Aircon Ceiling Drip, Concealed Drain Joint

Case Details

What We Checked

• Replaced drain section was intact and sealed — no leak at the new joint. The previous contractor's work was done correctly; the fault was simply not in that section.
• Water staining pattern on the ceiling tile indicated the drip origin was further upstream from the replaced section, based on the direction and spread of the moisture marks.
• Ceiling panel opened to expose the concealed portion of the drain run — a section that had not been inspected during the previous repair visit.
• One pipe joint behind the panel had separated by approximately two millimetres, leaving a hairline gap visible under close inspection with a torch.
• Water was weeping through the gap under test conditions — we poured measured volumes through the drain to confirm the joint was the leak source at flow rates matching humid-day condensate output.

The Diagnosis

The concealed drain joint had separated by roughly two millimetres over the ten-year life of the system. PVC drain joints in ceiling runs rely on solvent cement that bonds at installation, but thermal cycling — the pipe expanding when warm and contracting when cold, twice a day, every day — gradually stresses that bond. In this case, the joint behind the ceiling panel had crept apart just enough to create a hairline gap. On normal-humidity days, condensate flow was light and water passed the gap without pooling. On humid days, the evaporator pulled significantly more moisture from the air, producing a heavier condensate stream. That higher volume backed up at the gap, overflowed the joint, and dripped onto the ceiling tile below. The previous contractor replaced only the visible downstream section, which was fully intact — the actual leak was upstream, hidden behind the panel where it could not be seen without opening the ceiling.

What Fixed It

We cleaned both pipe surfaces at the separated joint to remove residual cement and debris, applied fresh PVC solvent cement to both faces, reassembled the joint, and held it under compression for the full cure time specified for the adhesive. While the ceiling panel was open, we inspected every other concealed joint along the run — all others were intact with no signs of separation, stress cracking, or discolouration that would indicate early degradation. We then closed up, ran the unit on full cooling at maximum fan speed for thirty minutes to generate peak condensate flow, and monitored the ceiling tile for any moisture. The repair held under load with no drip. We also poured additional water through the drain at above-peak flow to stress-test the joint. No section of the drain run needed replacement, and the ceiling work was limited to the single access panel we opened for inspection.

The intermittent drip stopped. The unit has run through several humid days since with no recurrence and no ceiling work beyond the access panel.

Why This Happens

Intermittent ceiling drips often point to concealed joints.

• A drain run has many joints behind panels that are easy to skip during a repair on the visible section. Fixing only what you can see leaves gaps further up the line. The leak comes back days or weeks later, once humid weather pushes more water past the gap.
• Drips that show up only on humid days point to a small gap — measured in millimetres, not centimetres. The coil produces far more water on humid days, and the higher flow spills past a gap that lighter loads drain through safely. That humidity pattern is the key clue that sets a joint gap apart from a blocked drain or cracked pipe.
• When a visible drain repair fails to stop a leak, the next step is tracing the full run joint by joint behind the ceiling panel. Open access panels and check each joint with a torch — not just the parts already in view. A pour-through test at peak flow confirms or rules out each joint.
• PVC drain joints in ceiling runs wear slowly from thermal cycling. The pipe expands when warm and shrinks when cold, twice a day, every day the unit runs. After seven to ten years, that stress can pull a glued joint apart by one to two millimetres. Hidden joints fail first because no one checks them during routine servicing.