Concealed ducted aircon: what the clean ceiling costs

A concealed ducted unit is an indoor unit built to be buried. The body hangs inside the ceiling void, and short ducts carry its air out to a grille or a linear slot. Nothing of the machine reaches the finished surface. The opening a room shows can sit well away from the body feeding it, and often not in that room at all.

Everything inside the casing is ordinary. A filter, a coil, a blower and a drain tray, arranged the way any fan coil unit arranges them. Capacity, brand and refrigerant behave no differently for being out of sight. Position is the whole of what changed, and position is what this page weighs.

One body does not have to feed a whole flat, though that is the picture most buyers arrive with. In condos here a slim body often serves a single room, or a pair of rooms opening off the same corridor. A single machine feeding every room through a branched duct tree is a larger purchase carrying its own consequences. What one shared coil means for every room hanging off it is set out in how ducted aircon systems work.

Against a ceiling cassette the difference is where the air gets handed over. A cassette takes its own square of ceiling and pushes air out of the panel it sits in, so the machine and the opening are one object. A concealed body splits them apart. That split is why the format exists, and why looking after it works differently.

Deep enough for the body, and then deeper again for everything leaving it. Daikin publishes its FDXM-F3 slim concealed range at 200 mm high, 620 mm deep, and either 750 mm or 1,150 mm wide depending on capacity. The datasheet for that range states it mounts in a ceiling void of 240 mm. That is the slim end of the market. Bodies built for heavier loads are taller, and the gap above the ceiling grows with them.

Height is the number people check, and width is the one that catches them out. A casing over a metre across and more than half a metre front to back needs a clear rectangle, not a gap. Where a ceiling was pushed down around structure or a run of services, that rectangle comes out smaller than the plan suggests. Anything already crossing that stretch of ceiling void was routed for its own convenience, not around a machine nobody had specified yet.

A published void figure covers the box and stops there. The duct leaving the body, the fall on its drain line, the lagging wrapped around both, and the rods carrying the weight are separate claims on the same gap. Each one is small on its own. Together they decide whether a datasheet number survives contact with a real ceiling.

Every millimetre of that depth comes out of the room underneath. On a fixed slab height, concealment gets paid for in headroom across whatever area the ceiling drops. So where the drop lands is a design decision with a comfort consequence attached. A low ceiling over a corridor costs almost nothing. The same drop over a sofa is felt every evening.

Duct is the price of the separation. Air leaving a wall unit is in the room already. Air leaving a concealed body still has a journey ahead of it, and everything happening on that journey happens where nobody can watch.

One body means one filter, however many outlets hang off it. Room air comes back through a return grille, crosses a single filter and a single coil, then gets pushed out again. Two bedrooms sharing a body do not have a filter each. They have one between them, doing the work of both.

The far outlet is where the arithmetic shows up. A branch that runs longer and turns more corners than its neighbour delivers less air, so whatever sits at the end of it runs warmer. Setting that split deliberately is what air balancing exists for, and it is a commissioning step with consequences of its own.

Filtration hits a ceiling here as well. A denser filter takes more out of the air and resists it harder, and that resistance comes out of the same allowance the duct route is already spending. Asking a concealed body to carry a finer filter than it was specified for is a request the fan answers by moving less air.

Get the number of rooms one body will feed written down before a layout is signed off, along with where the single return lands. A body sized for two rooms and later asked to cover three is a change made on a drawing that nobody goes back to. The room added last is the one that runs warm, and by then the ceiling has closed.

• •One filter, so a choked return weakens every outlet at the same time.
• •One coil, so a fouled face takes all of them down together.
• •One blower, so a fan fault empties the whole set of rooms.
• •One drain tray, holding water condensed on behalf of every room it feeds.
• •One setpoint at the controller, unless dampers were fitted to divide the air.

A servicing figure quoted per unit describes different work on the two formats. On a wall unit a technician arrives, drops the front flap, and the filter is in hand. On a concealed body the same task starts with a ladder, a covered floor, and a leaf that has to come down and go back up afterwards.

Filter position gets decided at renovation and lived with from then on. It sits either at the body itself or behind the return grille, and only one of those can be reached without going up into the ceiling. Which one a household ends up with is worth asking about while the layout is still a drawing, because it settles whether the routine item stays routine.

The room underneath joins the job. Covers go down, furniture shifts, and the space stops being usable while anyone is working overhead. In a bedroom that means the bed moved off its wall. In a shop or a clinic it usually means somebody working when the doors are shut.

Two servicing quotes for the same flat only line up once each one states what it assumes about getting in. A quote written against an opening that already exists and a quote written against a sealed plane price two different pieces of work. Making good a ceiling appears in exactly one of them. Who positions the opening, and which trade supplies the leaf, is the argument behind every false ceiling access panel.

Drainage usually gains a moving part. A body sitting at ceiling level has nowhere to fall to, so many slim bodies carry a pump inside them. Daikin lists a standard drain pump with a 750 mm lift across the FDXM-F3 range. What a condensate lift pump adds to ownership gets its own page. The refrigerant run went up there too, and concealed piping covers what reaching that later involves.

The look is a real gain and nothing else recovers it. A room with no box on the wall reads differently, and no amount of neat trunking gets a wall unit to the same place. Anyone weighing this should say that part plainly instead of arguing themselves out of wanting it.

It earns its place most cleanly where a ceiling was coming down anyway. Cove lighting, a bulkhead, a services run: once a plane is being built, putting the body inside it extends a decision already taken. The extra cost is the depth and the opening, and both stay cheap right up until the ceiling closes.

It earns it least in a bedroom with one clear wall and a flat ceiling. Building a ceiling to hide a unit that already had somewhere to go is a great deal of work for a wall nobody looks at. A ceiling cassette sits between the two, since the panel it shows carries its own filter and hands it down without anyone entering the void.

The condition attached to all of it is that access gets settled in the same conversation as the look. A household that wants the unbroken plane and refuses a visible leaf has chosen a ceiling somebody cuts open every time the machine needs attention. That is a defensible choice. It stops being defensible when nobody said it out loud at the time.