Drips only on humid days: mineral buildup narrows the drain

A wall unit in an older home dripped only on humid days. The owner feared a cracked drain pipe inside the wall that would need hacking. But a crack drips at a steady rate whatever the weather, so humidity-linked dripping points elsewhere.

By Team Snowflake | Updated 28 Jul 2026

Case summary

Hitachi Wall-mounted14 years oldLandedBishan, Singapore

Concern: Feared the drain pipe inside the wall had cracked and the wall would need hacking open to replace it.
Found: Mineral buildup at a joint further along the drain run had narrowed the pipe's bore, so higher condensate volume on humid days backed up toward the indoor unit
Key check: Traced the full drain pipe run and checked each joint for buildup before assuming the pipe was cracked
Result: The dripping stopped. The pipe now drains freely on both dry and humid days. The wall was left untouched and the original pipe stays in service.

What we were told

The living room aircon has been dripping on and off for a few months. It only happens on really humid days, and on dry days it is fine. The house is old and the pipes run through the wall. We are worried the drain pipe has cracked inside and the wall will need hacking open.

What we checked

Dripping that comes and goes with humidity means the drain copes with normal water but overflows once the volume rises. A cracked pipe leaks at a steady rate regardless of weather. The pattern pointed to a flow problem, not a break, so we traced the full drain run from the unit to the discharge point and checked each joint along the way for buildup.

The pipe run near the unit showed a steady, even slope with no dip or sag anywhere along it.
A joint further along the run had a hard, chalky buildup narrowing the inside of the pipe.
On humid days the extra water arrived faster than the narrowed joint could pass, and it backed up toward the unit.
After the joint was cleared, water flowed freely through the whole run at peak flow.

What we found

The drain pipe's slope had always been fine; the problem sat further along the run, at a joint where mineral deposits from the water itself had slowly built up on the inside wall. Layer by layer over the years, that buildup narrowed the pipe's bore at just one point. On ordinary days, the reduced opening still passed the unit's output without trouble. On humid days output rose sharply, and water arrived at the narrowed joint faster than it could pass through, so it backed up and overflowed from the drain pan. The pipe itself had no crack, no sag, and no loose bracket anywhere along the route.

What fixed it

The drain pipe was not cracked and did not need re-levelling. It had no splits, holes, or separated joints anywhere along the run. We opened the affected joint, cleared the mineral buildup from inside the pipe, and confirmed the bore was restored to its original width. We then poured water through the full run at peak flow to confirm no backing up. No hacking, resupporting, or pipe replacement was needed.

Outcome

The dripping stopped. The pipe now drains freely on both dry and humid days. The wall was left untouched and the original pipe stays in service.

What this case teaches us

Drips tied to humid weather often mean a narrowing drain, not a crack

A crack drips at the same rate every day. A drip that shows only on humid days means the drain cannot keep up with the extra water.
Drain pipes can narrow from the inside as mineral buildup collects at a joint over the years. A single narrowed point can hold back a rising flow.
Ask for the full drain run to be traced and each joint checked for buildup before anyone quotes to hack the wall or resupport the pipe.

Questions this case answers

Aircon drips only on humid days and stays dry otherwise: why?

The drain copes with normal condensate and cannot handle the extra. A crack drips at the same rate whatever the weather, so a pattern tied to humidity points at flow, not a break. Tracing the whole run in this landed home turned up a joint with hard, chalky buildup narrowing the pipe from inside.

Can a drain pipe get blocked without anything falling into it?

Yes. Minerals in the water itself deposit on the inside wall layer by layer, and over the years they narrow the bore wherever they gather. The opening left here still passed ordinary output without trouble. Only when humid weather lifted the volume did water arrive faster than the narrowed point could clear.

Will the wall need hacking to fix a drain problem in an older house?

It was not needed here, and that fear was what the owner arrived with. The pipe near the unit held a steady, even slope with no dip or sag, and no splits, holes or separated joints turned up anywhere along the route. Clearing the buildup from inside the affected joint restored the bore.

What should be checked before anyone quotes to open a wall?

The full drain run, traced from the unit to the discharge point, with every joint checked for buildup. After the clearing here, water was poured through at peak flow to verify nothing backed up. The pipe now drains on dry and humid days alike, and the original run stays in service.

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