Aircon Trunking Explained: What Runs Inside the Casing

Trunking is the plastic casing that hides the pipe run between the indoor unit and the outdoor unit. It is usually white PVC, roughly rectangular, fixed along a wall or under a ceiling line. On a quote it sits beside the piping, and in the flat it is the only part of the route most people ever see.

The word is local trade shorthand, which is why nobody stops to define it. Elsewhere the same casing gets called a line-set cover or a duct; here it is trunking, on every quote, in every message thread with a contractor, and in most complaints about how a finished install looks.

Calling it a casing rather than a pipe changes what you expect of it. A casing cannot leak, lose gas, or choke; everything a run appears to do wrong is done by something inside it. The plastic only decides how visible the problem becomes and how hard it is to reach.

The casing does have two jobs of its own. It keeps the insulation on the refrigerant lines out of direct sun and away from knocks, and it holds the bundle in a fixed line so nothing sags between fixings. Sun and physical damage age exposed insulation fastest, so the casing does real work even though it carries nothing itself.

A trunking run carries four things, not one. Two copper refrigerant lines, the foam insulation sleeved over each of them, the condensate drain line, and in most installs the interconnecting cable that lets the indoor and outdoor units talk to each other. The casing is a bundle holder.

The two copper lines are different diameters and carry refrigerant in different states. The thinner liquid line runs warm. The thicker suction line runs cold, which makes it the one that sweats when its insulation is thin, gapped, or crushed. Both are sleeved in closed-cell foam that has to stay continuous from end to end. A gap the width of a finger at a bend is enough to start a drip.

The condensate drain line is the odd one out because it works on gravity alone. It carries the water the indoor unit pulls out of the room air, and it needs a continuous fall from the fan coil to the discharge point. Nothing pushes that water along. If the route dips, the water sits.

The interconnecting cable usually shares the same casing. It carries power and the signal between the two units, so damage to it shows up as an error code or a unit that will not start, never as water. That is why a nicked cable behind a cover gets blamed on the aircon itself, and why renovation work near a run is worth flagging before it begins.

Trunking comes in a small set of standard widths, and the one chosen at install is close to permanent. Changing it later means stripping the whole run off the wall, so the size is picked on the day against the bundle it carries. The common profiles sit roughly in the 75mm to 150mm range, with wider sections where several pipe pairs share one route.

The failure mode is a run sized to the pipes rather than the bundle. Copper measures small; copper plus foam plus a drain line plus a cable measures a lot larger, and the foam is the part that gives. Force that bundle into a casing that is nominally big enough and the insulation compresses at the tightest point, usually a corner.

Compressed insulation stops being insulation. Closed-cell foam works because of the air trapped inside it, so flattening it to half its thickness does not halve its performance; it removes most of it. The cold line then sits against the casing with room air on the other side, and condensation starts at exactly that point.

The drain line suffers differently. It is flexible, so a tight run kinks it rather than crushing it, and a kink holds a small pool of standing water where growth starts and a choke eventually forms. A drain that blocks in the same spot after every clearing is often telling you about the casing around it, not the drain.

This is why the trunking question deserves an answer at quote stage on multi-split systems. A System 3 or System 4 pushes several pipe pairs along at least part of a shared route. If nobody has walked that route and sized the casing to its widest section, the decision gets made by whoever is on the ladder with the material already cut.

Trunking is not one continuous piece. It arrives as straight lengths with moulded fittings for internal and external corners, flat bends, tees, joiners, and end caps where the run finishes at a wall penetration or the unit. Each piece is a base fixed to the surface with a cover clipped over it.

Every fitting is a seam, and a seam is both an entry and an exit: humid room air gets in, and water gets out. That is why a wet patch almost never sits directly below the actual fault. Water inside a casing runs downhill along the base until it meets the first gap, then leaves there.

If someone quotes a repair based on where the stain is without opening the run above it, push back. The stain marks the lowest gap, not the source, and tracing upward from the mark to the first joint costs nothing.

Corners are where sealing quality shows first. A straight length is easy to close properly; an internal corner in a ceiling void, worked overhead with limited clearance and no one watching, is where a rushed job leaves the cover proud of the base or skips the sealant. Look at corners first on a finished run.

Once water actually shows, the question stops being what trunking is. A leak along trunking casing narrows to three usual sources, and telling them apart runs on where the mark sits and how long the unit had been running before it appeared.

A run can be surface-mounted on the wall, hidden above a false ceiling, or chased into the wall and plastered over. The three look different on handover day and behave very differently on the day something needs attention.

Surface trunking is the least attractive and the most serviceable. The cover lifts off, the fault is visible in minutes, and the repair stays confined to the section that failed. Most HDB installs run it along the ceiling line for that reason.

A false-ceiling route hides the run and keeps most of the access. The lines sit above a panel rather than behind plaster, so opening up means lifting a section of ceiling instead of breaking one. The trade is that nobody sees a problem developing: a drip that would mark a surface run straight away can wet a ceiling board unseen.

Chased and plastered piping is the tidiest and the least forgiving: there is no cover to lift, so reaching a joint means cutting the wall, making good, and accepting that the finish will never quite match. Concealment of that kind is reasonable when the run is short, straight and pressure-tested before it disappears, and a poor bet when the route has bends nobody has documented.

The question worth asking before anything gets covered is a plain one: if this run needs opening later, where does the opening happen and what gets damaged? A contractor with a ready answer has thought about the route; one who treats the question as pessimism has not.

Trunking condition is a readable summary of the install behind it. The casing is the only part of the route a homeowner can inspect without tools. It is fixed by hand, in the open, at the end of a long day, and whatever discipline the install had tends to show up in the covers and corners.

Almost every visible fault on a run is reporting on its contents, not the plastic. PVC does not sag by itself; it sags because the fixings were spaced too far apart or the bundle inside is heavier than the profile was sized for. It does not discolour by itself either, so a yellowed length usually means sun exposure the route was never planned for, and a stained length means water has been inside long enough to carry dirt with it.

Covers that will not sit flush are the most common finding and the easiest to read: a cover fighting the bundle inside it is the crushed-insulation problem announcing itself before any water appears. That sign is worth acting on early, because the drip that follows is avoidable and the work is smaller before anything has sat wet.