Aircon Plenum Box: The One Part That Gets Made on Site

A plenum box is a chamber wider than the pipes leaving it. Air arrives through one opening and leaves through several, at lower speed. Ducted systems carry them in two positions: at the discharge of the indoor unit, dividing flow into branches, and above a ceiling outlet, spreading a single duct across the face of the fitting.

None of it comes out of a carton. The Daikin FDXM installer reference guide states the position in one line: the ducting is to be field supplied. Its diagram labels the flange, insulation and aluminium tape the same way.

Mitsubishi Electric writes the job as a short list. Its PEAD ducted range asks for a canvas duct between unit and duct, incombustible material for duct parts, and full insulation on the inlet and outlet duct flanges. None arrives finished; they get cut, fitted and wrapped by hand above a ceiling.

So the box is the one component whose quality is settled entirely on site. The machine was tested before it shipped, and so was the diffuser; what joins them was folded and taped by whoever worked above that ceiling.

Every throw, pressure and noise figure in a diffuser catalogue was taken under one inlet condition, and the catalogues say so. Titus bases its data on ASHRAE Standard 70, which requires several diameters of straight duct at the approach. The sentence after it carries the weight: field installations seldom allow these conditions.

The measuring point is written down as well. Titus takes an outlet's total pressure as the sum of static and velocity pressures in the supply duct, one and a half duct diameters ahead of the outlet.

Price Industries arrives at the same place from the design side. Its guidance for quiet air distribution asks for smooth airflow at all duct elements, including branches, elbows, transitions and outlets, with five to ten duct diameters of straight ductwork between them. A ceiling void in a flat has neither the depth nor the length.

Both makers then name the same remedy, a fair measure of how common the problem is. Price asks for equalizing grids when non-ideal inlets cannot be avoided, and Titus lists the same item for direct diffuser connections. That is a bought part costing money and ceiling depth a tight void lacks.

The penalties are published, and they are not small. Titus puts the effect of joining a diffuser straight onto a duct at as much as 12 NC above catalogue levels, caused by regions of high velocity inside the fitting. That gap separates a bedroom somebody sleeps in from one they can hear.

Flexible duct at the inlet was tested as its own case. Titus reports that flexible ducts at a diffuser inlet increase pressure drop, increase sound levels and produce nonuniform air distribution. An ideal gentle 90 degree connection adds about 1 NC; a kinked one adds 7 to 9.

Air pattern moves with all of it, and that part carries no number. The same list records only that the distribution pattern can be greatly affected, varying by diffuser type. A warm corner under a correctly selected diffuser can come from the shape of the metal above it.

The equalizing grid figures show what an approach is worth in decibels. Titus reproduces two cases from the ASHRAE Handbook. With an equalizing grid, sound levels match the maker's rating; without one they run up to 12 dB higher. The second compares eight duct diameters of straight run against two, and the short version measures 12 to 15 dB above rating.

The proportions of the box decide where the air points. Titus tabulates the discharge angle of an end-fed linear diffuser against the ratio of slot area to duct area. At 0.4 the airstream leaves at 15 degrees; at 0.75 at 25, at 1.0 at 35, and at 1.5 at 40. Nothing changed but the relative size of two openings.

Getting a flat discharge back takes a part. Titus assures a 0 degree angle along the whole face by including straightening blades behind the face vanes or bars. Leave those out and the face still looks finished, while the air leaves at an angle nobody chose.

The machine end has a geometry rule of its own. The PEAD guidance warns that intake noise increases dramatically if the intake sits directly beneath the main body. It asks for the intake to be fitted as far from the body as possible, with an inlet duct of 850 mm or more.

Insulation on the box answers condensation before comfort, and the manuals give that as the reason. Daikin's FDXM guidance asks that the duct be insulated to prevent condensation, naming glass wool or polyethylene foam 25 mm thick. Mitsubishi asks for full insulation on the inlet duct flange and outlet duct, for the same reason.

Climate is what those instructions were written against, and the PEAD manual sets a threshold. Where air above the ceiling runs hot and humid, with a dew point above 26 degrees C, condensation may form in the indoor unit, and the manual asks for 10 to 20 mm of insulation across the whole unit surface. A closed void above a flat here is warm and still, and nobody measures its dew point after handover.

An uninsulated box behaves like a cold glass. Moisture in the surrounding air condenses on the metal and runs to the lowest point it can find, usually the ceiling board. The fault then presents as a stain or a drip in a room, with nothing wrong inside the machine.

Sealing is the second job, and a standard sits behind it. The 1995 edition of SMACNA's duct construction standard defines sealing as closing openings in ductwork and field-erected plenums and casings through which air leakage would occur. The words field-erected sit inside the standard's own definition, so a box made on site falls within its scope.

Whatever escapes goes into the ceiling, where nobody sees it. Cooled air leaving an unsealed seam still crossed the coil and was still paid for, and no room received it. Daikin is direct: wind aluminium tape around the intake side flange and duct connection, and make sure there are no air leaks elsewhere.

No code reaches into a flat to govern this. SS 553 sets out good practice for air-conditioned and mechanically ventilated buildings in Singapore, scoped to commercial, office and institutional buildings except hospitals. It contains a ductwork and other air passages clause, a seal requirements table and minimum duct R-values. A home falls outside the building types it applies to.

The construction standard says as much about itself. SMACNA records that duct systems for residences are not ordinarily subject to its provisions. Where its own pages are silent, it hands the choice over: the contractor shall select configurations suitable for the service.

So the answer to who decided the box's shape is always the same. It was not a maker, an engineer or a code, but the installer, working to whatever depth the ceiling left.

One request tests a quotation without argument. Ask for the outlet selection in writing, along with the approach condition it assumed and how the box and its joints will be insulated and sealed. A contractor who picked the outlet off a selection table already holds both on paper; where the answer comes back as a machine size, the path from machine to room was never priced.