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 in profile, and fixed along a wall or under a ceiling line. On a quote it sits as a line item beside the piping. In the flat it is the only part of the whole route most people ever see.

The word is local trade shorthand, which is part of 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, cannot lose gas, and cannot 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 are what age exposed insulation fastest, so the casing is doing 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 gets picked on the day against the bundle it has to carry. The common profiles sit roughly in the 75mm to 150mm range, with the wider sections used where several pipe pairs share one route.

The failure mode is a run sized to the pipes rather than to 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 inside face of 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 does not crush it, it kinks it. A kink holds a small pool of standing water, which is where growth starts and where a choke eventually forms. A drain that blocks in the same spot after every clearing is often telling you about the width of the casing around it, not about 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 separate moulded fittings for internal corners, external corners, flat bends, tees, joiners between lengths, and end caps where the run finishes at a wall penetration or at the unit. Each piece is a base fixed to the surface and 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 at a seam. Water gets out at a seam. That pairing 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. The check that costs nothing is tracing the run upward from the mark to the first joint, and opening that one first.

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 standing proud of the base or skips the sealant. Look at corners before you look at anything else 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. Reaching a joint means cutting the wall, making good afterwards, 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. It is 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, because 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 the job when the day is running long. Whatever discipline the install had tends to show up in the covers and the corners.

Almost every visible fault on a run is reporting on its contents, not on 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. A yellowed length usually means sun exposure the route was never planned for, and a stained length usually 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 is the one sign worth acting on early, because the drip that follows is avoidable and the work is smaller before anything has been sitting wet.