vertical. Slot diffusers carry a pattern controller in each slot, set to throw left, right or straight down.

Radial and twist diffusers put vanes across a circular face to spin the discharge, which Price describes as a twisting pattern for rapid mixing of room air. Industrial ranges run the other way: drum louvres and nozzles carry air a long way across a large volume, and neither belongs over a bed.

Three properties come off a single selection, and none is set alone: reach, pressure cost and room noise all move together, so pushing any one moves the other two.

One catalogue page shows the shape of it. Titus publishes performance for its PAS flush face supply diffuser, where a 24 inch module with a 6 by 6 inch neck runs across nine conditions. Airflow climbs from 75 to 350 cfm, and reach to 50 fpm on the four way setting goes from 4 feet to 15.

What that reach costs is nowhere near proportional. Total pressure along the row rises from 0.017 to 0.360 inches of water, roughly twenty one times over. Noise starts unlisted, meaning below NC 10, and finishes at NC 45.

Pattern is the one lever that moves reach without disturbing the other two. Hold that neck at 150 cfm and the table gives one total pressure of 0.066 inches of water and one noise figure of NC 19, yet reach still changes: 17 feet on one way, 13 on two way, 11 on three way and 8 on four way.

Neck size is the third lever, and it trades the opposite way. A 6 by 6 neck at 200 cfm sits on 0.118 inches of water and NC 28, while an 8 by 8 neck passing more air, at 222 cfm, sits on 0.052 and NC 19, reaching two feet less. More air, under half the pressure, nine points quieter.

So a fitting sized down to suit a ceiling grid pays twice: more of the branch's pressure allowance and more noise. Where a quotation names a diffuser size before anybody says how far the air must travel, it came off the ceiling tile.

A diffuser has no single performance. Its catalogue page is a grid, and each number on it carries the condition it was taken at.

Reach is published against three terminal velocities. Titus tabulates it to 150, 100 and 50 fpm, and states the tests were isothermal, meaning supply air at room temperature. A cooled room is nothing like that, and the distances assume jet attachment at a surface.

Noise carries assumptions of its own. Titus derives NC from octave band sound power minus 10 dB of room absorption, and a dash on the table means a figure below 10. An unlisted reading is a floor, never a promise about a room.

Speed through the neck settles nothing about noise on its own, and the makers say so. Price compares four linear diffuser designs at one 24 inch module, 380 cfm and a 700 fpm neck velocity, and NC across the four runs from 31 to 46. Its conclusion is blunt: neck velocity is a poor indication of acoustic performance.

The design point is not the only one worth checking. Price asks that minimum and maximum air quantities both be considered for reach, warning that outlets holding a good pattern at very low flow can still fall short. Titus asks the same, at maximum flow and at the lowest flow expected while occupied.

Published performance describes the article as it left the factory, tested in clear air. A flat can move it out of that condition without anybody touching the machine.

Price names ceiling layout, walls, partitions and boundaries as things that may obstruct the pattern and produce unacceptable velocities in the occupied zone. Reach also counts along a wall, because air arriving with speed left carries on down it.

Patterns can run into each other as well, and Price sets a rule for that case: where two patterns will meet, each outlet should be chosen so its reach equals half the distance between them. A partition built later moves that midpoint, and nobody reselects anything.

Controllers get left in the wrong position. On adjustable families, a previous fit out or tenant can leave a slot throwing at a wall, or a plaque dropped to vertical. The ceiling looks finished either way, and no marking says what position the face is in.

Paint over the face is worth reporting. No maker publishes a penalty for it, because a fitting is tested as it is made. A painted face is no longer the article those figures describe, and on a perforated diffuser the coating sits across the measured free area.

Swapped faces belong on the same list. A fitting changed during a renovation to match a new ceiling was picked against that ceiling, not the branch feeding it, so it may pass different air at a different pressure and pattern.