Ceiling suspended aircon: the format that needs no void
An exposed box under the ceiling reads as the dated option, chosen before cassettes existed. More often it is the answer to a ceiling with no void to hide anything in. What the structure above has to carry then becomes the real question.
By Team Snowflake | Updated 9 Aug 2026
The whole body stays in the room
A ceiling suspended unit fixes to the underside of a ceiling and keeps every part of itself below it. Casing, coil, fan and drain pan all sit in the room, in plain view. Air leaves the front face close to horizontal and runs away along the ceiling.
That single property separates it from the two formats it competes with. A ceiling cassette puts its body above the ceiling and shows one face. A concealed ducted unit puts the body and its ductwork up there and shows only a grille, sometimes in another room. Both need depth above the finished ceiling line. A suspended unit needs none, which is why it appears under hard slabs where no ceiling void exists to work with.
The visible box is bigger than most people picture. Daikin's FHA-A(9) installation manual lists body widths by class: 960 mm for 35 and 50, 1270 mm for 60 and 71, and 1590 mm for 100 through 140. Mitsubishi Electric's PCA-M·KA manual gives 960, 1280 and 1600 mm across the same ladder. The largest classes put over a metre and a half of box on open display.
So the format buys independence from the ceiling and pays for it in appearance. A wall-mounted indoor unit strikes the same bargain against a wall instead. What decides between the two is seldom taste. It is whether a wall exists at the height and in the position the room needs air from.
| Indoor format | Where the body sits | What the ceiling has to provide |
|---|---|---|
| Indoor formatWall-mounted | Where the body sitsOn a wall, high in the room | What the ceiling has to provideNothing above; the wall carries the load |
| Indoor formatCeiling suspended | Where the body sitsUnder the ceiling, fully in the room | What the ceiling has to provideStructure that can take the mass on four bolts |
| Indoor formatCeiling cassette | Where the body sitsAbove the ceiling, one face showing | What the ceiling has to provideEnough void depth to swallow the body, plus a finished ceiling |
| Indoor formatConcealed ducted | Where the body sitsAbove the ceiling, plus duct runs | What the ceiling has to provideA deeper void, duct routes and an access panel |
| Indoor formatFloor standing | Where the body sitsOn the floor against a wall | What the ceiling has to provideNothing above; floor area instead |
The rooms this format was drawn for
A ceiling with nothing above it is the strongest argument for this format. Shophouses, older blocks, workshops and shop units often have a structural slab as the finished ceiling. Building a false ceiling so a cassette can be recessed adds a trade, a cost and lost headroom. Spaces like that rarely have headroom to give away.
Tall rooms follow, and manufacturers publish limits against them. The Daikin FHA-A(9) manual rates the unit for installation up to 4.3 m on the 100 to 140 class. The 35 to 71 class is rated to 3.5 m. Those are model figures, not a property of the format, and both sit far above what a flat offers.
Distance is where the mounting position pays off again. A unit fixed under a ceiling starts its air at ceiling level, already aimed along the surface. Those are the opening conditions the coanda effect asks for, and a wall unit has to be placed carefully to get them. How far the air then travels is a throw distance question, answered by the room as much as by the box.
Against all of that, the case collapses in a typical flat. A bedroom with a free wall at the right height has no problem this format solves. A wall unit is smaller, simpler to fix, and its drain leaves at a similar height with a shorter run to an outlet.
Why height earns a smaller room
Mounting height carries a formal benefit under the R32 rules, and Daikin prints it in the same manual. The minimum floor area table splits into three cases for one refrigerant charge: ceiling-mounted, wall-mounted and floor-standing. At 4.6 kg of charge it allows 13.4 m² for the ceiling-mounted case against 20.0 m² for the wall-mounted one.
The variable behind the split is release height. A published table treating a high unit as safer on the same charge is crediting height, not this format, and a cassette collects the same credit. It matters here because the small, tall room is where the format tends to get considered in the first place.
Four bolts into the structure above
Every kilogram of the unit is carried by fixings into the ceiling. Daikin instructs the installer to use M8 or M10 bolts for hanging the indoor unit, then tighten the hanger bolts at four places. Mitsubishi Electric calls for suspension bolts of W3/8 or M10, procured locally. Its manual ships four washers and fixes the spacing between them: 917 mm on the M35 and M50, 1237 mm on the M60 and M71, and 1557 mm on the M100 through M140.
Both manuals make the ceiling itself a precondition. Mitsubishi prints it as a warning: mount the indoor unit on a ceiling strong enough to withstand the weight of the unit. The same manual specifies inserts of 100 kg to 150 kg each, and tells the installer to secure the suspension bolts or use angle stock braces or square timbers.
Daikin words the failure in two grades. Insufficient strength may result in the air conditioner falling down and causing injury. Short of that, the manual says it may lead to vibration of indoor units and unpleasant chattering noise. It warns separately that a weakly held unit can touch the ceiling and make that noise. The site-selection section tells the installer to investigate whether the place can withstand the mass, and to reinforce with beams before hanging where it cannot.
Read together, those instructions hand an owner a diagnostic order most people run backwards. A buzz or a rattle from a suspended unit is a fixings question before it is a fan question. Four anchors hold the whole load, and anything that shakes a building for years works on them: foot traffic, machinery, a roller shutter. Where a new noise arrives and nobody has looked up, indoor unit vibration is the cheaper cause to rule out.
Everything gets located before the unit goes up
The geometry goes onto paper before anything is lifted. Daikin supplies installation pattern paper, and the installer marks the hanging bolt positions, the piping outlet, the drain outlet and the wiring inlet from it. All those holes get drilled first. Mitsubishi uses the same method, and tells the installer to predetermine the length from the ceiling to within 100 mm.
That is why the drain side, the pipe side and the drop below the ceiling all stop being adjustable on day one. A wall unit can be nudged along its wall late in the job. A suspended unit is committed the moment the pattern paper comes down and the holes are cut.
What the fixings are up against
Fixings age differently from the machine they hold. A compressor or a fan fails on hours run. Anchor bolts loosen on cycles of movement, and those arrive from outside the aircon entirely. Nothing on the unit's own service schedule measures that, so the loosening stays silent until it turns audible.
That is why a support check belongs at the front of any service visit on this format. It costs a look and a spanner. Skipping it means cleaning a coil on a unit whose four anchors have been working loose for a year.
Where the water goes when the ceiling is solid
The drain leaves a suspended unit near ceiling height, which is the highest point in the room and the worst place to start a gravity run. Both manuals still treat gravity as the default. Daikin requires the drain piping to be as short as possible with a downward inclination of 1/100 or more. Mitsubishi specifies VP-20 pipe at 26 mm outside diameter, with 1/100 or more downward slope.
Whether that fall exists is a site question, settled before anything is ordered. Daikin allows the drain to leave right rear, right side, left rear or left side. Routing it left means moving the socket cap and insulation across to the right socket. Mitsubishi handles the same choice with a rubber plug pushed into the unused port. The side gets chosen once, against where the nearest outlet actually sits.
No usable fall means a pump, and Daikin lists the drain pump kit as an optional accessory. That wording is the honest reading of the part. It goes in because the geometry refused, not because the format includes one. A condensate lift pump then puts a powered component and a float switch into a run that had neither.
Quotes on these units go wrong at exactly that point. A drip from a suspended unit gets called a failed drain pump over the phone, and plenty of these installations were never fitted with one. Ask first whether a pump exists. On a gravity run the fault sits in the fall, the supports or a blockage, and the drain gradient is the thing to check before any part is ordered.
| What the drain run meets | What the manuals require | What it means for the install |
|---|---|---|
| What the drain run meetsA fall the whole way to the outlet | What the manuals require1/100 or more downward slope, in both manuals | What it means for the installGravity works, and there is nothing to power |
| What the drain run meetsAn outlet higher than the drain socket | What the manuals requireGravity cannot climb | What it means for the installA drain pump kit, listed by Daikin as optional |
| What the drain run meetsA long horizontal run | What the manuals requireSupports at 1 to 1.5 m so the pipe cannot deflect | What it means for the installA sag becomes a low point that holds water |
| What the drain run meetsWarm indoor air along the pipe | What the manuals requireInsulate all drain piping running indoors | What it means for the installBare pipe sweats and the drips read as a unit fault |
| What the drain run meetsSewage giving off ammonia odour | What the manuals requireDo not connect the drain piping directly to it | What it means for the installAmmonia travels back and corrodes the heat exchanger |
The check that proves the fall
A water test closes the drain work, and it is the only proof the fall actually works. Daikin has the installer pour about 0.6 litres into the drain pan from the air outlet and watch it flow. Mitsubishi asks for about 1 litre through the same route.
Ask whether that test was done, because it is cheap at install and awkward afterwards. A gravity run that was never proven can behave for months, then back up in a humid stretch when the unit makes more water than usual. The fault reads as new. It dates from the day the pipe was clipped up.
Access from below, and who this is really for
Access is the difference an owner feels at every service visit. The whole unit is in the room, so filter, grille and panels come away from below with nothing else opened up. Daikin's suction grille releases on fixing knobs, two places on the 35 and 50 classes and three on 60 through 140. Mitsubishi's intake grille works on two or three holding knobs, and each side panel comes off on one screw. That manual also caps the grille at 120 degrees, past which the hinges can be damaged.
Set that against the formats it displaces. A cassette and a concealed ducted unit both put part of every job above a finished ceiling, which turns ceiling void access into its own problem and occasionally its own trade. Here the question never arises, because there is no void to reach into.
One Daikin instruction about access reads oddly until the reason lands. Do not bond the drain socket and drain hose, the manual says, because maintenance and inspection for the heat exchanger and others cannot be conducted afterwards. Glue at that joint saves a minute during install and costs a coil inspection for the life of the unit. Worth naming at handover, since nobody sees it once the panels are back on.
Put the whole published envelope together and it points at trade spaces. Ceilings rated to 4.3 m, bodies up to 1.6 m wide, fixings engineered on site into a structural slab, and a discharge Daikin wants at 2500 mm or more above floor level on a high mounting. Those are shop, workshop and office dimensions. Nothing in either manual rules out a home, and the test is identical in both places: whether a void exists above, and whether a wall sits where the room needs air from. Two noes make this the answer. A yes to either usually points elsewhere.
The gap around the box
Clearance is what makes that access real, and Daikin publishes numbers for it. The manual asks for 300 mm or more at the suction side and 30 mm or more at the sides. Where extra space is available, it adds that servicing can be conducted more easily if 200 mm or more is secured.
A unit pushed tight into a corner still cools. What it costs is every visit afterwards, and that bill lands on the owner, not the installer. Worth measuring the gap before the position is agreed, since nothing about it changes once the bolts are in.
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